For many years I have been using Deep Sky Stacker to align and stack my short exposure frames to give the result of a longer exposure. The latest 64-bit version seems very stable. More recently, two new stacking programs have become available; Starry Landscape Stacker (~£38.99) and Sequator (free, but a contribution by paypal for a cup of coffee or two is suggested). The former is only available for the Mac and the latter for Windows. They both have the ability to treat the sky and any foreground separately – great for wide angle night landscapes. In this case, a fixed tripod is used with exposures short enough to prevent star trailing. The sky area is selected leaving the foreground unselected. The programs will then stack the sky area accounting for the movement of the stars across the sky but stack the foreground with no movement. As a result, the stars within the sky will be sharp as will be the foreground. Clever.

There are several Mac apps that combine a sky chart with telescope control for compatible GoTo telescopes. They include, for example, Equinox, SkySafari, TheSkyX and Starry Night. DeepSkyStacker is for astrophotography that simplifies all the pre-processing steps of deep sky pictures and is specialized in dealing with RAW files from. Alignment and stacking.

Thoughts about Deep Sky Stacker

I currently do not have any wide angle data to try out, but decided to test Sequator with a set of Nikon raw files that I have extensively tried out with Deep Sky Stacker. The raw frames had been taken using a full frame Nikon D610 attached to a CFF Telescope’s 127 mm apochromat refractor employing an aspherical oil-spaced triplet objective. The D610 was controlled over a USB cable from an Android tablet using the program qDslrDashboard and took 56, 20 second, exposures at an ISO of 800. No darks or flats were taken. The telescope has a 3-inch focuser but, having a 2-inch barrel, could not be expected to cover the full frame sensor. As a result, the field was significantly vignetted but the two clusters were well covered. I was not using a field flattener, but the stellar images were perfect across the unvignetted field of view.

In an extensive article, ‘Deep Sky Stacker: could it be worth first converting raw files into Tiffs? A case study imaging M35 and NGC 2158’, I came to the conclusion that the best results were obtained when the .NEF raw files were first converted into Tiff files. It could be well worth reading this article. I discovered that the result when stacking the Tiff files derived by Raw Therapee gave a far better colour to the bright blue stars within the cluster than that derived directly from the .NEF raw files in Deep Sky Stacker and that the star images were somewhat tighter. This could possibly be that Raw Therapee uses a more sophisticated de-Bayering algorithm or that Deep Sky Stacker overexposes them when processing the raw files so increasing their size and reducing their saturation. So, along with quite a number of other astrophotographers, I have been first converting my raw files into Tiff files before stacking in Deep Sky Stacker.

Sequator

So I was very interested to see how Sequator handled the raw files. It has a very simple interface and is far easier to use by beginners. There is no need, for example, to first set the star detection threshold. It is also far faster: for this set of 56, full frame, raw files, Sequator took just over 2 minutes using my i7 laptop whereas Deep Sky Stacker took over 13 minutes.

The Sequator opening screen has a set of commands at its left which should be applied downwards in turn. A double click on the top button allows one to select the frames that will be aligned and stacked. For a stack like this, there is no need to specify a reference frame with Sequator selecting one from the middle of the stack itself.

Perhaps surprisingly, before the aligning and stacking takes place one needs to select the name of the resulting Tiff file and the location into which it is to be placed.

There are various stacking options, for example, the ‘Remove dynamic noise’ mode will attempt to eliminate hot pixels. I suggest that you try these out with a small number of frames to see what suits your data best. The ‘High dynamic range’ option tries to prevent parts of the image ‘blowing out’ so helping to preserves the star colours and does a great job of suppressing the significant light pollution that is within the frames.

The Sequator manual (https://sites.google.com/site/sequatorglobal/manual) gives a description of all the stacking options. Having tried out various alternatives (possible as the stacking process is so quick), I came to the conclusion that, for this data set, the best final image came when selecting none of them.

Having set up the program options, one simply presses ‘Start’ and watches a log of the progress.

After the ‘Successful completion!’ statement (note the elapsed time) as seen in the first image below, one clicks on ‘Close’ and the stacked result appears as in the the second image below.

This looks exactly how the resulting Tiff file will appear when opened up in, for example, Adobe Photoshop or GIMP, in great contrast to the image that appears having stacked the frames in ‘Deep Sky Stacker’. Surely it would be easy for Deep Sky Stacker to give a similar image. A minor point, but one up to Sequator.

The output file below shows the significant light pollution from my town location.

