Image Processing in Photoshop

Bringing Your Time and Effort to Life

You’ve just spent hours, or even days or weeks collecting data for your latest object. You stack up all of your light frames with your calibration frames (darks, flats and BIAS) in DeepSkyStacker and let it do it’s thing. What comes out is a very dim view of your object, or a even what looks like a red hazy image with no object at all. If you did the first part of the job right, your data is in there, I promise. How do you pull that data out in the most effective way possible? Well, I’m sorry to say there is no magic bullet because everybody’s data is different. However, there are methodologies, workflows, and tools to help you along.

I’m constantly finding new techniques and methods and have upgraded to Photoshop CC, but there are some basic steps to carry through all images.

I don’t think one specific tutorial is the most effective way to learn image processing, though I have used them for ideas. With a tutorial, you’ll have a tendency to follow the it exactly to process your data-set that doesn’t match the data used in the tutorial. Instead I prefer to follow a set of steps to create a successful workflow. Everything I reference has to do with Photoshop. I’m using CS5, which is ancient by now, but it does the job. GIMP is a free tool that has a lot of the same functionality, but I can’t speak to it with any type of authority having not used it.

The two images below were created from the same data. The data was collected on the first night I ever imaged. The image on the left was processed when I was green to the hobby and clueless. The image on the right was processed using my current workflow methods. Neither of these are a completed data-set and require additional integration time, but it’s representative of a bad workflow vs. a good one.


Layers Make a Huge Image – They Also Make a Successful Image

For every major step in processing, you should be stamping a new layer (Shift+CTRL+ALT+N+E). There are two main benefits to this:

  1. Your workflow is non-destructive when using this method. Your history only holds so many steps and you can’t go back beyond what’s listed, but if you created layers for all the major steps you have the flexibility to go back to any point and quickly review all of your steps to see where something might have gone wrong, or mark what you did right.
  2. By using layers, you can also adjust the opacity, which in turn adjusts the effect of your change. By adjusting the opacity of a layer, you can feather it in to make a drastic step that can ruin an image a subtle change that enhances your image.

Noise is Your Enemy- How do You Avoid it?

We’re all very anxious to pull out the details of the object we’ve already spent so much time acquiring. My first advice, slow down! You popped into the curves and pulled your first aggressive stretch. That wasn’t enough, so why not another aggressive stretch? Getting too aggressive out of the gate will introduce and enhance existing noise from the start and doom the rest of your workflow. Go slow and gentle with the curves. Use the opacity function I already talked about with your curves layers to feather them in. I usually only do three curves, they look similar to the images below, and in the order they are presented. First an aggressive stretch, then a moderate s-curve stretch, followed by a gentle stretch. Each curve is a new layer, and the levels are adjusted between each stretch. Depending on the quality of your data, you might get away with another very mild s-curve stretch.


Get a Handle on Levels

Slam the levels to push the data to the left of the histogram window and get a black sky. How hard is that, right? The problems with this method is that the sky isn’t black, and you’ll be introducing additional noise when you try to bring out the darker details of your object using curves. There are some tricks to levels that will help you get the most out of your hard work.

  1. Use the eyedropper tool to mark what will be the darkest spot of your image by holding the SHIFT key while clicking in a dark spot. This spot should be between stars and not contain any nebulosity from your object. Now on your infotab (click Window–> Info form the menu bar), you’ll see values for Red, Green and Blue. 
  2. Don’t slam the levels all the way down to make your dark point as dark as possible. I rarely set my “black” point below 30,30,30, and never below 25,25,25. You may initially like the contrast dropping the levels to 20, or even below, but in the end, you’ll clip data containing the darker details of your image.
  3. Adjust your levels independently for each color. There are many ways to balance color in an image, but you should use the baseline of equalizing your dark point by adjusting each color (Red, Green, Blue) independently. It’s easier to just push over the one slider on the RGB setting, but you will end up magnifying any color balance issues and make them harder to fix later. Balancing using levels also helps mitigate some of the effects of light pollution. See the image below for an example.
    Levels 1
  4. Keep an eye on your levels and adjust accordingly after each action. Your color balance can get away from you in a hurry if you’re not always watching the histogram and adjusting your levels to even it up.

