Image Processing in Photoshop: Advanced Blending, Star Size Reduction
Copyright 2005-2014, Dick Locke.  All Rights Reserved.  Contact and Image Use Information 

Andromeda Galaxy
: Believe me, they don't start out looking this good!

This page serves as my reference for Layer Blending and other Photoshop techniques to remember.  I'm always looking for better ones, contact me if you have suggestions or questions.  My normal Astronomy Image Processing Workflow is here.

Mosaic Seam Blending - My Reference for Blending Prime Focus into Wide-Field Images

This is the approach I take when blending in a "small" prime focus image into a larger, wide-field image.

  1. Make the wide-field image bigger by 150% or so (optional, depending on image scales).
  2. Get very flat-field versions of all your images.   You can blow out your prime focus stars somewhat as they will be much smaller than your wide-field stars when blended.  You may want to take steps to reduce star size (note references below) in your wide-field image, but don't do it too much or Registar will have problems.
  3. Registar or other alignment software is critical.  Registar easily handles blending wide-field astronomy images with my prime focus shots. 
  4. If working with very large files, work on two images at a time only.
  5. Register and align images in Registar.
  6. Calibrate if the smaller image to the larger image (this is a Registar function to match colors between the images).  You definitely want to calibrate if padding a union of the images; it doesn't matter if you only care about the intersection.
  7. Crop/Pad so images are same size.  The small image will be padded with black.  Save the resized and calibrated prime focus image.
  8. Open Aligned & Calibrated images in Photoshop.
  9. Select all on the smaller image, then copy the image to the clipboard.
  10. Paste the smaller image on top of larger image.  At first, all you can see is what you pasted.
  11. Click layers mask icon at the bottom of the layers palette to add a mask. A white box shows next to the prime focus layer.
  12. Alt-click on mask (white box) to make it active.  The image goes white.
  13. Paste smaller image into mask.  (It should still be on the clipboard.)  A black and white version of the small image is now shown, surrounded by black.
  14. Open another window to see the image manipulation changes.  In CS3 Window->Arrange->New Window for (name).  Arrange the two windows so you can see them.  The new window shows the end result of your image, while the mask window allows you to perform the necessary manipulations.
  15. Go back to mask image window (make it the active window.)
  16. Select black border (color range) in the mask, then select inverse to select the smaller (real) image as a mask. 
  17. Now you begin to manipulate the mask. 
  18. You may want to blur the mask.  The light/bright areas of the mask are now controlling what shows through from the prime focus image.
  19. If you make the mask brighter (e.g., by using curves) more of the prime focus image will be blended.  The white is transparent while grey is less so.  Black areas of the mask don't allow the prime focus image to be shown.
  20. Manipulate the mask to taste, don't worry too much about the seams.
  21. Hide the seams: Use a black brush, 20% opacity, then paint along the edge of the mask to hide the seams.
  22. Make final adjustments to the mask or to the opacity in the layers palette.
  23. Save a layered version in case you want to come back later.
  24. Flatten and save a non-layered version.

Result of above technique: Antares and Rho Ophiuchi Area

References, Star Size Reduction and Layer Masking

Sometimes you want want to reduce the size and prominence of stars in astronomy images.  The ImagesPlus "Star Size and Halo Reduction" feature worked pretty well on M16.  Some additional links with other techniques are below.  There doesn't seem to be a magic formula for star size reduction that works well for different kinds of images, you have to experiment.

Jerry Lodriguss' Layer Mask TechniqueMore Photoshop Tricks including shrinking stars
Selecting StarsMatt's Star Shaping
PS Action to Remove StarsNoel Carboni's Astronomy PS Actions (has a shrink stars function)
Ken Crawford's Processing Tutorials 


Key Elements of Layer Mask Blending

  1. This is a great technique to blend elements of any two images together where controlling the relative brightness of an an area of the image is the goal (think the core of M42 or other bright nebula, or even star sizes)

  2. Copy an image with a more controlled bright area and paste it on top of the image with a too-bright area

  3. Create a layer mask by clicking on the layer mask icon

  4. Alt-click the layer mask to open up a window for the mask

  5. Paste in one of the images you are working with, recognizing that the white/bright areas allow the top image to come through in the blend, while the dark areas will be ignored

  6. Blur and/or use curves to manipulate the layer mask to smoothly blend the two images together

  7. You can open one window showing the layer mask, and one showing the blended image, for reference.

Paint in a correction layer steps reference:

Color samplers-> select (eyedropper palette) 

Duplicate Layer


Merge Layers


Background Layer to Normal Layer

Double-click background layer

Channel Blending

Channel Blending 2

Color Rules

  1. Magenta between Red and Blue (redish blue) kills green

  2. Cyan btw blue and green kills red

  3. Yellow btw green and red kills blue


6/5/2005: Flat fielding technique (astrophotography)

Ron Wodaski posted to following to one of the news groups describing a way to eliminate vignetting.  I used this technique above.  Jerry Lodrigus has documented another useful technique, but the disadvantage of Jerry's is that you never can get it exactly right throughout the whole frame.  The technique below allows you to do that.  From this page: Orion Area, 300mm

I've documented a simple way to created synthetic flat fields using
Photoshop in my book "The New CCD Astronomy." It works for both film and
CCD images.

The basic idea is to use an image that doesn't have extensive nebulosity in
it, and to apply Photoshop's Dust and Scratches filter with an appropriate
radius that will remove most of the stars in the image. You can use the
Clone Stamping tool to clean up any very large stars or small patches of
nebulosity that remain.

Then apply a strong Gaussian blur (usually in the range of 10-30; depends
on what's left) and you will have a very smooth and very accurate pseudo
flat-field image. The nifty part of this is that it works well for
full-color images. You can apply it in a single step by putting it in a
layer above your image, and setting the bland mode to difference. Adjust
the blending % to somewhere in the range of 80-95 (usually) to get just the
right amount of subtraction.

(DL note here: I'm wondering if the above is a mistake, as I am using more 10-15% blending than 80-95% as noted above.  Please let me know if I'm missing something :-)

This methods works well for light pollution gradients, too.

See my Current Astronomy Image Processing Workflow

My Photoshop "Basics" Page

Astronomy Pictures: Dick Locke's Astrophoto Gateway page....

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Copyright by  Dick Locke.  All Rights Reserved.
Contact and Image Use Information
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