Image Processing

Image Processing

This is a brief overview of how I typically stack and process images. I will attempt to update this overview as my workflow adjusts and changes

Stack in Deep Sky Stacker:

• Light Frames
• Calibration Frames:
  • Dark Frames
  • Bias Frames
  • Flat Frames
  • Dark Flat Frames

Basic Outline

• Convert image to 16bit / Crop
• Initial Levels
• Color Balance
• Initial Stretch – light
• Gradient (optional)
• Remove Stars / Create Star Mask (Optional)
  • Adjust color and size of stars
• Levels
• Raw Camera Filter
  • If stars are removed, clarity and texture become options to use
• Noise
  • Should only be done lightly
• Levels / Stretch
  • If you use Astrophotography Tools Actions, Enhance DSO
• Contrast adjustments
• Noise (if needed)
  • Again, very light
• Final Crop (optional)

Process in Photoshop – Note, all new layers created by duplicating the previous layer

• Open Image
• Slight crop and change image to 16bit
  • Use Exposure / Gamma
• Initial Levels (New Layer)
  • Bring shadows to edge of peak, bring highlights in to line
  • Create Threshold Layer –
    • Layer -> Adjustment Layer => Threshold
      • Slider to highlights, use Eyedropper Tool
        • Eyedropper – Select Color Sample Tool with 5 by 5 Average
          • Select two to three spots void of stars (and nebulosity as possible)
          • Select center of bright star
    • Delete Threshold Layer
• Color Balance (New Layer)
  • Image -> Adjustments – > Levels
    • Adjust each level individually to reach balance based on info from Color Sample Tool
      • R G & B should be low (15 – 10) on dark spot and on star try to reach 248 across the board. Lower then 10 is fine, just do not clip the histogram peak
• Stretch (New Layer)
  • Stretch by Levels (RGB at once) or Curves (normal or S-Curve)
    • This is personal preference and I usually try both ways
  • Do not do a full stretch – goal is to get nebulosity visible for star removal
• Gradient (New Layer) – Optional
  • I use GradientXTerminator – Detail = Coarse, Aggressiveness = Low and experiment with checking the Balance background color box
• Starless (New Layer)
  • I use StarXTerminator, you can use Starnet+
    • Starnet+ you will need to flatten image and save as a 16bit Tiff file and run it separate. Undo flatten and open file to add starless image as a new layer on top of existing layers
• Stars Only (New Layer)
  • This layer should be off the gradient layer and sit just below the starless layer
  • Image -> Apply Image
    • Set Layer as “Starless,” set Blending to “Subtract,” set Offset to 2
• Stars Only – Working (New Layer)
  • Deselect “Starless” Layer
  • Duplicate Stars Only Layer, keep below Starless
  • Fix stars
    • Here you have several options – you should just have the stars visible.
    • I use Camera Raw Filter (Filter -> Camera Raw Filter) to lower the saturation in magenta to -100, red to -60 and orange to -40. This is because I use the Optolong L-eNhanced Duo-Band Filter. Other filters or not using a filter may need different options. Experiment!
    • I then use “Make Stars Smaller” from Astronomy Tools Actions, and I typically run it twice
• Move “Stars Only – Working” Layer above “Starless” Layer
  • Set Blending to “Linear Dodge (Add)
  • Reselect “Starless” Layer
    • You should now have a “combined” image again
• Clarity (New Layer) –
  • Should be below “Stars Only – Working” Layer
  • This is where you will begin processing the image to how you like, creating new layers as you go. Remember to make sure your new layers remain below the “Stars Only – Working: Layer
  • Camera RAW Filter
    • Can adjust Texture, Clarity, Dehaze and Contrast
• Levels (New Layer)
  • Adjust Levels to bring highlights forward to the line in the histogram. Do not exceed 200 in the highlight number box
• Raw Camera Filter (New Layer
  • Adjust exposure, contrast, highlights, shadows, whites and blacks. Can adjust vibrance and saturation as well
• Noise (New Layer)
  • Here I use NoiseXterminator at 60%, detail 10
• Noise 2 (New Layer)
  • Here I use Deep Sky Noise Reduction in Astrophotography Tools Actions
• DSO (New Layer)
  • Here I use Enhance DSO Reduce Stars in Astrophotography Tools Actions. Obviously, if stars are removed it will do nothing to the star layer. You can also use levels to boost from the highlights and adjust from the black point
• Boost & Boost Copy (New Layers)
  • Deselect the star layer and create a layer mask using Select -> Color Range -> Highlights. I use 89 Fuzziness and 88 Range. Invert and hit “add mask” in layer panel. On the Mask, move curves to expand the black areas as desired. Use Gaussian Blur at 2.0. Go back to the regular image and move curves down slightly until the desired adjustment is made
  • Merge Boost Copy down into Boost
• Raw Color (New Layer)
  • Adjust color and saturation as desired in Camera Raw Filter)
• Clarity 2 (New Layer)
  • Adjust texture, etc for the final time if desired
• Contrast & Contrast Copy (New Layers)
  • Here I use Local Contrast Enhancement from Astrophotography Tools Action and lower the opacity to 50% or lower in the layers panel. You can use Photoshop’s contrast if you want
  • Merge Contrast Copy down into the Contrast layer
• Noise (New Layer)
  • Final minor noise reduction to reduce noise created by final level boost or stretching
• Final Crop – Optional
• Save a working 16bit TIF copy, save a JPG copy

Notes:

• Keep an eye on the histogram and make sure not to clip into the blacks
  • Clipping into highlights is not as big of a deal but try to avoid it
• Do not over do sharpening or noise reduction – it will create artifacts. Depending on your setup and how the images were taken, not all noise will be able to be removed, nor can all details be sharpened effectively
• Do not over use vibrance and saturation, you will lose detail, especially in nebulosity