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:
Registax is a free program used by astronomers to stack, align and average multiple images such as would come from a web cam. These techniques allow astronomers to obtain high quality images of planets, stars etc using relatively cheap equipment. Skype is software for calling other people on their computers or phones. Download Skype and start calling for free all over the world. The calls have excellent sound quality and are highly secure with end-to-end encryption. What other software is available out there for Lunar and Planetary stacking? I use Registax v5.1 for stacking RAW files from the DSLR but it is a pain in the base. V6.1 (or whatever it is now) won't stack RAW files, DSS doesn't seem to want to stack anything without stars in it. Stacking programs like RegiStax and Autostakkert! Will take many 8-bit images and create a stacked 16-bit image capable of further processing. But they can only do this if there is sufficient random noise in the signal to allow it to effectively calculate the intermediate grey levels by an averaging process.
- 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 astrophotography software for stacking both starry landscape and deep-sky images. It can also be used to create star trails.
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.
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
Program Like Registax Free
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.
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.
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.
Overview:
Setting up the telescope for a night of imaging is only half of the equation. Capturing the images and post-processing them can be time consuming but the end result can also be quite rewarding. Depending on the equipment and software used there are many methods to capture and process the image data. The following steps are simply the way I capture and process my images using a Meade LX90 and DSI Pro II imaging camera.
My Equipment Setup:
The following is an example of my equipment setup when preparing for a night of imaging:
- Position and level tripod.
- Attach LX90 OTA (Optical Tube Assembly) to the tripod and fasten securely.
- Attach Dew Shield to Optical Tube as needed.
- Re-level scope.
- Run Power. This can be either 110v current, or 12V DC power depending on the location.
- Setup folding work table.
- Setup Laptop computer.
- Attach the appropriate focal reducer and spacer to the DSI depending on which objects I am planning to image.
- Attach the DSI imager to the telescope and connect it to the laptop using a USB cable.
- Position counterweights to achieve balance at approximately 30 degrees.
- Turn on the telescope and ensure that the Autostar hand controller has current information such as Location, Date, Time, etc.
- Prior to darkness, Calibrate the Sensors, and Train motors on the telescope for more accurate tracking (if necessary).
- Once dark, turn on the telescope and perform an automatic (2-star) alignment. I use a 9mm Illuminated Reticular eyepiece for this although I can also use the DSI and laptop to align as well.
- Boot up the laptop and start the Meade Autostar Envisage program.
Once the telescope has been powered on and aligned it can then locate objects in the sky. The Autostar controller contains a database of over 12,000 astronomical objects. An object is selected, and the go-to button is pressed. The telescope will now locate that object and center it in the field of view then track the object. Once this is performed the object can then be imaged.
Why I prefer a Monochrome camera over a Color CCD camera:
• A mono CCD has every single pixel sensitive to shades of black/white.
• A color CCD has a series of RGB (red/green/blue) pixels across its surface. So when a photon hits a mono CCD, it will 'excite' all of the pixels in the area it hits. The same pixel hitting a color CCD will only excite the RGB pixel that responds to color of the photon.
• The mono CCD is more sensitive to light since all of its pixels are identical and respond to all photons. The color CCD will have pixels that only respond to the color hitting them.
• While a mono chip can create a better image, if you want to get a color image out of a mono CCD, you will need to use filters and take a series of 3 or 4 images (RGB/LRGB) then stack them. So, while the color CCD certainly is more convenient, the mono CCD puts out the better image.
Image Capturing with Autostar Envisage:
There are a number of various programs that can be used to capture images.
•For Lunar, Solar, and Planetary imaging I prefer to use the Meade Autostar Envisage and K3CCDTOOLS programs.
•For Deep Space imaging with the Meade DSI Pro II I use the Meade Autostar Envisage and Stark Labs Nebulosity programs.
Here's an example of the Meade Envisage program:
(Click on the image to enlarge)
Using Envisage to capture images can be somewhat complex but the more it is used, the more familiar the user will become with the program. Images can be captured in a number of file formats but I have found that the best format to capture with is FITS. More on this later.
My Save options are to 'Save All Uncombined Images'. Once the image has been located, centered, focused, and adjusted, clicking the Start button will begin the process of capturing images. By selecting the 'Save All Uncombined Images' option, each image will be captured separately and a combined image will be produced as well. Separate images can then later be combined using various programs like Registax,Astrostack, Meade Autostar Suite's Envisage, or DeepSkyStacker (which is my personal choice).
Image Capturing with Nebulosity:
Here's an example of the Stark Labs Nebulosity program:
(Click on the image to enlarge)
Nebulosity is designed to be a powerful, but simple to use capture and processing application for a wide range of astronomy CCD cameras. Many cameras are supported for capture and images from just about anything can be processed (support for many FITS formats, PNG, TIFF, JPEG, CR2/CRW, etc). Its goal is to suit people ranging from the novice imager who wants to create his or her first images to the advanced imager who wants a convenient, flexible capture application for use in the field. In it, you get a host of purpose-built, powerful tools to make the most out of your images (e.g., Digital Development Processing, traditional alignment/stacking (equatorial and alt-az), Drizzle alignment/stacking, Bad Pixel Mapping, LRGB tools, real-time tricolor histograms for color balancing, star tightening via edge detection, adaptive scaling of stacks, Canon CR2/CRW RAW Bayer matrix loading, 32/96-bit accuracy, etc.)
