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

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:

• 1.  Use adjustment layer for extreme corrections

• 2.  X to set foreground color to black

• 3.  Make sure curves layer is clicked

• 4.  Alt-Backspace to fill Adjustment layer's mask with black (image returns to original look, black mask hides correction)

• 5.  X to set foreground color to white

• 6.  Brush tool

• 7.  Paint in correction

• 8.  (X to make black foreground color to erase)

• 9.  Can change layer mode if needed

Color samplers-> select (eyedropper palette) 

Channel Blending

• Get a copy of the channel you want:

•     Display the channel

•     Select All

•     Copy

•     New Document  (CTRL-N)

•     Paste

•     Drop down a layer (CTRL-E)

• Display only channel to change (channels palette or CTRL1 for Cyan or Red, e.g.)

•     Kbd shortcut ~ to show composite color while leaving pre. Channel active.  CTRL~ to access other color.

• Image-> Apply Image to mix in any other channel

• Blending modes Normal, Darken (disallow blend if would lighten image)

• Change opacity until you're happy.

Channel Blending 2

• Duplicate Layer (in RGB)

• Apply channel to RGB composite

• Luminosity blending mode, in Layers palette

• Opacity, adjust to taste if too harsh

• Normal mode, -> change to Luminosity ->or Darken

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

See my Current Astronomy Image Processing Workflow

My Photoshop "Basics" Page

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