## Working with previews in PixInsight

When working in PixInsight you probably make use of Previews a lot. And I really mean a lot! Most processes can take quite a bit of trial and error to get the right settings and fine tune them for the optimal result. We run these processes on previews so that it is much faster to see the effects compared to running it on the complete picture. Furthermore, we can use multiple copies of the same preview and apply the process with different settings so we can really compare the results of our fine tuning in great detail.
In this post I want to give you some quick tips for working with previews in PixInsight so that you may use them more efficient and convenient.

I must admit I don’t always do this myself, but I ran into problems because of it more than once. Nothing more annoying than to select the wrong preview for a process. Happens to me quite frequently in ColorCalibration for instance. It helps to consistently adjust the identifiers of your images and previews. Just double click the preview tab in the image side bar or right click and choose ‘Identifier’ and give the preview a descriptive name.

Cloning previews
In order to compare different operations on the same preview we need to create copies of the same preview. There are a two ways of doing this;
– right click the preview and choose ‘clone preview’
– drag the preview tab within the same sidebar

Please note that if you copied a preview which had some process applied to it, the new preview won’t have this process applied. So this way you can compare the ‘before’ and ‘after’ by simply comparing these two previews. Apply another process with different settings to compare the effect of the changed settings.

Copying zoom level
Note that the zoom will not be cloned and will be reset in the new preview.
In order to copy the zoom level, simply drag the preview tab from the preview with the zoom level you want to copy on top of the other preview tab.

Reset preview
If I ran a process on a preview which didn’t give good results I used to copy the preview to get a new one without a process applied and deleted the old one. This is quite inconvenient and actually there is a much simpler way to deal with this; you can simply reset the preview; hit Ctrl + R (or Cmd + R on mac) or right click on the preview tab and choose ‘reset’.

Cycle through previews quickly
In order to compare 2 or more previews you probably click the preview tabs and try to spot and judge the differences. When you do this by clicking the preview tab you (subconsciously) are looking away from the preview image itself for a short while to make sure you click the in the right spot on the preview tab. To prevent this and make it more easy to ‘blink’ the previews, simply use the keyboard short cut: Ctrl -> and Ctrl and Cmd Create new image from preview
Simply drag the preview tab outside the sidebar of the image you are working on. A new image will be created with just the preview. A very easy way to make different crops of the same image for instance!

Duplicate preview on another image
Sometimes you want to compare the same preview on different images. I use this to compare stacking results sometimes or maybe you want to compare the results of different settings on a preview by looking at them side by side at the same time instead of ‘blinking’ through the previews. It can be really annoying to get exactly the same preview on the other image. But again, there is a really easy way to do this; simply drag the preview tab from the sidebar of one image to the sidebar of the other image.

Duplicate multiple previews on another image
You might want to copy all previews on one image to one or more other images. For instance if you want to compare the previews of luminance with R, G and B versions of the image. You can drag an drop the previews like explained before, but this is tedious.Luckily there is a script that can do this for you in 1 go; PropagatePreviews. You can find it under Script -> Utilities -> PropagatePreviews.

Make sure you have selected the image that contains the previews you want to copy when opening the script. Simply check the previews you want to copy and select the images you want them to copy to. Unfortunately it doesn’t copy the Identifier you used on the previews.

Aggregate previews in one image
When you want to show the comparison of different previews or want to check the effects of different settings side-by-side you can aggregate multiple previews in one new image. Go to Script -> Utilities -> PreviewAggregator.

In the script window you can simply select the previews you want to aggregate in the new image.

I hope you found these tips on working with previews in PixInsight helpful. Please let me know in the comments below if I forget some useful preview tips or if you have any questions.

## ¿Qué es un eclipse lunar?

El eclipse lunar es un fenómeno astronómico que se produce cuando la Tierra se sitúa entre el Sol y la Luna, proyectando su sombra sobre ésta y oscureciéndola.

El eclipse de Luna es uno de los eventos astronómicos más hermosos y se puede observar a simple vista sin necesidad de telescopios. Durante el eclipse podemos apreciar como la sombra de la Tierra avanza sobre la superficie lunar, primero reduciendo el brillo de esta a medida que se sumerge en la zona de penumbra y luego adquiriendo una tonalidad rojiza cuando se adentra en la sombra.

La Luna gira alrededor de la Tierra en un ciclo de 27,5 días que denominamos periodo sidéreo pero durante este tiempo la Tierra se ha movido también alrededor del Sol por lo que para que volvamos a ver a la Luna en la misma fase tienen que pasar los 29,5 días del periodo sinódico.

Los eclipses de Luna se pueden ver desde cualquier parte de nuestro planeta si es de noche y suelen tener una duración de varias horas.

## ¿Por qué no se produce un eclipse Lunar cada Luna llena?

La Luna gira alrededor de la Tierra en una órbita con una inclinación de unos 5º con respecto al plano orbital de la Tierra alrededor del Sol, por este motivo la Luna no siempre pasa por detrás del cono de sombra de la Tierra, a veces pasa un poco por arriba y otras por debajo de ésta. Para que se produzca el eclipse lunar nuestro satélite debe encontrarse en el nodo o punto de intersección entre su órbita y el plano orbital terrestre.

Solo se producen entre dos y cinco eclipses lunares al año de los cuales los eclipses totales se producen dos veces cada 3 años. También es interesante saber que cuando se produce un eclipse de Sol, 15 días después se puede observar también un eclipse de Luna.

## ¿Qué tipos de eclipse de Luna hay?

Dependiendo de la cómo la sombra de la Tierra se proyecta sobre la Luna podemos hablar de 3 tipos diferentes de eclipse:

### Eclipse total de Luna

Un eclipse total lunar se produce cuando la sombra de la Tierra oculta toda la superficie de la Luna. Es el eclipse lunar más vistoso en el que la Luna se oscurece totalmente adquiriendo tonalidades rojizas, marrones o incluso más oscuras.

### Eclipse parcial

Hablamos de eclipse parcial de Luna cuando solo una parte de de la Luna se ve ocultada por la sombra de la Tierra. Hay que tener en cuenta que durante los eclipses totales también se produce una fase de parcialidad antes y después del eclipse total.

### Eclipse penumbral

El eclipse penumbral es tan solo un leve oscurecimiento de la Luna debido a que la sombra de la Tierra no oculta a nuestro satélite y esta solo atraviesa la zona de penumbra que rodea a la sombra. Dentro de los eclipses penumbrales podemos encontrar la circunstancia de que toda la superficie de la Luna quede bajo la penumbra o que solo una parte de nuestro satélite se vea afectado en una parte. Estos últimos son los eclipses más difíciles de apreciar ya que la luna solo ve reducida una parte muy poco significativa de su brillo.

Se suele utilizar la escala de Danjon para medir la oscuridad de los eclipses lunares. Esta escala fue propuesta por André-Louis Danjon en 1921 y va desde el valor L=0 para indicar mayor oscuridad a L=4 para menor oscuridad.

• L=0: Eclipse muy oscuro, la Luna es casi invisible en la semitotalidad.
• L=1: La Luna adquiere tonalidad gris oscura o pardusca, hay pocos detalles visibles.
• L=2: Eclipse rojizo o rojo pardusco con área central más oscura, regiones externas muy brillantes.
• L=3: La Luna adquiere una tonalidad roja brillante, habitualmente con un borde amarillento.
• L=4: Eclipse anaranjado o cobrizo, muy brillante, a veces con un margen azulado.

Estas diferencias de tonalidad pueden ser producidas por el grado de inmersión de la Luna en el cono de sombra pero también por la presencia de partículas en la atmósfera de nuestro planeta. Por ejemplo, las erupciones volcánicas o grandes incendios forestales pueden producir eclipses más oscuros.

## ¿Qué podemos ver durante un eclipse de Luna?

Hay algunos fenómenos ópticos muy interesantes que podemos observar durante un eclipse lunar. En primer lugar el paulatino oscurecimiento de nuestro satélite pero también los diversos cambios de tonalidad a media que la Luna se adentra en la sombra de la Tierra.