Plugins and Tool-sets

Plugins and tool-sets are available in abundance for Photoshop. I use two on every image, and a third on several. Purists may think using these is cheating. Instead, I prefer to think it’s helping me because I’m lazy.

  1. Astronomy Tools Action Set is a series of astrophotography specific tools that takes an action consisting of several steps and combines it into a one-click function. It is not a free tool-set, and comes in at $21.95 (US). Well worth it in my opinion. I use several functions on every image, such as Space Noise and Deep Space Noise Reduction, Make Stars Smaller, and my favorite, Enhance DSO and Reduce Stars. These should be used on new a layer so you can adjust the opacity.
  2. Gradient Xtermonator is pretty self explanatory. No matter how many flats you take, if you have any light pollution whatsoever, you’re likely to have some gradients in your images. This one is not cheap at $50 (US), but it’s another one that I use on every image I process. There are several manual methods and tutorials abound on the Internet to take care of gradients, but I can never get them to work as well as this tool does.
  3. Hasta LaVista Green (HLVG) is a tool that removes extra green that creeps into images with a nebula or a galaxy containing blue hues. This tool is specific to the object you’re imaging and I don’t use it all that often. However, it is free!! A few objects I’ve used it on to kill the green that seems to creep in on the blues are the Eastern Veil Nebula (NGC6992), The Orion Nebula (M42), and the Andromeda Galaxy (M31)

Order Is Important, but Flexible

The order of steps is important, but not always the same. I usually do all my curves at the start, but there are times that I push off the third curve until after I fix gradients. I might reduce the stars toward the middle of processing, and depending on how I want the image to pop, it might be one of the very last steps – or I may remove all of the brighter stars all together. Take your time and make your changes less drastic and more subtle. They add up in the end. The differences are VERY subtle in the beginning, but magnified in the end. The images below are again from the same data – about an hour of total integration on the Eastern Veil Nebula. Again, not a complete image. The steps in processing were pretty much the same with slight changes in the order and subtle changes in some of the action’s intensities. The exploded view of a brighter section of the nebula should show the difference in detail and color.


The list that follows if very flexible, and also very general. In the end, your best results will come through experimentation on your own. These general steps should help you get where you want to be.

  1. Crop and set mode to 16-bit color
  2. Set black-point and adjust your levels
  3. Perform your curves
  4. Fix your gradients
  5. Adjust your levels
  6. Color balance
  7. Decide if you want bright stars or small stars and adjust accordingly
  8. Sharpen (Unsharpen Mask)
  9. Make it fuzzy with Less Crunchy More Fuzzy if you have the Astronomy Tools Action Set (yes, that does seem to contradict itself)
  10. Select a color range and increase saturation slightly if you feel the colors are too flat.
  11. Enhance DSO and Reduce Stars is a great step at this point if you have the Astronomy tools action set. If not, and you feel like your object is a little too dim, You can do a very subtle curve and adjust the opacity and levels.
  12. Adjust your levels
  13. Smart Sharpen or the Local Contrast Enhancement. Again, on a stamped layer so you can adjust the opacity.
  14. Noise Reduction
  15. Get the black-point to where you want it
  16. Check if any gradients cropped up and correct them
  17. Depending on the object, add a synthetic luminance layer. Switch the image mode to Lab Color (don’t flatten), click on the lightness layer, select all and copy. Change the image back to RGB (don’t flatten) and paste what you copied as a new layer, setting the blending mode to luminosity. You can change the opacity and/or pull the curves down to reduce the effect if you need to.
  18. One last balance, level set, and you should be all set.

This definitely does not encompass every step you’ll take processing an image. There is more to it, but these are the basics. There are good and bad methodologies, and good and bad images. That said, always remember that this is all largely artistic, so what matters most is how it looks to you, and what you’re happy with.

I know processing can be difficult and frustrating. You seem to do the same thing for the same data and get different results. I’m still there all the time. Keep your layers, backtrack when you need to, and try to have fun. I love space and astronomy, and I love art. Knowing the potential of the final product is what keeps me staring at a computer screen after I’m at my wits end.

If you want to follow a tutorial, I recommend the one linked below, created by Trevor Jones at AstroBackyard. Trevor is a graphic artist by trade who is more than happy to share his knowledge and has helped me along a great deal.

AstroBackyard Image Processing Tutorial (Soul Nebula)

Clear Skies, Bleary Eyes – KA

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.