I then used my standard ‘light pollution removal’ technique in Adobe Photoshop: The image was duplicated and the ‘Dust and Scratches’ filter applied with a radius of 30 pixels. The filter ‘thinks’ that the stars are dust and removes them!

Some of the brighter stars in M35 and the small cluster NGC 2158 still remained in the image. These were cloned out from nearby regions and then a Gaussian Blur applied to give a very smooth rendition of the light pollution.

The two layers were then flattened with the ‘Difference’ blending mode selected so removing the light pollution.

This image was stretched using a curves function.

The Sequator result shows bluer stars within the M35 cluster which are slightly tighter than those from Deep Sky Stacker when processing the .NEF files and gave a similar result as when Deep Sky Stacker processed Raw Therapee Tiff files. I was disappointed that, in both cases, the colour of the red giants, very obvious in a single frame, had not been retained.

To bring out their colour, I first produced an enhanced image of the brightest stars in the image derived from a single frame.

I then ‘added’ this image into the main image using the ‘Screen’ Blending mode to give the final (cropped) result.

I also used Sequator to stack the Raw Tharapee derivedTiff files. I found no obvious difference between this and the result from stacking the raw files and so assume Sequator is using a quality de-Bayering algorithm.

Though I was able to achieve a similar result when stacking Tiff files in Deep Sky Stacker, it took a few minutes to derive the Tiff files and then 13 minutes to align and stack them. In contrast, the complete Sequator process took just over 2 minutes. This makes me think that for this observing project Sequator was the better option.

You should try it!

Astrophotography is closer to science than art, and there is no such thing as “getting it right in camera.” This means you cannot simply point the camera at the sky and snap away.

In astrophotography you cannot avoid post processing your images, so stacking and editing your images serves three main purposes:

1. Reduce noise and deal with light gradients and vignetting.
2. Improve signal to noise ratio.
3. Reveal the faint details in the image.

Image stacking is the technique used to improve the signal to noise ratio, and it is the only noise reduction method that will boost the image details rather than smear them out.

In this article, we will discuss some of the most popular software available for astrophotography image stacking.

Note: Don’t miss the detailed video at the end of this article, It was created to help show you how to quickly start using some of the stacking software mentioned in this article.
Click here to skip to our Image Stacking Demo Video.

What Does Stacking Photos Mean?

The concept behind image stacking is simple, but to appreciate how it works, there are a couple of things we have to consider:

1. A stack can be visualized as a pile of images all stacked one on top of the other;
2. Each digital image is formed by a set of pixels, all having a certain value: dark pixels will have a lower value than the bright ones;

In the simplest form of image stacking, the pixels values for all images in the stack are averaged to produce a single image.

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What is the purpose of stacking photos?

The result is a single image with improved signal to noise ratio, i.e., with better details and lower (random) digital noise and better details.

The scheme below illustrates the concept.

If the considered digital noise affects the pixel values randomly across the stack, then the result of averaging the stack is that the random component of the noise to the pixel value is significantly reduced.

ISO noise and Luminance noise and Chrominancenoise are examples of digital noises that are random.

The image below shows a real-life example from stacking 30 images from my Sony RX10 bridge camera taken at ISO 6400. As you can see, the original images showed a greater deal of noise (grain) than the stacked one.

The More Images You Stack, The Better

The more images you stack, the cleaner the resulting images are, as shown in the comparison below.

While Image stacking creates a cleaner image, it often softens the image: digital sharpening techniques are then used to recover sharp looking details.

Finally, bear in mind that the progression of image quality is not linear.

If stacking 4 images improves the image quality of 50% respect what you got by stacking only 2 images, to improve a further 50% the image quality from stacking 50 images, you may need to stack 300 images or more.

Image Stacking And Movement

If nothing moves between shots, like in the previous real life example, implementing image stacking is very simple: just group the images and average them to smooth out the noise.

With a moving subject, grouping and averaging the images will not only smooth out the noise, but also the subject itself.

This is the same principle for which long exposures of passing traffic and crowd result in a street image without cars nor people.

This effect is amplified with the number of images used, and the moving subject could simply disappear from the stacked image.

To resolve the issue, you have to align the images based on their content before stacking.

Due to image alignment, you may have to trim the edges of the stacked image to get rid of artifacts, but your target will not be lost.

Note that while in theory you can stack images of a static scene taken with the camera on a tripod, in reality, those images will probably differ at the pixel scale due to micro-movements. It is always beneficial to align the images before stacking.