En ocasiones es posible ver un borde azulado junto a la umbra. Esa tonalidad azul es producida por la capa de ozono de nuestra atmósfera que absorbe la luz roja.

También podemos apreciar que la Luna adquiere cierta profundidad o sensación de 3D a medida que avanza el eclipse. Durante la fase de Luna llena ésta se muestra plana ya que no hay sombras sobre su superficie. Durante el eclipse la Luna si que muestra perspectiva de profundidad debido a las diferencias sutiles de iluminación. Su observación con prismáticos es una experiencia única que no te dejará indiferente.

También es posible observar el impacto de meteoros en la superficie de la Luna durante los eclipses. Estos se muestran como pequeños destellos luminosos en la superficie. No es habitual pero nosotros lo vimos personalmente en una ocasión y son varios los registros fotográficos de estos eventos.

Otro fenómeno muy llamativo es la paulatina aparición de estrellas en el cielo a medida que la Luna va perdiendo su brillo. Es fácil observar incluso ocultaciones de estrellas por nuestro satélite durante la fase de totalidad.

## ¿Cuánto dura un eclipse lunar?

Los eclipses lunares duran varias horas y las diferentes fases se determinan por sus sucesivos contactos con la sombra o penumbra. Un eclipse total puede durar hasta 6 horas. Las fases de un eclipse total son las siguientes:

• P1 (Primer contacto): Inicio del eclipse penumbral. La Luna alcanza el límite exterior de la penumbra.
• U1 (Segundo contacto): Inicio del eclipse parcial. La Luna alcanza el límite exterior de la umbra.
• U2 (Tercer contacto): Inicio del eclipse total. La Luna se adentra completamente en la umbra.
• Máximo del eclipse: Fase central del eclipse y de mayor ocultación de la Luna. La Luna está en su punto más próximo al centro de la umbra.
• U3 (Cuarto contacto): Fin de la fase de totalidad. El punto más externo de la Luna sale de la umbra.
• U4 (Quinto contacto): Fin de la parcialidad. La Luna sale de la umbra terrestre.
• P2 o P4 (Sexto contacto): Fin del eclipse penumbral. La Luna sale completamente de la penumbra terrestre.

En un eclipse parcial no se producen las fases U2 y U3 y en un eclipse penumbral no hay fases U1, U2, U3 ni U4.

La duración de las diferentes fases es muy variable y depende de la distancia de la Tierra a la Luna que no es constante debido a la excentricidad de la órbita. El eclipse será más largo cuanto más cerca del apogeo se encuentre la Luna.

## ¿Cuando se podrán ver los próximos eclipses totales de Luna?

Aquí puedes consultar algunos eclipses totales pasados que hemos vivido personalmente y las fechas de los próximos eclipses de Luna:

Eclipse total de Luna del 28 de septiembre de 2015 y crónica de observación desde Cubas.

31 de enero de 2018: Visible solo en Australia y Asia.

Eclipse total lunar del 27 de julio de 2018 y crónica de observación desde Corral de Almaguer.

Eclipse total de Luna del 21 de enero de 2019 y crónica de observación desde Madrid.

26 de mayo de 2021: Visible en América, Australia y Asia.

Eclipse total de Luna del 16 de mayo de 2022 y crónica de observación desde Carranque.

8 de noviembre de 2022: Visible en América del norte, Australia y Asia.

14 de marzo de 2025: Visible en América, Australia y Asia.

7 de septiembre de 2025: Visible en Asia, África y Europa.

3 de marzo de 2026: Visible en Nueva Zelanda, Australia y Asia.

31 de diciembre de 2028: Visible en Europa, África, Asia, Australia y Pacífico.

26 de enero de 2029: Visible en América, Europa, África y Asia.

20 de diciembre de 2029: Visible en América, Europa, África y Asia.

## Control Your Consumption – Astroniklas

When you’re planning to head out for astrophotography, one of the things in your check lists it to figure out your power consumption (or it should if you haven’t figured that out yet!). Some people rely on batteries, others again have a steady power supply from a plug in their homes/outdoor observatory sites, and finally some would rely on a power generator.

To figure out how much power your equipment consumes per hour, there’s a simply calculation method. If you know your Watt-hours and Volts (most astronomy equipment is powered by 12 Volts) to Ampere-hours you can use a simple formula to discover the amount. Supposedly your Watt-hours is 240 then we get;

$Ah = \frac{Wh}{Volts} => \frac{240}{12} = 20Ah$

Now, to convert how much time would that give us, create a simple list of all your equipment and how many Amperes each one of them consumes. In my case,

• Mount (NEQ6 Pro): 4 Amp
• Cooling Camera (ASI1600GT): 2 Amp
• RCA Dew Heaters (1 Amp each): 2 Amp
• Lakeside Focuser: 1 Amp
• EAGLE Pro (Mini-PC + Power Management Unit): 1 Amp

That would give us a total of 10Ah. Supposedly I’m using a Duracell battery of 20Ah, then my power consumption would end up discharging my battery source after 2 hours (20 Ah / 10 Ah).

Instead, in my case I would then need a steady powersource for much longer than that. In average my observing sessions are no less than 3 hours (on mediocre nights) or even up to 4 or 5 hours when there are really beautiful night skies making it worthwhile to stay up longer.

A battery would be enough to just observe visually, but definetely wouldn’t take me a long way for astrophotography. And to make matters worse with batteries, they shouldn’t go below 20% of their total capacity if you want them to be long lived, or say goodbye to an expensive battery after just a few sessions!

I’ve decided that, for my own personal gain to buy a power generator that would provide me a reliable power source for many hours at end, without risking killing any expensive batteries, damage my equipment or to abandon a beautiful night sky. Additionally it gives me great independence from anything when it comes to sudden power outages, people around, or ending up running out on battery sources.

The downside is of course a solid power generator would become heavy to carry around (the one I’m looking at is 48 lbs) and the other downside is of course its loudness (~50 dB which corresponds to light rainfall) when its operating. You’ll also need a long cable to avoid having it too close to cause vibrations during your astrophotography session.

Ultimately nothing beats a steady power source offered by a wall outlet… But you can’t ask to have everything right?

## Best star trackers for astrophotography 2022

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The best star trackers for astrophotography have changed the scene forever by counteracting the rotation of our planet. Until only a few years ago a long exposure of over about 10 seconds caused stars to blur. That made it very difficult to extract much data from deep-sky objects such as nebulae, but also from the Milky Way. Cue the invention of the star-tracker, which is basically a shrunken equatorial mount, but designed for cameras instead of telescopes.

Like an equatorial mount, a star tracker needs to be aligned (often with the help of a smartphone app) with the north celestial pole (the star Polaris) in the northern hemisphere or the south celestial pole in the southern hemisphere. It then keeps your camera in sync with Earth’s rotation. That way it counteracts the rotation of the Earth and keeps the target object still in a composition, thus allowing blur-free long exposures.

## The best star trackers for astrophotography 2022

While most star trackers are a compromise between their own weight and their payload, the Benro Polaris is both super lightweight (at 3.3lbs / 1.5kg) and super-supportive, taking a mighty 15lbs/7kg of gear (the highest carrying capacity of any star tracker mount so far). It achieves that by using precise high torque motors and a waterproof IPX6 rating. That helps explain the very high price. It’s the first star tracker to offer built-in DSLR control and a built-in micro SD card slot. Remarkably, the Benro Polaris can even be controlled via the cellphone network. Its huge 2500 mAh battery can be recharged via USB-C while alignment is via any objects from a choice presented on a smartphone app. However advanced the best star trackers appear, there’s evidence from this electric tripod head that their days are numbered.

The incessant creep of light pollution means it’s now almost inevitable that you’ll need to travel to find the darkest night skies possible. Even if you don’t travel internationally, finding dark skies often means hiking into backcountry areas away from other humans. That necessitates a star tracker that strikes the right balance between its own weight and what it can support.