DSI Pro II with attached filter bar:
If I am creating a color image, I capture 4 sets of individual images. They are Luminance, Red, Green, and Blue using the provided filters that come with the camera. See the following image which shows the camera, filter bar, and focal reducer:
(Click on the image to enlarge)
Image Stacking:
My image processing program of choice is DeepSkyStacker. This freeware program is an improvement over the Meade Image Processing program and provides additional features which makes my image processing quicker and with better results. DeepSkyStacker can be downloaded from http://deepskystacker.free.fr/english/index.html.
(Click on the image to enlarge)
The screenshot above shows a number of raw images that have been added to the DeepSkyStacker program. The individual images are of M1, The Crab Nebula. In the example above, this group of Luminance (mono) images will be stacked together in DeepSkyStacker to create one composite image. The procedure will then be repeated for my individual Red, Green and Blue images. Once completed, I will have four composite images that will then be aligned with each other in Autostar Image Processing then post-processed in Photoshop to create the final image. Note how faint the Crab Nebula looks. That is because the image above is only one single raw image. Stacking many frames on top of each other, will bring out more detail. Combining all four sets of stacked images together in Photoshop and post-processing will bring out even more detail.
The individual settings of each person using DeepSkyStacker varies. The following are the settings I use when stacking individual images. The version I use is 3.2.2 so other versions may differ slightly.
(Click on the images to enlarge)
Register Settings - Actions Register Settings - Star Detection Stacking Parameters - Result Stacking Parameters - Light Frames
Stacking Parameters - Alignment Stacking Parameters - Intermediate Files Stacking Parameters - Cosmetic Stacking Parameters - Output
Once I have stacked my four sets of individual images into 4 combined images, I need to align them with each other for final post-processing in Photoshop. I can do this using DeepSkyStacker, or I can stack them using Meade's Image Processing program which is part of the Meade Autostar Suite software package.
The following are the settings I use when stacking the 4 combined images. These vary slightly from the DSS windows above:
(Click on the images to enlarge)
Register Settings - Actions Stacking Parameters - Result Stacking Parameters - Intermediate Files Output files to be combined in Photoshop
Autostar Image Processing:
With Meade Autostar IP astronomical image processing software one can perform many of the same image processing tasks that a professional astronomer would do on a large institutional computer. With Meade Autostar IP you can:
• Enhance high resolution images using advanced image processing techniques.
• Determine stellar magnitudes directly from electronic images, and a number of other powerful features.
One can also use Meade's Autostar Image Processing program to stack the 4 combined images. Image Processing will align all four combined images with each other and create four aligned images so that each combined image will line up with the next. The way to do this is:
• From the main Processing window, select Group/New and select your 4 combined images (L,G,R, and B).
• Select Group/Align/Two Star. You are looking for a star pattern from all 4 images. These patterns are usually somewhat consistent across the entire image. In the example below similar patterns of four stars exist throughout the image.
• Using your mouse draw a box around a 4-star pattern. Go to the opposite side of the image and draw a second box around another 4-star pattern. The four unaligned images will now automatically process.
Example of 4 separate combined images before aligning.
Program Like Registax Movie
(Click on each image to enlarge)
Once the 4 unaligned images have processed with each other Image Processing will automatically create four aligned images into the same directory as the four unaligned images. If the first alignment is not successful, or additional aligning is needed, process the four aligned images and create a new set of four images using the same process as above. Once the images are aligned, the result will look something like the following example:
The four aligned images are now ready to be post-processed in Photoshop.
Photoshop Processing:
FITS Liberator:
Like all of the above steps, there are a number of different tools available to process one's images. With post-processing, Photoshop is used by many because of the quality of the software, and the ability to bring out the rich detail of an image. The following will not be attempt to guide one through each and every step within Photoshop. Rather the following steps will serve as a guide for processing an image. More detailed links to step-by-step processing will be included below.
Programs Like Registax
I post-process my images using Photoshop CS2. My images are captured in FITS format.
What are FITS images?
FITS stands for `Flexible Image Transport System' and is the standard astronomical data format endorsed by both NASA and the IAU (International Astronomical Union). FITS is a file format used for many years in astronomy to store and move images from telescope-based instruments. It has been designed to provide capabilities needed by astronomers but is not a generally known format such as JPEG or TIFF.
In order to process FITS images, either download a free FITS Liberator plug-for Photoshop (v2.2), from spacetelescope.org, or download a standalone version (v3) that does not require Photoshop from The European Homepage For The NASA/ESA Hubble Space Telescope. Since I post-process in Photoshop I prefer the v2.2 plug-in. The system requirements are as follows: