Post-processing is the final step in the Astrophotography process and often the most confusing. There are a myriad of different programs to use to view, align, stack, and edit your photos. Here, I will describe my current process for taking the black and white FITs and making them into a full-fledged color png/tif/jpg.

Note: I am very much still learning best practices for getting the most out of the data from the CCD camera. My current process is definitely not the best and I would highly recommend reading or watching tutorials on the google (specifically the youtube, more specifically AstroBackyard and Chuck’s Astrophotography) if you plan to make a lot of photos.)

Aligning and Stacking

Software

There are two options for this stage: Nebulosity 4 or DeepSkyStacker. Nebulosity 4 is much more manual and you will need to do your processing at the Goodsell Observatory Lab (unless you have Nebulosity 4 on your personal computer for some reason?) but gives you more control over the process and allows you to visualize the aligning and stacking process (somewhat). DeepSkyStacker is a free, Windows-Only software that is much more automatic and greatly faster than Nebulosity 4 but does not give you the granular level of control nor the visualization of what is happening.

Nebulosity

DeepSkyStacker

  1. Eventually, I will have a video explaining the process that you might find here
  2. After downloading the software, click on “Open Picture Files” (Upper Left Side). Select all of your Light FIT files for all RGB
  3. Then click on “Dark Files” and select all of your dark FIT files.
  4. On the left side, click “Check all” then click “Register checked pictures” (This one is the first red option)
  5. Deselect “Stack after registering”
    • If you are having trouble registering the photos, click “Advanced” and lower the “Star detection threshold” (I generally have a lot of trouble stacking)
  6. Go through your photos and either rename bad photos or remove them from your list
    • I prefer to rename them so I know which ones are bad in the future
    • Bad can be defined as having a plane streak or a shift in alignments
  7. Select one photo that is both good (a high “score”) and one where you like the composition. Right click this frame and click “Use as reference frame”
  8. Now for these steps, you will need to repeat them for each channel
    1. Select all photos of one channel (by “checking” them)
    2. Select all of the dark frames
    3. Hit “Stack Checked Pictures”
    4. My default setting for the stacking method is “medium” for both light and dark frames
      • Take note of the number of frames you are “stacking” (this won’t stay the same by the end)
      • Feel free to play around with other stacking methods and other settings!
    5. You will now see your stacked image. Do not make any changes using the sliders at the bottom (this will be done in a separate software)
      • At this point, look at the top left underneath the filepath of your images. It will show you the actual number of images this stack contains (which sometimes might be less than what you started off with)
    6. Click “Save picture to file” and name it something very descriptive (eg. M42_2x2_Red_15s_20x_DSS_Recommended.Tiff)
      • Trust me, the more descriptive your naming scheme, the better off you will be later on
      • I often save as a 32bit integer Tiff file but I haven’t tested as a 16bit integer so I’m not sure if there are any benefits to 32 over 16
    7. Click “Registering and Stacking” in the top left
    8. Deselect your current lightframes and repeat for the next channels

Combining Channels

Software

Gimp

This is a great, free, & cross-platform image editing software. A lot of what can be done in Photoshop can be done in this software. However, I use Photoshop (alongside Lightroom for my student work study) and so because I am more familiar with Photoshop, I will talk about using that.

Adobe Photoshop

  1. Open each channel in their own tab (select “Assign Working Gray: Gray Gamma 2.2” as default for each) (I usually then organize them by channel order [L R G B])
  2. For each Channel:
    1. Perform a Levels Adjustment
      • You should notice that most of the curve is on the left of the histogram
      • Bring the middle slider/triangle toward the right edge of the curve and select okay
    2. Perform a Levels Adjustment (Yes, again)
      • You should notice that most of the curve is now toward more of the center of histogram
      • Bring the left slider/triangle toward the left edge of the curve and select okay
    3. Potentially repeat the levels adjustment again but I usually just do it twice
    4. If your image is a 32 bit: Click “Image -> Mode -> 16 Bits/Channel”
    5. Change “Method” from “Local Adaption” to “Exposure and Gamma”
    6. Repeat for each Channel
  3. Go to your Luminance Image and select all (CMD+A or Ctrl+A), copy (CMD+C or Ctrl+C), then click “File -> New” (We do this because it will automagically size your new file with the proper dimensions)
  4. Change “Color Mode” from “Grayscale” to “RGB Color”
  5. You should see a “Channel” window/studio/thing on the right side (if not, go to either “View” or “Window” in the Menu Bar and look for it)
  6. Go to your Red Channel image and select all and copy all
  7. Go back to your new file and select the red channel from the “channel” window/studio/pane/thing and paste the image (CMD+V or Ctrl+V)
  8. Repeat for Green and Blue
  9. Select the “Layers” window/studio/pane/thing
  10. Rename your “Background” layer as “RGB” (or something else if it suits your soul)
  11. Go to your Luminance Channel and select all and copy all
  12. Paste your Luminance Image as a new layer
  13. Rename your new layer as “Luminance”
  14. In the “Layers” window/studio/pane/thing, there is a drop-down that defaults to “normal”. Change this to “Luminosity”
  15. Congratulations! You now have a color image!

Editing

I do want to preface this section by saying I am still learning the best practices for editing images. Described below is my current method and most likely definitely does not represent the best way to do it. This section will also deal in the abstract as it will vary greatly by image.

Adobe Photoshop

  1. I usually start with a “Color Balance” adjustment on the rgb layer. I typically adjust shadows, midtones, and highlights to make the image as neutral as I can
  2. I often add a curves adjustment by making three points on the curve of the histogram. (One of the left edge, one in the middle, and one on the right edge). I will adjust these to make an “S-shape” with the curve (this is called an S-Curve…I think?)
  3. If the object is colorful, I will add a Vibrance and Saturation adjustment
  4. I might add an extra Levels adjustment

This section is really hard to describe. Apologies. You will learn by practice. Just make edits that you personally like. Look up other pictures of your object to get a sense of what others have done. And a great practice tip is to make an image you are happy with, save it, then start again and try different edits to see the difference (sometimes, I will go back to old sets of images and restart the process. This is why it is important to save the FITS plus have a good naming scheme so you know what the images are)