Cue the Sky-Watcher Star Adventurer Mini, affectionately known as SAM, which can take a payload of 3kg yet it is relatively easy to squeeze into a camera bag. It’s not the sleekest device ever, and nor is its SA Console app up to much. However, as we found during our Sky-Watcher Star Adventurer Mini review, once you get used to its foibles SAM is reliable and relatively easy to use. It’s possible to get accurate long-exposure images of up to about four minutes, which makes SAM a great compromise product. Accessories include a counterweight and declination bracket to increase the payload.

The priciest and one of the best star trackers around for astrophotographers is the iOptron SkyGuider Pro. Many star trackers are made for landscape photographers wanting to save on weight when out in the field searching for wide-angle compositions that include the night sky. But there are plenty of astrophotographers that only want to use telephoto lenses to capture light from distant deep sky objects. That means bigger payloads and longer exposures, which is what the iOptron SkyGuider Pro is designed for.

Able to take about 11lbs / 5kg, it can support long lenses or even a small telescope, making this a product that in some ways behaves more like a motorized equatorial mount, though its wedge lacks a little precision. Another downside is its use of a counterweight to reach that higher capacity than average, which adds a further 3lbs/1.35kg to the product. Aligning using its electronic polar finderscope and iOptron Polar Scope app, like most of its rivals this star trackers also tracks the Sun, Moon and allows 1/2-speed motion time lapses at night.

Even smaller and more nimble than the SAM is the great value Move Shoot Move, a star tracker that’s suitable only for wide-angle lenses. That’s partly because of its limited payload of 6.6lbs / 3kg and partly, as we discovered in our Move Shoot Move star tracker review, because it’s just not the most accurate star tracker around.

While that might sound like a deal-breaker, it’s actually a plus if you intend only to take wide-angle images of the Milky Way and starfields. For such photos a rough alignment with Polaris is all you need, something that can be done easily and quickly using an included green laser pointer.

The Move Shoot Move isn’t going to accurately track Polaris for more than about two or three minutes (though the wider and lighter your lens the longer it will remain accurate enough). But if you have a reasonably fast wide-angle lens none of that is going to matter much. If you have a telephoto lens though, look elsewhere.

While iOptron’s SkyGuider Pro is aimed at deep sky photography, the pared-down and more compact iOptron SkyTracker Pro is aimed more at wide-angle nightscapes. Its payload capacity, at 6.6lbs/3kg, is a lot less than its stablemate and at 2.5lbs/1.1kg it also weighs less. As such it’s more suitable for those wanting to carry a star tracker in their camera bag during trips and travel.

It has a wider appeal than just nightscapes since in addition to tracking objects in the night sky it can also follow the Sun, Moon and has a half speed for motion timelapses. As a bonus, its internal battery can run for 24 hours. Accessories include a counterweight and declination bracket to increase the payload.

The Vixen Polarie isn’t for deep-sky astrophotography. In the world of star trackers, it’s always a trade-off between size and versatility, and the Polaris compact size means it can support a payload of just 2kg. Therefore, it is best used with not only wide-angle lenses but fairly lightweight models, though using a mirrorless camera body will give you more flexibility. Alignment is via a supplied compass, a built-in latitude meter and a polar sight hole, so you will have to know how to find Polaris and/or the south celestial pole.

On hand to help are both red light illumination and the Vixen PF-L Assist app for smartphones. As well as long exposure astrophotography the Polaris can track the Moon and the Sun (the latter useful for solar eclipses) and its half-speed allows motion time-lapses at night. Its short two-hour battery life can be augmented by instead attaching a portable battery to its micro USB slot. Optional accessories include a counterweight to boost the payload to 6.5kg, a polar axis scope and a time-lapse adapter.

## Best star trackers for astrophotography 2022: What to look for

However, star trackers — which sit between a tripod and a camera — are not all the same. They have varying weights and designs but also manage different payloads. While some are ideal for telephoto lenses pointed at specific targets, others can only handle wide-angle lenses for capturing the Milky Way. Both the maximum payload and the accuracy of star trackers vary. They are often fiddly and time-consuming, but at their best star trackers can deliver addictively good images.

As well as weighing your camera body and lens before making a purchase do remember to take into account the added weight of a couple of ball-head mounts and the load-bearing ability of your tripod. If in doubt, go for bigger capacity mounts because as a rule of thumb it’s best to have your rig’s total weight about half the capacity of the mount.

Here’s everything you need to know about the best star trackers available for astrophotography and night-scape photography.

## How we test the best star trackers for astrophotography

In order to guarantee you’re getting honest, up-to-date recommendations on the best star trackers for astrophotography to buy here at Space.com we make sure to put every star tracker through a rigorous review to fully test each instrument. Each star tracker is reviewed based on a multitude of aspects, from its construction and design, to how well it functions as an imaging instrument and its performance in the field.

Each star tracker is carefully tested by either our expert staff or knowledgeable freelance contributors who know their subject areas in depth. This ensures fair reviewing is backed by personal, hands-on experience with each star tracker and is judged based on its price point, class and destined use.

We look at how easy it is to set up, whether the star tracker mounts are reliable and quiet if a star tracker comes with appropriate accessories and also make suggestions if a particular star tracker would benefit from any additional kit to give you the best astrophotography experience possible.

With complete editorial independence, Space.com are here to ensure you get the best buying advice on telescopes, whether you should purchase an instrument or not, making our buying guides and reviews reliable and transparent.

## I’m a Sony fan and these are the Black Friday lens deals that are on my radar

November is here, which means the best Black Friday camera deals are now ramping up. But you don’t need to limit your attention to cameras – because plenty of lenses will also see some big discounts over the next few weeks.

Photography isn’t a cheap hobby or career at the best of times, so finding some great discounts can make all the difference, particularly when we’re going through a cost of living crisis.

Black Friday 2022 is on November 25, so we still have a few weeks to go before we know for sure which lenses will be on offer. This is therefore a great time to do some research around what could work for you and your shooting style, so that you know where to look when the day arrives.

I’m currently looking to upgrade to one of the best full-frame cameras, so I’ll need some full-frame lenses to go with it. I primarily shoot seascapes and landscapes, so lenses with a wider focal length will be at the top of my wishlist, followed by a telephoto for those tighter shots and perhaps a prime lens or two.

Whatever style of photography you do, there’s bound to be a lens for you on offer – not to mention that Sony is also offering winter cashback with many UK retailers, giving you the opportunity to save even more money. The company hasn’t said how long the cashback offer will last, but calling it “Winter Cashback” suggests it’s going to be around a good while.

## Which Sony lenses were reduced last Black Friday?

Last year’s deals can offer a good guide to what might also be discounted this year. In 2021, we found some great discounts on the best Sony lenses at Adorama, Amazon and B&H, while UK retailer Wex was also offering fantastic savings on Sony lenses. Generally, the more expensive the lens, the bigger the saving, so this could be a great time to splurge on that big lens you’ve always wanted, or to stock up on a few smaller ones while the prices are low.

### FE 70–200mm F2.8 GM OSS Lens

Another lens worth checking out on Black Friday is Sony’s beast of a telephoto lens – the 70-200mm F/2.8. If you don’t mind the fact that it’s massive and very heavy, this is a great lens to add to your kit, as the quality is just incredible. I’ve shot with this lens before and you can crop a 200mm image down so much and not sacrifice any image quality or detail at all.

### Sony 16-55mm f/2.8 G Master APS-C Lens

For any Sony shooters who don’t want to make the switch to full-frame just yet, this incredible 16-55mm f/2.8 G Master Lens is probably one of the best lenses you can put on your camera. I bought this model earlier in the year and the quality is astounding. It’s currently on Amazon UK for £1,039, which is full price, however I have seen it drop as low as around £800 in the past, so there’s a good chance it could go lower during Black Friday. All of Sony’s lenses also hold their value really well on the used market, so you could definitely sell it on for a good price when and if you do switch over to full-frame.

