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 Image Processing using

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      Since acquiring my wide field Orion StarShoot Pro V2 color camera I have been plagued by a red doughnut showing up  in my images when I stretch them to get some dim object to show up. I had tried every thing I could think of in Nebulosity and Photoshop to try and eradicate this artifact with less the satisfactory results. Click on the image at right to see the artifact (dim red doughnut in the center)

     In hopes of getting some expert help I contacted the members of the various Yahoo tech groups. As luck would have it, John Reed of the Stark-Labs group has a process that  he uses in Nebulosity which eliminates the problem almost completely. There may be a slight gradient remaining which can be eliminated in Photoshop.

     The trick to the process is to  de-Bayer the master flat ( convert it to a color image) and then save the RGB components of that  flat as separate files giving you  a redFlat.fit, GreenFlat.fit and a blueFlat.fit  These flats will be applied to  the de-Bayered image subs once they have been separated into their RGB component files.

John's procedure using Nebulosity follows:

1. Remove Bias from all raw light frames
2. Remove "Bad Pixels" (or subtract darks if you prefer) from each raw.
3. De-Bayer the pre-processed raws to RGB fits.
4. Use the "Batch Conversion" to convert them to separate colors.
5. Previous to all this I have made a master flat and De-Bayered it, then
converted it to to separate colors.
6. Using the "Batch Multiple Sets" , tell the program to apply the green flat
to all the green lights, red to red and blue to blue.
7. Stack each pre-processed color set as you would with normal RGB lights (a
bunch of files at this point).
8. Save the results of each stack: green.fit, red.fit, blue.fit.
9 Align green.fit, red.fit, blue.fit with the "save each file" checked.
10. Under "Image" use "LRGB Color Synthesis" to combine them back to an RGB fit
11. Process with DDP etc. as normal.

I have found that the green center is almost completely gone unless I really
stretch too far. Another trick I have learned is when you do your final work on
Photoshop, duplicate the main layer and make the top blending mode "Luminosity".
Use gradient Xterminator to flatten the bottom layer, leaving the top unchanged.
When I say flatten I would use "Coarse" and "Strong" for your settings. This
will remove any color gradient that is left. Finally flatten the layers. Noel
Carboni has a neat Photoshop Action that more or less does the same thing.

 Thanks to John for sharing this information. Now I can get some decent images.

The same image after using the above process. Click to enlarge


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