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:

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Reduce noise and deal with light gradients and vignetting.
Improve signal to noise ratio.
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:

A stack can be visualized as a pile of images all stacked one on top of the other;
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.

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:

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.
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.
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,

A star field from a fixed tripod.
A deep sky object from a tracking mount.
The Moon handheld.
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 a

Software For Mac Os X