### Sony 20mm F/1.8 Prime Lens

The 20mm f/1.8 prime lens is a very popular lens among Sony shooters because, although it’s a prime lens, it’s actually quite versatile. While it wouldn’t necessarily be a good choice for wildlife or sport photography, it’s a great lens for any type of travel or photojournalism and it’s also a very popular option for landscapes and cityscapes. Many photographers I know also use this lens when getting into astrophotography, as it’s wide enough to capture foreground and a big chunk of night sky, while the fast aperture means you can let a lot of light in. While professional astrophotographers would probably choose a 14mm lens instead, this model would be a great way to dip your toe into this style of photography. If this lens is reduced on Black Friday, it’s definitely one to consider picking up.

## Longer exposures versus multiple shorter exposures

Which is better: many short exposures or fewer long exposures? 120 x 1 minute exposures or 10 x 12 minute exposures? Questions similar to this one get asked really often and they probably receive all kinds of different answers. People talk about the camera, faintness of the signal, dithering, stacking, light pollution etc. etc. So many factors creep into the discussion that sooner or later it will seem like a personal choice, almost like a matter of opinion. Is this really a matter of opinion and/or complicated? Or can we find some definitive answer to the question if more shorter exposures are better than fewer longer exposures?
Actually, it turns out that although there are formulas for ideal (theoretical!) exposure times, personal preferences and opinion and practicality will come into play in reality when determining the exposure you want to be using.
Knowing how the subexposure time will impact the SNR of your stacked image will help you to determine the exposure time you choose to use. Please bear with me while we explore the way to determine this optimum exposure time, which will include quite a bit of math. I find that going over the math and actually calculate the SNR for a few different scenarios helps greatly in my understanding of this matter. So I encourage you to follow along with the math. For those who are mostly interested in the key take aways I’ll start with those in a short summary, before diving into details and the math further on.

## Key take aways

When you are imaging under light polluted skies, you should not worry about your exposure times. It will make very little to no difference if you are using subexposures of 30sec. or 3minutes. It’s the total integration time that matters.
If you are imaging under dark skies, you will benefit from longer exposures in terms of SNR. However, it’s not worth it to go to extremes and keep in mind the cost of throwing out a subexposure due to poor tracking for instance. Only increase exposure time if you can reliable track accurately for that long.
If you don’t use darks and/or want to use dithered (bayer) drizzel integration, make sure you get at least 10 to 15 subexposures.

Now that we’ve learned not to worry about subexposure time under light polluted skies and give subexposure time priority under dark skies, let’s see why this is the case and how we arrive at these conclusions.

## Read noise and background sky flux

To determine the optimum exposure it basically comes down to these two factors; read noise and the background sky flux.
The read noise is the noise that occurs within the electronics of the camera when we convert the electrons coming from the sensor into digital units (ADU) and store them. The background sky flux is the signal coming from the light pollution, moon light and/or air glow. The background is never completely black and the value of the background signal is the background sky flux.

If the read noise is swamped by sky background noise it becomes irrelevant and it doesn’t matter how long our subexposures are

If we wouldn’t have read noise, it wouldn’t matter if we take one very long exposure or many shorter exposures with the same total integration time. If we wouldn’t have background sky noise but do have read noise, the longer exposure would always have better SNR than many shorter exposures.
In reality we always have to deal with some sort of mixture of these two scenarios. There will always be some background sky noise and read noise is always a factor to take into account. Especially for us DLSR users. So how should we take those two factors into account? Well, it comes down to determining the point where the sky background noise will make the read noise irrelevant. Whenever this is the case, it doesn’t matter any more if we take many shorter or fewer longer exposures.
So if you are imaging under strong light pollution this probably applies to you. Alternatively, if you are using narrowband imaging and/or if you are under truly dark skies, it will be practically impossible to reach this point and so the longer exposures will always be better.
Now let’s research why this is the case and look at some test results;

## Adding multiple exposures and SNR

Let’s look at what exactly happens to the SNR when we add multiple exposures and when we expose longer. We will dive into some math here and I will try to do this step by step and as clear as possible. I know lot’s of articles will skip steps and rewrite formulas without explaining how or why which I’ll try to avoid.

The SNR is simply the Signal divided by the noise. Sounds simple, but with different noise sources we need to dive in a bit deeper and see how we add noise. But first let’s define the term Signal exactly: S = signal per second (s) * time of the exposure (t) * number of exposures (N). So
To be clear; this is the total amount of signal recorded over multiple exposures.
Next let’s consider the noise sources we are dealing with here: Object shot noise, sky (shot) noise, dark current noise and read noise. Whenever we are dealing with detecting photons we deal with shot noise. This shot noise is the square root of the number of photons. This noise is also building up over time just like the signal. So in terms of the object shot noise, it is the noise associated with the signal of the object(s) we are imaging and is the square root of the signal.

For sky shot noise it is the square root of the background signal coming from the sky over time.

For the dark current noise it is the square root of the thermal signal (dark current) build up.

The read noise is different in the sense that this doesn’t build up over time but it does occur once for every exposure. This is important to realise. The total read out noise (RonTot) is:

We can sum random, uncorrelated noise by adding them quadratically so we get the following formula for the total amount of noise:

Remember the fact that the shot noise is the square root of the Signal source. So if we add this quadratically we can rewrite the Noise in terms of the signal like this:

Since the number of exposures (N) is present in each term we can rewrite this as:

or

So the formula for the SNR is:

Notice we have N as a term present in both parts of the division. Since we can rewrite the above formula into:

Now we have written the formula down for SNR in this useful format, we can explore what the impact of the background sky flux and the read noise is on the SNR.

### Scenario with no read noise

Maybe you start to see now what we said earlier; if we wouldn’t have read noise it wouldn’t matter how long we expose as long as the total exposure time is the same. Let’s just take out the read noise (RN) out of the above equation. Without read noise the SNR will be:

Since we have t present now in every term we can group it:

which is the same as

and as we’ve seen before we can write this down as

Now we can see it doesn’t matter for the SNR how we fill in values for N and t as long as N*t = the same value. So only total exposure time matters (N*t) and not how we divide it in subexposures.
But wait, what about really faint signal? Don’t you need very long exposures for that? Well, not in this case where we don’t have read noise. You just need (very) long total exposure time. If we look at the above formula this is clear, but if we think of stacking and consider that we are averaging there this might seem less intuitive all of a sudden. Because, the average of a few electrons collected by many frames is smaller than the average of a few electrons in one frame right? Well, sure that is correct of course. However, this is only considering signal. And it is not useful to talk about signal alone, we always need to talk about SNR. That alone determines if you will have detected signal that will stand out in an image or not.
And if we consider the fact that SNR = simply signal / noise you’ll realise it doesn’t matter in how many frames we detected the signal, since we’ll only end up dividing both terms of the division which doesn’t change anything ((a*b)/(a*c) = (a/a) * (b/c) = 1*(b/c) = (b/c))
So yes, it’s true that if read noise wouldn’t exist it doesn’t matter what exposure time you use and how many exposures you take, all that matters is the total integration time. And even with read noise included in the formula, you can see that once the other values are much much bigger than the read noise, the same will apply; the read noise becomes (almost) irrelevant and we are left in the situation where it doesn’t matter what exposure time you use.

once the other noise values are much much bigger than the read noise, the read noise becomes (almost) irrelevant and we are left in the situation where it doesn’t matter what exposure time you use.

### Scenario with little to no sky noise

Alternatively, consider the scenario where we are under a truly dark sky with no light pollution, no moon light and only a little sky glow; we have very little background sky signal (Sky_s). Let’s consider the SNR formula again:

Now let’s say the DarkCurrent_s = 0.15e-/sec (which I found reported for the Nikon D7000), Sky_s = 1e- / sec and read noise = 3e- (Nikon D7000 @ ISO200). If we take a very faint signal that’s similar to the sky flux we will see the following SNR values for different number of exposure times but same total exposure time:
Let’s compare scenarios with a total integration time of 120 minutes and compare 120x1min and 12x10min.

