Image Stacking in Astrophotography [Reduce Noise]

Image stacking is how you reduce noise from your precious photo. Yes, noise…those seemingly random specks that dot the landscape of your photo. What is it? How it get there? And most importantly how to I get rid of or control it? For those of you concerned with the detail in the image you create, I will try to answer those questions and more. If you’re new to astrophotography, this is aimed to give you new knowledge of an obstacle to overcome. There are several ways to reduce noise. Let’s take a look.

What is Noise?

Noise is not seemingly random, it is random. The specks or dots can be random color changes, changes in brightness, or look like the grain in film photography. The photo below has an example of noise in a photo.

What you’re seeing is low signal to noise ratio. The signal comes from the letters in the word plus the background and the noise is everything else. Because the noise is so high, the actual color of the background is difficult to see. The letters are missing some detail around the edges and corners.

The noise comes from several areas in astrophotography with both the smartphone and the DSLR camera:

• HIgh ISO camera setting
• High Sensor Temperature

Plan to Reduce Noise

As far as temperature goes, check the weather. Cold nights are good, but summer nights are grueling for a camera sensor. I use a DSLR camera and my Google Pixel. Both sensors keep heat during summer nights. During warm summer nights, take 50 photos and stack them. Plan to take a lot of photos. You’re going to see a theme here.

You can also modify your camera to add cooling for any time of year or purchase an astrophotography camera (CCD or CMOS). I suggest taking a lot of photos and stack them.

Brighter targets don’t require a high ISO setting. ISO 400 or 800 are typically acceptable. I recommend an ISO as high as you can go based on brightness of the target and light pollution conditions. Too high will wash out your signal as well. This means that you’ll have some noise to remove from your photos. To reduce noise, take a lot of photos and stack them later.

Image Stacking and What is it?

Stacking is the process of aligning and combining several short exposure photos. Once aligned, the images are averaged together to create an image with higher signal to noise ratio than the individual photos. I use software to stack the images. Listed below.

• Deep Sky Stacker – For deep sky objects
• Registax – Planetary objects
• Autostakkert – Planetary objects

There are many more including those above. The results of stacking can take a photo from this:

to this:

However with image stacking, the alignment is achieved using the stars. This requires precise polar alignment or the software will struggle to align the photos. Assuming you have this, what kind of differences can you see in your photos? Let’s explore a few of my results.

Shown above are two separate capture sessions. The left is at 13 frame stack with significant noise. The right is a 31 frame stack and looks much more clear. The detail in the Andromeda galaxy is visible in comparison. Zoom in and the noise reduction results are eye popping.

These Pleiades results look to have more subtle noise issues, but zoom in and they become more apparent. Of course the 16 frame stack on the left is the one with the noise issue. Zoom in to view the wispy clouds around the stars and you’ll see the difference.

Image Stacking Workflow

My process for stacking photos begins with a review. Inside of Deep Sky Stacker, I can look at each photo in detail. I’m looking for any egg shaped stars, cloud cover, planes or space junk within the photos. Any I find are removed from the list. I have started experimenting with RawTherapee to complete my pre-review of the photos. RawTherapee also allows me to make changes to photos before stacking. I’ll write about these results when they are available.

Quick rundown of the workflow:

• Add photos (lights)
• Review lights and remove bad frames
• Add calibration files (darks, flats, bias)
• Click Check All button
• Nex Click Register Check Pictures
• Click Advanced on the Register Settings Dialogue box
• Click Compute the number of detected stars button
• Adjust the slider based on the results (100 stars < target < 500 stars)
• Click on Recommend Settings and adjust based on recommendations
• Click OK to stack

If you click on compute the number of detected stars and you still cannot get greater than 100 stars at 2%, then PUNT! The pictures have problems that keep the software from recognizing the stars in the photos. Try again and adjust your capture settings. After this workflow is complete, I take the photo and process in Startools. There’s still some noise to remove.

Reduce Noise in Startools

After image stacking, I begin post processing. However, you’ll find during this process additional noise reduction is still required. I’m still learning Startools, but it’s great at removing noise. It tracks the noise created during your post processing. Once I have an image I like, I complete it with noise reduction. Setting up the final opportunity to reduce noise.

Gimp Noise Reduction

In addition to image stacking and Startools, I address noise one final time in Gimp. Most astrophotographers use other software like Photoshop. I’ll get there one day. None the less, Gimp has filters that reduce noise. Under Filters, Enhance, choose Noise Reduction. Because the filter exists, that doesn’t mean it has to be used. Check the results as it will remove some of your desired data.

With these steps, my photos have begun to contain less noise than prevoiusly experienced. Hopfeully these steps can help you in your astrophotography journey. Clear Skies!