Astrophotography Image Stacking – Astro Stacking

Hopefully you’ve been out shooting and applying what you’ve learned about astrophotography. For most there’s a fairly big learning curve with astrophotography. I was always pretty good with the computer, electronics, and the mechanical hardware, but learning to process the images was a huge challenge. Hopefully I can share what I’ve learned to help speed up your learning process.

There’s a lot to learn when it comes to taking the images from the camera to making a final image for display. You’ll find that 99% of the deep sky images that you shoot will require some form of post-processing. But before we even discuss doing any processing, let’s discuss how to best shoot the scene.

In the previous blogs, I’ve hinted about a technique that will let you get the most out of your astro images. Shooting very faint moving targets can be pretty challenging. It takes fairly decent equipment to get the really faint stuff, but beyond this, it’s important to properly photograph the subjects. There is one valuable technique that will help tremendously with processing and make the most of your data. This technique is stacking.

Let’s take a look at stacking in very basic terms. Shooting faint targets makes for generally noisy images. This is true  for astrophotography as well as regular photography. This means that the photos look grainy and lack the silky smooth transition. In astrophotos, noise will disturb the transition from the target object to the dark regions. But if you shoot many photos of the same subject and stack them together, the result is far better than that of a single frame. The noise and graininess is filled in and the image will appear much smoother and complete. When I was going for the best quality images, I would generally shoot for between 10 and 20 hours of open shutter time. But again, these were for my very best deep sky images on professional level equipment. For me, that meant shooting over many nights and stacking all the data in the final image. I was shooting exposures that were ½ hour long,o I needed fewer frames. But the end result was a lot of data, that when assembled, resulted in very good data sets.

If you’re just starting out it’s not necessary for you to shoot this much. But generally the more you shoot the better. There’s a big difference that can be seen immediately in the final image. There is a point of diminishing returns, but most astrophotographers will never come close to this limit. So if you can start with shooting a couple hours you’ll end up with fairly decent data. But even shooting and stacking 10 images will be better than one single frame. The better the data, the easier it is to process into the final image.

How do we begin…?  Once you have your mount aligned (see my previous blogs) the target framed and the lens or telescope focused, you can start shooting your images. Shoot the same subject, over and over. I generally use a computer or an intervalometer to take the work out of this. This allows me the ability to walk away and let the camera shoot until it’s done. Just be aware that you may need several batteries or an AC adapter for your camera. This is especially true in the cold. For your first outing, try to shoot for at least an hour of open shutter time. That means if you’re shooting 5 minute shots you’re going to want 12 of these to make an hour. It’s generally best to shoot with an exposure as long as possible, but not so long that the image becomes saturated with light fog or you begin to get star trails. I generally tried to shoot until I reached about 25-75% on the camera’s histogram. But this depends on the target and from where I’m shooting (and how much light pollution is present). Just keep in mind that 1 hour is not a magical number. Shoot more, if you have time and patience. This will make the post-processing after the stack easier and the final image even smoother.

Once you have the stack, what’s next? You need to process all these images into a single image. This is possible in Photoshop and there are some really great videos and information on the topic. So I’ll leave this learning process to those interested in doing the stacking in this manner.

The real benefit is doing the stacking in a program that is meant for processing astrophotos. There are many programs that are available to do this, some are even available for free. I used a program called MaximDL which is a high-end piece of professional astrophotography processing software. In addition to doing some processing, it also handles camera control, filter wheel control, focusing, guiding and many other aspects of shooting deep sky images. In a complex setup, it’s very beneficial to have control of everything in a single piece of software. However for those just starting out, look at getting Deep Sky Stacker (DSS). It is an excellent stacking program and is available at no cost. This allows you to practice shooting and processing images without investing a lot of additional money in software.

Be sure to take a look at the excellent instructions on the DSS website and online. It is fairly powerful and capable producing nice images. It will also allow the addition of calibration frames (discussed below), which is another very powerful feature for noise control. I generally found that I liked doing the stacking in DSS and then doing the remainder of the processing in Photoshop or similar image processing program. But that’s totally my preference. Each photographer should investigate the best workflow and combination of programs to use to produce the final image.