How To Shoot For Exposure Stacking Your Images

Image stacking can be done with any camera and even camera phones and with images in both RAW and JPEG format.

Nonetheless, some things can be done to improve the final result:

1. Lock the focus, so that the camera will not hunt for it between images. This will also help to keep the focus consistent through the shooting sequence.
2. Keep the same settings, in particular shutter speed, aperture, and focal length: you don’t want to change the camera field of view during the sequence, nor the brightness of the images or the depth of field.
3. If you are shooting on a tripod, disable image stabilization. If you want to shoot handheld, do so only for short sequences at very high shutter speed.

Image Stacking In Astrophotography

Related:Astrophotography Software & Tools Resource List

As said previously, image stacking is a standard technique implemented in any astrophotography editing workflow for,

1. A star field from a fixed tripod.
2. A deep sky object from a tracking mount.
3. The Moon handheld.
4. A starry landscape from a fixed tripod or tracking mount.

Every astronomy image will benefit from image stacking.

List Of Photo Stacking Software For Astrophotography

Here is a list of software used in astrophotography for image stacking.

Adobe Photoshop

Complete Image Editor | Commercial – Subscription Plan Photography Bundle $9.99 / Month | Mac OS X, Windows

Pro

• Versatile
• Available for Mac and Windows
• In bundle with Adobe Lightroom CC, Bridge, Camera Raw, and web space
• Many action packs and plugins available for astrophotography

Cons

• Subscription Plan only
• Can’t be used to calibrate light frames
• Stacking capabilities are somehow limited

If you are interested in photography, chances are you know Adobe Photoshop is the standard in the industry and does not need introductions.

With Adobe implementing a subscription plan for their applications, if you are using Lightroom CC for your everyday photography, your plan subscription will also include Photoshop CC and Bridge CC.

And for astrophotography, Photoshop is what you need. Lightroom cannot stack your images nor perform the histogram stretching, two crucial steps in the editing workflow for astrophotography.

In this article, we have already covered in detail how to stack astrophotography images with Photoshop.

Sequator

Deep Sky And Starry Landscape Stacker | Freeware | Windows

Pro

• Free
• Easy to use
• Fast
• Suitable for both Starry Landscapes and Deep Sky images
• Can create Star Trails

Cons

• Windows only
• Limited set of options
• Not suitable for Planetary astrophotography

Sequator is an easy-to-use and intuitive astrophotography software for stacking both starry landscape and deep-sky images. It can also be used to create star trails.

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While not as advanced as other stackers, it nonetheless allows you to calibrate your light frames with dark and flat calibration frames. It also allows you to remove light pollution, reduce noise, and perform other simple tasks on the stacked image.

Starry Landscape Stacker

Starry Landscape Stacker | Commercial, $39.99 | Mac OS X

Pro

• Fast
• Easy to use

Cons

• Mac Os X only
• Does not read RAW files

If you are into starry landscapes and you are a Mac user, Starry Landscape Stacker is a must-have.

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Easy to use, it allows you to stack and align the sky and the foreground independently by letting you easily mask the sky.

Unfortunately, the software lacks the support for RAW formats, thus forcing you to convert your RAW images in the more heavy TIFF format.

Aside from that, it works very fast and the final image is of good quality. You can also save the sky only, which is useful to further edit the shot in Photoshop or similar editors.

Starry Sky Stacker

Deep Sky Stacker | Commercial, $24.99 | Mac OS X

Pro

• Fast
• Easy to use

Cons

• Mac Os X only
• Does not read RAW files
• Basic

Starry Sky Stacker is Starry Landscape Stacker brother and it has been created to stack deep sky astrophotography images.

As Starry Landscape Stacker, Starry Sky Stacker is very easy to use and intuitive, although very basic.

If you are a casual star shooter and a Mac user, this could be a good choice for you.

Deep Sky Stacker

Deep Sky Stacker | Freeware | Windows

Pro

• Free
• Easy to use
• Fast
• Full light frames calibration
• Features Comet stack modes
• Can Drizzle
• Many advanced stack options and methods available

Cons

• Windows only
• Post-processing is quite limited
• Not suitable for Starry Landscapes nor for Planetary astrophotography

Deep Sky Stacker, better known as DSS, is arguably one of the most widely used software to calibrate and stack astrophotography images.

With DSS, you can fully calibrate your images with Darks, Flats, Dark Flats, and Bias calibration frames for the best results possible. Light frames are analyzed and scored by quality so that you can decide which percentage of best images you can stack (Best 75% by default).