120x1min:

12x10min:

Ok, so SNR is higher indeed as expected. What if we take this to the extreme and just take 1 image of 120min?

1x120min:

Hmmm, that’s a really small improvement over the 12x10min exposure. Clearly this is a case of (quickly) diminishing returns. I made a graph showing the SNR gains compared to a 30sec exposure SNR for exposures between 30sec and 120min to show the benefit of exposing longer in this scenario:

This graph paints quite a clear picture I’d say. In case of a dark sky, the gains in SNR while exposing longer is quite big in the beginning and reaches a 10% improvement already at 3,5minutes exposures compared to 30sec exposures. The improvements tail off quickly as well though, reaching only a further 1% improvement at 8 minutes compared to the 30sec exposures. To be clear; in this scenario, the difference between exposing 3.5minutes and 8 minutes (with the same total integration time) is even slightly less than 1% improvement.
Please note that these gains are dependant on the signal coming from our target object as well. So if we would take a much fainter object with a flux of only 0.2e- compared to the 1e- we just saw, we get the following graph:

Wow! we can see the same strong curve with diminishing returns, but the SNR improvements for a fainter signal are clearly much much higher! Let’s look at the same minute marks as before: using 3.5 minutes exposures compared to 30sec exposures gives you an SNR improvement of 26.3%. Going from 3.5 minutes to 8 minutes gives you a further improvement of 2.86%. So even though we still see strong diminishing returns, the improvements remain significant up till longer exposures as before.
In this scenario, the improvement in SNR between 8 and 15 minutes still is 1%.
Please note that we’re talking about signal coming from the sensor here. So this includes scenarios with slow optics as well as using fast optics on very faint signal.

The role of read noise in SNR
To make it really clear what the role is of the read noise in the scenario’s we just ran through, let’s take a look at the role of read noise specifically and how it adds up when we add more exposures.
Let’s look at the SNR again for the same situation as described before, 12x10min subs versus 120x1min subs and rewrite it a bit just to see what is happening to the noise terms:
For the object shot noise, sky noise and dark current noise we get the following:
12x10min:

which is;
=

and for the 120x1min this then is;
=
So you see this is exactly the same, just as we could expect and have seen before in the scenario without read noise.

Now, let’s see what is happening with the read noise; which is simply
12x10min:
120x1min:

So the read noise is growing with the square root of the number of exposures in our integration, while all the other terms simply grow by total exposure time alone. So for a given fixed total exposure time, the read noise will be smallest with the least number of exposures.

Read noise is growing with the square root of the number of exposures in our integration

To see it’s impact in the total noise let’s run the actual numbers. Remember, uncorrelated noise adds up quadratically, so the total noise we get in these situations is;
12x10min:
120x1min:

Now we can clearly see how big the impact is of the read noise in this scenario.
Next let’s see what these numbers and the impact of read noise looks like in case of a bright sky. Let’s say sky background flux is 50e-.
For the time dependant noise sources we get:
=

12x10min:
120x1min:

This is a totally different situation and the read noise could simply be considered irrelevant.
In fact, this looks very much like the hypothetical situation without any read noise we saw earlier.

So we can conclude that the exposure time only is relevant when the read noise is relevant. And the read noise is only relevant if the sky is dark enough.

The exposure time only is relevant when the read noise is relevant. And the read noise is only relevant if the sky is dark enough.

## Determining optimal exposure time

Now we’ve seen the scenarios above you might wonder what would be applicable to your specific situation. As we just concluded this will be dependant mostly on the brightness of the sky you are imaging under.
There are formulas to determine the optimum exposure using the read noise and sky flux as input. There is also a script in PixInsight which you can use to give you an ‘ideal exposure length’. (Scripts->Instrumentation->CalculateSkyLimitedExposure)
However, as we’ve seen in the graphs before the benefit in SNR is one of diminishing returns. This means it is not possible to give one absolute answer to the question what the optimal exposure time is. Assumptions need to be made about how much contribution of read noise to the total noise you will tolerate. And the differences in this assumption is often huge (factor 2 differences). Furthermore they don’t take practicalities into account. So I’d like to just show you a few more scenarios and the SNR corresponding gains for longer exposures compared to using 30sec exposures in a situation where we use a total exposure time of 2 hours. I’ve listed the ‘95% improvement mark’ for exposure time for each sky brightness.

The most obvious thing we can learn from this chart is that there is a huge difference how much you benefit from longer exposures under a dark sky versus brighter skies. Furthermore, the 95% improvement mark seems to be awfully close for all scenarios. However, I’m not sure how useful this number is since the next step up in exposure after this mark under a dark sky will still give you 0.05% increase while this improvement is only 0.0012% in the brightest scenario. To make this even more clear; for the brightest scenario the SNR for 30sec. exposure was 11.83, while all the way at the end with one exposure of 120 minutes the SNR was 11.86. So we could safely consider this a scenario where subexposure length is completely irrelevant.

Remember we saw earlier that the SNR improvements were much larger if we are dealing with faint signals. So let’s look at the SNR improvements for different sky brightness with the previously used faint 0.2e- signal

The impact of the sky brightness is very clear again in this chart. Although we do benefit more from longer exposures when dealing with fainter signal also from brighter skies, the difference is negligible for very bright sky and is really huge for the really dark sky.

Based on these graphs we can conclude that if you are imaging under a light polluted sky you should not worry much about your exposure length. Every sky brightness between 5e- and 50e- basically has a total SNR improvement between 0.2% and 2% for which you’ll reach 80% of maximum SNR increase already at 3 minute exposures.

## Other practical considerations to take into account

Next to the read noise and background sky flux, we need to take some practical factors into account as well when we want to determine the optimal exposure length. The ability to guide accurately for longer exposures for instance is of course really important, as well as the cost of loss of data if you need to throw out a subexposure. Remember that the total integrated exposure time is most important for SNR, so there is a real significant cost when you need to throw out subexposures.
Furthermore you might want to use (bayer) drizzle integration, which will need a minimum amount of (dithered) subexposures to give proper results.
These are all very important things to consider which might change things completely in terms of the optimal exposure time for you personally.

## Light pollution and narrow band filters

I got asked a lot about the influence of filters. I haven’t tested this out myself, but the book The Astrophotography Manual covers a comparison between different light pollution filters and concluded that in his test it didn’t effect the background sky flux all that much (which was surprising). As always, test it out to know for sure, but I think the effect of light pollution filters is minimal on the read noise contribution to the overall noise.
Narrowband filters on the other hand will basically change even a light polluted sky to a dark(er) sky in general. So with narrowband filters you will benefit a lot from longer subexposures.
Of course there will always be exceptions to this, like OIII imaging with full moon, and exact details will depend on the speed of your scope and the quantum efficiency of your camera for instance. However, the generalisations will hold true in most situations.

## A comparison from a dark site

Let’s look at the following data that I shot while I was in Namibia. There is practically no light pollution there and this was shot without the moon present. The only bit of background sky flux was the sky glow and perhaps a bit of zodiacal light.
The data was shot during 3 different nights so some variation is to be expected based on conditions for that particular night. There were no noteworthy differences between those nights so I have no reason to believe this is influencing the comparison much.
I had used different exposure lengths: 8, 12 and 15 minutes. For this comparison I made three integrations that all had the total integration time of 120minutes. So 15 frames of 8 minute exposures, 10 frames of 12 minute exposures and 8 frames of 15 minute exposures. No processing is done to these integrations. Just a STF applied to make the data visible.
Based on what we learned before we would expect the fewer but longer exposures will have the better SNR. Let’s see if this was the case:

Hard to tell from this wide shot, so let’s zoom in a bit.

Here we can already see some difference in the amount of noise. Let’s zoom in further to have a better look.