One really great feature of DSS is the comet stacking routine. Processing comets is even more complicated as the comet is typically in a different location in each frame. Some move slow enough not to have to worry about it. But others can move significant amounts in each frame. This typically takes some crafty processing to get a decent image. DSS takes a lot of the work out of it. This image was processed in DSS and Photoshop.

Coat hanger Asterism (CR399)  and Comet Garradd

When beginning the stacking process, the images need to be quality sorted first and then aligned (or registered) first. The quality sorting can be done automatically in DSS, but I generally liked poking through the images and picking out the ones that were blurred from movement, or had clouds or planes. The registration or alignment will adjust the images up and down and also in rotation in order to bring all the frames in perfect alignment and then stack them together in one of several stacking methods. I generally prefer one of the median stacking methods.

Many of my astrophotos, including the comet photo above, were shot with professional level equipment. This equipment cost about half what my first house cost. To be fair I wanted to show what can be done with a DSR and Lens (or small telescope), so I re-processed some of my earliest images in DSS, knowing what I know now. These were shot with an Astro-modified, 6.3MP Canon 300D (Digital Rebel). This is one of the earliest DSLR’s. It was noisy and did not generally produce very clean astro images. But even with this old camera, the data was very usable and produced some fairly decent images.  We’ll take a look at a few of these below:

My first modified DSLR for Astrophotography

 

Stacking Examples

Here are some examples of images right out of the camera and also some processed images. The first is a single frame that shows the Heart & Soul nebula (IC1805, IC1871, NGC 869 and NGC 884) as well as the double cluster. The top is out of the camera the next is after stacking and processing.

Unprocessed, right out of the camera

Stacked and post processed 

The difference in these is drastic.  In fairness, the single frame image was fogged by heavy light pollution.  But this is a problem that will plague the majority of astrophotographers.  The only way to combat this is to shoot from dark sites away from the city lights.

This next example is not as drastic. The top is out of the camera, the bottom is stacked and processed. Also included are crops of a single frame and stacked and processed images.

Single Frame and crop of the Rosette Nebula (NGC 2237)

Notice the missing details in the crop of this image.

Stack/post processed image and crop of the Rosette Nebula

The stacked image makes is much cleaner and much of the missing data has been filled in.  Also note the better detail that is visible in the crop of the Rosette.  This is the real benefit of the stacking method.  One thing that you need to keep in mind with processing astrophotos is that it’s an incremental process. No single step is going to make a magical image from junk. Each step will add a tiny improvement, and with enough tiny steps you’ll end up with a very pleasing image. If you’re stacking many photos, most pieces of stacking software will take quite a while if you’re computer isn’t up to the task (like mine). So be patient and just let it run until it’s completed the registration and stacking processes.

Here’s another example of a single frame vs a stack.  This one is of the Horsehead Nebula (B33) in Orion.

Single Frame and crop

Stack/post processed image and crop

It’s fairly easy to see the benefit of stacking when shooting astrophotos.  One more advanced technique that will help reduce the noise in your stacks is called dithering. Basically this is moving the camera a couple pixels in a random direction after every frame. When using a median stacking method, objects in a different location on each frame will be eliminated. So using the stars as the alignment reference, the galaxies, nebulae or other subjects will remain in the same place. But hot pixels, satellites, planes, noise and other random effects will be in a different location, with respect to the stars, so these are eliminated when stacked. There are many guiding or tracking programs that will do dithering automatically. But even with a manual shutter release, it can help tremendously if you manually move the mount between exposures. It seems like a hassle, but dithering will add a fairly significant level of improvement. None of the images above (except the comet image) used dithering.

Another helpful addition is to add calibration frames. These will serve to help remove additional noise and other artifacts from the images. Dark frames will help remove hot pixels, Bias frames reduce read noise and flat frames will help clear up any dust spots or other specs that are caused by looking through the lens or telescope. There is a superb description of this in the FAQ section here. The newer more modern cameras tend to provide better noise and hot pixel control, so calibration might not be needed. But at the very least, flat frames should be used to ensure the removal of artifacts caused by a dirty lens or sensor. It will also help reduce any vignetting that occurs in the images. Remember: incremental improvements.

In the final installment of this Astrophotography series, we’ll discuss some of the details of going from a rough stacked image to the final image. This is where a lot of the magic happens so I hope you’ll stay tuned. In the meantime get out and shoot. See you soon.