[APML] a short note on film flat fielding

[Wei-Hao Wang <wang@ifa.hawaii.edu>]

ALOHA,

Last month several of us mentioned doing flat fielding on film images.  
I tried it and I think it is successful.  What I did is following.

1. I used the same telescope and camera setup (as taking astro images)
   to take a few exposures on daytime sky.  Each exposure was made on
   different directions so the average of them should not contain too
   much sky gradient.  Film used for flats is different from the film 
   for astro images (I don't think the film matters).
   
2. The sky flats were scanned and averaged.  RGB channels were merged 
   into a single greyscale image to further increase the S/N.  This 
   forms a "flat field model."
   
3. On a real astro image, I manually selected several background areas 
   (where there are no stars and nebulas) and measure their pixel 
   values.  At exactly the same positions on the flat field model, I 
   measured the pixel values.  By comparing the values from the real 
   image and from the flat field model, I can "twist" (in a nonlinear
   fashion) the flat field model so that it has the same brightness 
   distribution as the background of the real image.  
   
4. Divide the image with the twisted flat field model.  Then the image
   should have a uniform background.  3 and 4 were done independently
   in each RGB channel, though using the same greyscale flat field model.
  
I'm applying this technique on the images I took last week.  So far so
good.  It is slightly faster than my old method and does an excellent
job on correcting the non-axisymmetric component of the vignetting 
pattern.  It also produces less artifact.  

Probably after two days, I can show the results of my Orion mosaic
(now with 12 frames) using this new technique.  If I have time next 
month, I will write a more detailed report with some examples on this 
technique.

Cheers,

Wei-Hao


______________________________________________________________________
Wei-Hao Wang  :)

Institute for Astronomy at University of Hawaii

Address:                       Phone: 808-956-9867                  
2680 Woodlawn Drive            Personal Website:
Honolulu, HI 96822             http://www.ifa.hawaii.edu/~wang
______________________________________________________________________



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