The differences in SNR are clearly visible now and we see indeed what we expected: the 8x15min. integration looks (much!) better than the 15x8min. integration.
This is even better visible in the dark region of Barnard 44A:

So visually we can already draw the conclusion that we were right: longer exposure has better SNR in this situation where we are clearly read noise limited. Now let’s check the numbers to confirm.
First let’s look at the amount of noise:

Noise is higher in longer exposures which is expected. The question is; did the signal grow more than the noise?
The SNRWeight measure (to be clear; this is a relative measure and not reflective of the actual SNR differences)

So yes, it clearly did.

Unless you are under dark skies, the subexposure length won’t matter much once you are using 2 to 3 minute exposures.

## Conclusion and final considerations on sub exposure time and number of exposures

With all that we learned and the simulations we looked at and the real world dark site test we have quite some information about the optimal subexposure length. However, I bet you still are wondering how this impacts your particular situation and what the optimal subexposure length is. That will remain difficult to answer exactly, and much of the details go right out of the window if we need to take other factors like drizzle integration and guiding errors into account. However, I feel the most important conclusion probably is the fact that the exposure length is only relevant when the read noise is relevant. And the read noise is only relevant when you are imaging under a dark sky.
With most moderately to strong light polluted skies, the subexposure length won’t matter much once you are using 2 to 3 minute exposures.
Let me know in the comments below if you agree or disagree or still are left with questions at this point!

## Comprar una estrella es caer en un fraude

A lo largo de mis años como astrónomo aficionado son muchas las personas que me han preguntando dónde comprar una estrella o peor aún, me confiesan que han pagado dinero para comprar una. Lo cierto es que detrás de la venta de estrellas hay un intento de estafa.

Muchos de los anuncios de empresas que venden estrellas incluyen un certificado de autenticidad pero lo cierto es que el único organismo con potestad para poner o cambiar nombre a una estrella es la Unión Astronómica Internacional (IAU) y ésta se rige bajo criterios científicos sin ánimo de lucro así que olvídese de que pongan el nombre de su persona amada a una estrella, no va a pasar. Como explican en su página web son muy estrictos con la forma de nombrar objetos celestes y se desvinculan totalmente de las practicas comerciales que consisten en vender nombres de estrellas ficticios.

Si quieres regalar una estrella a una persona querida puedes hacerlo por tu cuenta con un folio y una foto o un dibujo de la estrella que tu quieras, será igual de original y tendrá la misma validez legal que ese supuesto certificado expedido por una empresa privada en Internet, pero mucho más barato.

El timo de la venta de estrellas comenzó en el año 1979 cuando apareció la primera empresa que ofrecía estos servicios, la International Star Registy (ISR). En su página web afirman ser los únicos que tienen registro en la oficina de derechos de autor de los Estados Unidos como si eso fuera algo que les diera relevancia o autoridad. Lo cierto es que nosotros mismos podemos crear el «Atlas de estrellas de Cielos Boreales» y registrarlo como un libro más en tal registro y apuntar los nombres ficticios de nuestro catálogo como cualquier otra publicación de ciencia-ficción. Nadie, excepto nuestra propia conciencia, nos impide cobrar 100 o 200€ por añadir tu nombre al de una estrella en nuestro catálogo personal, no estaríamos haciendo nada ilegal.

Estas empresas ofrecen todo tipo de merchandising para aumentar sus beneficios; cuadros, pulseras, colgantes, tazas… durante estos últimos 40 años han ganado millones de dólares aprovechándose de los incautos que se dejaron engatusar. A menudo han usado incluso a populares actores de Hollywood para promocionar sus servicios.

## Otros regalos con estrellas más éticos

Hay otro tipo de regalos basados en las estrellas que son un poco más éticos. Es lo que hacen algunas empresas y creadores particulares al vender cuadros con la representación de la esfera celeste en un día determinado para conmemorar una fecha especial como un aniversario, por ejemplo.

En este caso no te están ofreciendo ningún certificado ficticio, es solo una imagen de cómo se veía el cielo en una noche determinada. Puedes hacer esto tu mismo ayudándote de programas como Stellarium o hacer un planisferio fijo para una fecha determinada.

Otras empresas más originales venden productos que han estado casi en el espacio. Es lo que hace EarthtoSky, que pone colgantes y otros avalorios en globos de helio que llegan a la estratosfera para luego caer de nuevo.

En cualquier caso ¿Por qué regalar una sola estrella si puedes regalar millones de ellas? Regala un telescopio y estarás haciendo el mejor regalo posible para cualquier amante de las estrellas.

## ZWO’s ASI1600GT Camera – Astroniklas

I remember in my early days my old astronomy club had a monochrome CCD camera from SBIG, which was used for scientific projects. The club opened its doors to students allowing them to discover new supernovae and other cool features in the night sky involving photometry.

Years passed by and as an amateur astronomer I’ve dealt with DSLRs for the most part of my observing sessions. While DSLRs can be suitable for using them both at day and at night, unfortunately they can’t compete with dedicated astronomy cameras.

CMOS technology has advanced more and mover over the years as well, bringing their sensors to a very competitive level vs. CCD.

In a market the recipe to success is very simple and that comes with manufacturing cost. While CCD manufacturing has struggled enormously to lower its costs, CMOS has prevailed in the technology area and made itself more dominant over the years. Availability and cost efficiency are the two major factors in its success. Both sensors eventually convert light to electrons so the end result will be the same.

Thus, it brings us to the point of my blog entry here. I’ve recently acquired ZWO’s monochrome ASI1600GT. A camera that has a very effective cooling capability, high reliability, built-in filter wheel and lightweight. ZWO has done an awesome job and provided amateur astronomers with a very competitive and strong camera. I can’t wait for the California weather to offer me the chance to try it out very soon!

With my purchase I’ve also acquired a set of OIII (Oxygen), SII (Sulfur), H-alpha (Hydrogen) 7nm narrowband filters with a set of LRGB filters from SVBony as well.

## How To Photograph the Total Lunar Eclipse

Are you hoping to capture a photo of the upcoming total lunar eclipse on November 8th? If so, you are not alone. Amateur photographers and astrophotography enthusiasts around the world will do their best to take pictures of the upcoming lunar eclipse using a wide variety of camera equipment.

A total eclipse of the moon is a truly breathtaking astronomical event that anyone can appreciate. The best part about it is that you do not need expensive astrophotography equipment or special filters to take a great picture of the total lunar eclipse. It’s all about using the best settings on the camera you are using (even if it’s a phone).

I recommend practicing your moon photography skills before the night of the upcoming lunar eclipse, so you don’t waste precious time fiddling with camera settings during the celestial event. With that out of the way, let’s get to the key information you need to take a great picture of the total lunar eclipse.

## Fast Tips:

• Practice your camera settings on the moon before the night of the lunar eclipse
• If you are using a smartphone through a telescope, use a smartphone adapter to hold it in place
• Use your cameras manual or ‘pro’ mode for full control over settings like ISO, Aperture, and Exposure
• Capturing the moon during totality is often easier to accomplish due to less extreme lighting variations
• Use a tracking equatorial mount when shooting at high magnification (star trackers work great)

## How to Photograph a Lunar Eclipse

Over the years, I have photographed a number of total lunar eclipses using a variety of cameras – from my smartphone to a dedicated astronomy camera. The key to a great image isn’t the specific camera you use, it’s all about magnification and the correct settings

Without enough ‘reach’, the moon will appear small and lack the details you are hoping for. I recommend capturing the lunar eclipse with at least 300mm of focal length or more, which means an astronomical telescope or telephoto camera lens is best.

Then, it’s all about choosing the best camera settings to capture such a challenging subject in terms of light conditions. The moon will change in brightness as it goes through the different stages of the eclipse, and you must adjust your camera settings accordingly.

What about those of you that don’t own a telescope or a long lens? The good news is you can still capture a great nightscape-style shot at a wider field of view. However, these types of photos look best if the moon is closer to the horizon while eclipsed.

A total lunar eclipse captured in the early morning hours using a DSLR and standard kit lens.

