Digital APO (C.A. Mask Technique, By: Tony Licata)
C.A. or chromatic aberration, caused by non-APO, refractive optics, can be subtracted from astro images using image processing software. C.A. shows up as Blue halos around star spots. In this example I’ll be using PhotoShop to create a mask that can be used to fix the image.
Here’s how. (Description for advanced users.)
1)Create separate B&W images of the red and blue channels.
2)Blur them slightly, and subtract the red file from the blue file, creating a masking pattern for the C.A.
3)Then subtract 100% of this pattern from the blue channel of the original image.
4)Add about 10 to 15% of this pattern to the green channel.
If you are not familiar with PhotoShop, here is a detailed description.
1)With the rectangular marquee, drag out a selection box around the entire image.
2)From the “Channels” display, under “Windows”, click on the word “Blue” to display only the blue channel.
3)Click “Edit”/Copy” then “File”/”New”, and Click “OK” to display a blank image.
4)Click “Edit”/ “Paste”. You should now see a copy of the blue channel.
5)Click the “Layers” tab to the left of “Channels”.
6)Click the arrow button in the upper right corner of the Layers window, and select “Flatten Image”.
7)Select “Filter”/”Blur”/ “Gaussian Blur” and use the slider to perform a mild blur to the image, select “OK”.
8)Click back on your original image and repeat steps 2 through 7, only this time use the “Red” channel for step 2.
At this point you should have two images you’ve created called “Untitled-1” (Blue), and “Untitled-2” (Red). You will now subtract “Untitled-2” from “Untitled”-1 as follows.
9)Select “Untitled-1” and click “Image”/”Apply Image”. While in the dialog box select “Untitled-2” as the Source, with blending set to “Subtract” and click ”OK”. (The image should now look like a bunch of doughnuts for stars.)
10)Now click on the original tri-color image, and display all the channels, but select only the bluechannel. This is done by clicking “Blue”, as before, then check the box to the left of “RGB”, to display the others.
11)Click “Image”/”Apply Image”. Use “Untitled 1” as the “Source”, (which is now your pattern). With blending set to subtract, click “OK”.
12)Select the green channel only, and display RGB by checking the box as in step 10. Then click, “Image”/”Apply Image”. Use “Untitled 1” as the “Source”, with blending set to “Add”. This time set the opacity to “15%”. (Default is 100%).
That is All! Ok it seems like a lot of steps but with a little practice it can be done in less than 2 minutes! For that you can save thousands, by not buying that “Super APO” you thought you needed.