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
file.
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 |