Wide-angle nightscape images that include a large portion of the night sky including an eclipsed moon can be done using a DSLR and tripod. For a 30-second exposure, a tracking mount is not necessary. At a focal length of 18mm or wider, star trailing will begin to show after about 20-25 seconds, so just keep that in mind.

To capture the stars and constellations in the night sky, an ISO of 800 or above is recommended. However, this exposure will likely record the eclipsed moon as a featureless ball of light.

To properly capture both the starry sky and a detailed moon, you will need to capture exposures of varying lengths and blend them together into a composite image. This is because the moon is much brighter (even while eclipsed) than the surrounding starry sky.

A composite image can be made by masking the area of your night sky exposure and blending in a shorter exposure of the moon with surface details. This technique will take some time and experience to master, but the results can be amazing.

## When and Where is the Lunar Eclipse Happening?

For a celestial event like this, a little planning goes a long way. You’ll definitely want to know exactly when the lunar eclipse is taking place, and where it will be in the sky from your location.

For example, you may have to travel to a location with a low western horizon for a total lunar eclipse occurring in the morning if your backyard is full of tall trees.

Lunar eclipses are visible from different parts of the world at different times. There are many times when a lunar eclipse is taking place on the other side of the earth that you are unable to observe.

Here are some helpful resources to help you plan for the lunar eclipse:

### Upcoming Lunar Eclipses (NASA)

Date Eclipse Type Visible From
November 8, 2022 Total Asia, Australia, Pacific, Americas,
May 5, 2023 Penumbral Africa, Asia, Australia
October 28, 2023 Partial Americas, Europe, Africa, Asia, Australia
March 25, 2024 Penumbral Americas
September 18, 2024 Partial Americas, Europe, Africa
March 14, 2025 Total Pacific, Americas, Europe, Africa
September 7, 2025 Total Europe, Africa, Asia, Australia

## The 7 Stages of the Lunar Eclipse

There are 7 stages of a total lunar eclipse, and many amateur photographers like to capture the event in each stage. This can later be made into a composite photo showing the transition of the moon as Earth’s shadow covers it. A time-lapse video is another excellent way to capture each stage of the eclipse.

The maximum eclipse stage is when most photographers want a great shot. This is when the moon turns “blood” red and the surrounding night sky becomes much darker from our point of view on Earth. It is an unforgettable experience for those lucky enough to witness this moment.

### Stages of the total lunar eclipse:

1. Penumbral Eclipse begins
2. Partial Eclipse begins
3. Full Eclipse begins
4. Maximum Eclipse
5. Full Eclipse ends
6. Partial Eclipse ends
7. Penumbral Eclipse ends

An interesting thing happens when the moon is completely eclipsed by the shadow of Earth. Not only does the moon turn to an eerie reddish hue, but the stars and constellations surrounding the moon begin to appear as they would on a moonless night. Capturing a scene like this requires careful planning and execution.

## Ways to Photograph the Total Lunar Eclipse

Here are 6 different ways to photograph the lunar eclipse, depending on the equipment you own:

## Examples and Best Practices

There are many ways to photograph the total lunar eclipse, but for the best results, I recommend using a DSLR camera and a small refractor telescope on a tracking mount.

This will allow you to get an up-close shot of the moon in each of its phases in detail. Some of the most incredible images of the lunar eclipse I have ever seen were captured this way.

If you do not own a telescope, you can use your longest focal length camera lens to pull the moon in close. For the photo of a nearly total lunar eclipse below, I used a Canon EF 400mm F/5.6 telephoto lens.

The 2021 Partial Lunar Eclipse on November 19, 2021. DSLR and 400mm lens.

An equatorial tracking mount, such as a star tracker is the best way to take a clear photo of the moon during an eclipse when using high-magnification optics. This essentially freezes the moon in place for an extended period of time.

When you have compensated for the rotation of the earth, your subject is no longer moving, and you have many more options to choose from in terms of camera settings. Now, you can dial back ISO settings and f-stop if necessary and let a longer exposure time collect the light.

This makes everything easier because the Moon will stay ‘still’ in the image frame while you adjust your camera settings based on the current stage of the eclipse. During the first stage of the eclipse, the moon will be very bright, whereas, during totality, it will be much dimmer.

Below, you will see the camera and telescope I used to take a crisp photo of the total lunar eclipse that occurred in September 2015. This telescope has a focal length of nearly 500mm, which was enough to reveal some amazing details on the lunar surface.

The camera and telescope used to capture a total lunar eclipse. Canon EOS 70D and Explore Scientific ED80.

## Using a DSLR and Telescope

A DSLR camera (or mirrorless camera) and telescope can provide an up-close view of the eclipsed moon in detail. The prime focus method of astrophotography is best, as the camera sensor’s focal plane is aligned with the telescope. You can directly attach a DSLR camera using a T-Ring adapter (see below) to utilize the telescope’s native focal length.

The prime focus method requires that the telescope tracks the apparent rotation of the night sky to avoid any movement in your shots. To learn more about the process and equipment involved in deep-sky astrophotography, have a look at a typical DSLR and telescope setup.

A DSLR camera and T-Ring Adapter attached to a telescope

If your goal is to capture an up-close view of the moon during the eclipse, there are many benefits to this technique. A small refractor telescope will have an adequate amount of focal length (magnification), offer precision focus, and have a stable base to attach to an equatorial telescope mount.

With the camera connected to the telescope, experiment with different exposures and ISO settings in manual mode, using live-view to make sure you have not under/overexposed the image.

The shortest exposures will only be useful during the partial stages of the lunar eclipse, as the lunar eclipse is beginning and ending. This is a challenging phase of the event to capture in a single shot, as the shadows and highlights of the image are from one end of the spectrum to the other.

A remote shutter release cable will help to avoid camera shake in your image.

When the moon enters totality, you will need to bump up your ISO, and/or your exposure length to reveal the disk of the moon as it becomes dimmer. Use a timer or external shutter release cable to avoid camera shake if possible.

Ideally, you’ll keep the ISO as low as possible for the least amount of noise. With an accurately polar-aligned tracking mount, exposures of 2-5 seconds will work great.

To record the lunar eclipse with a DSLR camera, no filters are necessary. A stock DSLR camera is best as the additional wavelengths available with a modified camera are unused in moon photography.

Canon EOS 7D, Explore Scientific ED80 Refractor, Sky-Watcher HEQ5 Tracking Mount.

Without a tracking equatorial mount like the Sky-Watcher HEQ5, a 2.5-second exposure like the one above is impossible. Even 1-second of movement at this focal length will record a blurry image if the telescope or lens is not moving at the same speed as the moon.

The benefit of shooting a long exposure during the maximum eclipse (totality) is that you also record the starry sky behind the moon. To do this in a single exposure on a normal full moon is not possible as the dynamic range is too wide.

A dedicated one-shot-color astronomy camera is more than capable of taking a brilliant photo of the eclipse as well. The computer software used to control these devices have countless options to control the Gain and exposure settings of these cameras.

For projects like this, I personally enjoy the freedom and simplicity of a DSLR. Camera settings such as ISO, exposure, and white balance can easily be changed on-the-fly as the eclipse is taking place.

## Without A Tracking Mount

Since the moon is very bright, it is possible to take a fast exposure (1/500″ or faster) of the moon without tracking. You will still want to use an optical instrument such as a telescope or long lens, and without tracking, it will be tricky.

Even at 10X magnification, the moon will slowly move across the eyepiece as you look at it through the telescope. This a subtle reminder that the earth is always spinning, and why astrophotography is so challenging overall.

Thankfully, unlike dim deep-sky objects like nebulae and galaxies in the night sky, solar system subjects like the moon are incredibly bright. You can take an ultra-fast exposure of the moon through a telescope that is still sharp, without tracking.

Many visual observers enjoy the affordability and performance of a Dobsonian Telescope like the one shown below. They are a fantastic choice for anyone interested in astronomy, and why I consider them to be the best telescope for beginners

It is possible to produce a comparable close-up image using a digital camera or smartphone through the eyepiece of a non-tracking telescope such as a Dobsonian, using the eyepiece projection method. For the best results, use a smartphone adapter that allows you to secure your phone to the telescope.