A very interesting feature is that with DSS, you can easily combine images taken during different imaging sessions, to produce images of higher quality.

Autostakkert!

Planetary Stacker | Freeware | Windows

Pro

• Free
• Easy to use
• Suitable for Planetary, Lunar and Solar images
• Stack full planetary disk and lunar surface close-ups

Cons

• Interface a bit confused
• It does not offer wavelet sharpening
• Windows only

Autostakkert!, also known as AS!, is a very popular free software among the solar system astrophotographers. With AS! it is easy to stack both images showing the full Planetary (or Lunar or Solar) disc and images showing lunar surface close-ups.

The interface is a bit confusing, particularly in the beginning, but it is easy to navigate through the different steps for the stacking.

Unfortunately, AS! does not offer wavelet sharpening, which is a widely used technique in planetary and lunar astrophotography. For this, you can load your stacked image in Registax, another freeware software for Windows only that, sadly, is now “abandoned-ware.”

Lynkeos

Planetary Stacker | Freeware | Mac OS X

Pro

• Free
• Has deconvolution and wavelet sharpening
• It is probably the only freeware planetary stacker for Mac OS X

Cons

• Not very intuitive
• Somewhat slower than Autostakkert!

Lynkeos is perhaps the only freeware planetary stacker software for Mac OS X, sparing you from turning to Windows for using Autostakkert!.

The interface is quite intuitive to navigate, but not when it comes to performing the different tasks.

On the other hand, it offers a deconvolution method and wavelet sharpening, a must-have for a planetary stacker. Definitely worth having a look at it if you are a Mac user.

SiriL

Deep Sky Astrophotography Editor | Freeware | Mac OS X, Windows, Linux

Pro

• Free
• Cross-Platform
• Active development

Cons

• A bit convoluted and not as intuitive as other stackers

SiriL is a freeware, cross-platform, astrophotography package that will let you calibrate, stack, and develop deep sky astrophotography images.

While not as easy and intuitive as Sequator or DSS, it offers a lot of options and produces good results. There is an active community, and it is under constant development.

Astro Pixel Processor

Deep Sky Astrophotography Editor | Commercial $60/Yr Renter License Or $150 Owner License | Mac OS X, Windows, Linux

Pro

• Full-grown astrophotography package
• Fairly easy to use
• Mosaics are created with ease and are of great quality
• Active and constant development
• Cross-Platform
• 30-days Trial period
• Affordable yearly subscription

Cons

• Only for deep sky astrophotography
• No Comet stacking mode

With Astro Pixel Processor (APP), you step in the realm of full-grown astrophotography packages, with many advanced options and methods to calibrate, stack, and post-process your deep-sky images.

Compared to PixInsight (PI), the software benchmark for the category, APP is cheaper and way easier to use, which makes it one of the best PI alternatives.

If you decide to buy it, you can choose between the renter’s license for $60/yr, to always get the latest version of APP, or the owner’s license for $150, but you will have to purchase the license again for major update releases.

PixInsight

Astrophotography Editor | Commercial – €230+VAT | Mac OS X, Windows, Linux

Pro

• It has all you need for astrophotography
• 45 days trial period
• A lot of tutorials and information available

Cons

• Expensive and without subscription plan
• Extremely steep learning curve
• Long and convoluted process
• Needs a powerful computer

When it comes to astrophotography, PixInsight is the software of reference against which all others are measured. It offers everything you may possibly need to produce pro graded images, and it is objectively the best software in the field.

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But user experience can be frustrating, as the learning curve is very steep, the editing is long and convoluted, and your computer must be quite recent and powerful to make it run smoothly.

The €230 + VAT price tag is also quite steep: sure it is worth every penny, but this makes PI be even more the software of choice for professional and keen amateur astrophotographers.

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A Comprehensive Demo About Image Stacking

In this video, I show you how easy it is to wet our feet with image stacking.

This is particularly true if you use Starry Landscape Stacker, Sequator, Deep Sky Stacker and Autostakkert!, as I showed in the video below.

Conclusion

Image stacking is one of the crucial steps in the astrophotography editing workflow.

You’ll need the appropriate stacker for each type of astrophotography: starry landscapes, star trails, or deep-sky and planetary images.

In this article, we have covered the most popular astrophotography stackers available on the market, both freeware and commercial.

And while Windows users have the more extensive choice, some notable stackers are available for Mac and even Linux users.