## Photographing a Lunar Eclipse with Your Phone

This type of astrophotography is often referred to as the eyepiece projection method. To do this, you’ll simply position your digital camera or smartphone into the eyepiece of the telescope. This method usually requires a fair amount of trial and error, but you may be quite surprised with your results.

An eyepiece smartphone adapter may help to steady your shot of the lunar eclipse. Although you’ll have much less control over exposure and record less detail, this technique can be used with a non-tracking telescope as a traditional Dobsonian telescope like the one pictured above, or a smaller tabletop model.

The moon is one of the few subjects that are relatively easy to photograph with a non-tracking mount compared to deep-sky astrophotography. However, the transition phases of the eclipse can be difficult due to the changing lighting conditions and exposure levels.

I recommend capturing the lunar eclipse during its maximum phase if you’re using this method. You likely won’t be able to capture a well-exposed image using the camera’s auto-exposure mode. Experiment with your camera’s manual settings that allow for variations in shutter speed.

I have had great results using the Celestron NexYZ smartphone adapter when photographing the moon. This model features a 3-axis design that allows me to line up the camera on my bulky Samsung S21 Ultra phone with the eyepiece. It clamps onto the eyepiece itself and is much more secure than models I have used in the past.

Use a smartphone adapter to line up your camera lens and secure your phone.

## Camera Settings

Once you have secured your phone in the adapter, and the camera lens is lined up with the eyepiece, you can start experimenting with settings. To fully control the exposure, it is best to use manual mode (often called ‘pro’ mode) rather than the standard auto setting.

Chances are, when you are pointing at the moon with your smartphone and telescope, it will appear very bright, and your camera will have trouble finding the correct exposure to show the lunar surface details. To fix this, adjust the basic camera settings like exposure length, ISO, and f-stop to properly expose the bright moon through the eyepiece.

A shorter exposure time 1/500′ and a moderate ISO setting of 400 is a good place to start (see below). If the moon looks too bright or too dim using these settings, make small adjustments to the exposure time until it is well exposed to reveal the moon’s surface.

Use manual focus mode to ensure that the moon is in critical focus, rather than relying on the autofocus capabilities of your phone camera. This can be tricky to get right, but keeping the camera steady via the smartphone adapter will make this a lot easier.

Capturing the lunar eclipse using the ‘pro’ mode on my smartphone.

## Using a Telephoto Camera Lens

If you don’t own a telescope, a telephoto camera lens with at least 300mm of focal length will work well. At longer focal lengths like the ones necessary for a close-up of the moon, you must use fast exposure to capture a sharp photo of the moon. This is because the Earth is spinning, so you’re essentially trying to photograph a moving target.

The image below was captured using a Canon EOS 70D and a Canon EF 400mm F/5.6 Lens.

The final stages of the partial eclipse phase are challenging to photograph because there is a bright highlight on a small portion of the moon. For the photo below, the camera settings included an ISO setting of 6400 and a shutter speed of 1/8.

A tracking telescope or camera mount such as the iOptron SkyGuider Pro (pictured below) is recommended. An equatorial mount that is polar aligned with the rotational axis of the Earth will allow you to take longer exposures, and get more creative with your camera settings.

Owners of astronomical telescopes for astrophotography usually own an equatorial telescope mount, and this is an ideal configuration for moon photography. This allows the user to enter any celestial object into the hand controller, and the mount will automatically slew to that object once it has been properly star-aligned.

An iOptron SkyGuider Pro camera mount with a DSLR and 300mm Lens attached

The key to capturing details of the moon’s surface in your lunar eclipse photo is reach and exposure. By this, I mean that you need enough magnification to show the detailed craters of the moon’s surface, and a fast enough shutter speed to not blow out any of the highlights in your image.

To do this, a precise exposure length must be used. One that preserves the data in your image while also bringing enough of the shadowed areas forward is ideal. For my photos, I found an ISO of 200 and an exposure of 1/200 to work quite well. This was enough to showcase a starry sky behind the eclipsed moon.

I use Adobe Photoshop to process all of my astrophotography images, including photos of the moon and our solar system. Adobe Camera Raw is a fantastic way to edit your images of the lunar eclipse because it gives you complete control over the highlights and color balance of your image.

Adobe Camera Raw offers powerful tools to edit your photos of the Total Lunar Eclipse

## Capturing a Lunar Eclipse Without a Telescope

If you are simply using a point-and-shoot camera or a DSLR and lens on a stationary tripod, you can still take an amazing photo of the lunar eclipse. This is often a great way to capture the landscape and mood of the moment. The photo below was captured back in October 2014 using a Canon EOS 7D and an 18-200mm lens on a tripod.

This is a wide-angle shot captured at 18mm, while the inset image was captured at the lens’s maximum focal length of 200mm. A zoom lens is handy for photographing the moon at varying magnifications.

When capturing the lunar eclipse without a telescope, you’ll want as much manual control over the camera settings as possible. “Auto” mode, flash, and autofocus won’t work on a photo of the total lunar eclipse. Adjusting individual parameters such as exposure length and ISO is essential to properly expose the moon.

Practice taking shots at night beforehand, so that you are ready when the eclipse happens. Ideally, find a location that includes some interesting foreground and background details to capture a dramatic scene on the night of the event. In the case of the lunar eclipse shown above, it took place in the early morning hours as the moon was setting.

## What is Happening During a Lunar Eclipse?

Do you understand why a lunar eclipse happens? There are two types of lunar eclipses: partial and total. As you know, the Earth orbits the sun, and the moon orbits the Earth. During a total lunar eclipse, the Earth is sitting directly between the sun and the moon.

Although the moon is covered in Earth’s shadow, some sunlight still reaches the moon. When the moon enters the central umbra shadow of the Earth, it turns red and dim. This distinctive “blood” color is due to the fact that the sunlight is passing through Earth’s atmosphere to light up the disk of the moon.

A diagram of what happens during a total lunar eclipse – NASA

Unlike a solar eclipse, observing a total lunar eclipse is completely safe to do with the naked eye. This natural phenomenon can be enjoyed without the aid of any optical instruments, although binoculars can really help to get an up-close view of the action.

Camera settings used for my lunar eclipse photo

This article was originally posted in January 2019, and updated on November 4th, 2022.

## Astrophotographer to give tips on night sky pics

The Andromeda Galaxy as captured by award winning astrophotographer James Cahill.
JEC ASTROPHOTOGRAPHY PHOTO

MOUNT DESERT — Astrophotography is no longer limited to NASA and other organizations with big telescopes. Recent improvements in technology have enabled more amateurs to take amazing photographs of the night sky, from near objects such as the moon and the planets, to galaxies, nebulae and other objects in outer space.

On Tuesday, Nov. 8 at 6:30 p.m., the MDI Photo Club will be hosting a Zoom talk by award-winning amateur astrophotographer James Cahill. The public is welcome to join the presentation in which Cahill will share the equipment and techniques needed for night sky photography and show some of his stunning deep space photographs.

Cahill is an IT specialist by day and amateur astronomer and astrophotographer at night. His work is shot primarily in Bucks County, Pa., but he also visits dark sky areas in Maine, Pennsylvania and other states on the East Coast. His work in astrophotography ranges from capturing nearby solar system objects to deep sky objects and wide field shots of the Milky Way.

Cahill started with a webcam, imaging the moon and other planets. After that, he was hooked, adding equipment to start taking pictures of objects farther away like nebulae, galaxies and star clusters. Pairing a digital camera and using the telescope as a “lens” has enabled him to take pictures of what has always been his passion –the night sky.

Cahill recently received the patron’s award for the 2020 Phillips Mill Photography Exhibition, in New Hope, Pa. His astrophotography can be viewed on Instagram @jec_astrophotography.

Non-members of the MDI Photo Club who are interested in attending this presentation should send an email to [email protected] to receive the Zoom link.