Re: [APML] a short note on film flat fielding

[Dean Jacobsen <deanjacobsen@earthlink.net>]

Wei-Hao,

That looks like a very interesting solution to a problem that I couldn't 
solve myself a few years ago.

Please keep us updated on your progress.

Dean Jacobsen
www.astrophoto.net

At 04:54 PM 2/23/2004 -1000, you wrote:
>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
>______________________________________________________________________
>
>
>
>_______________________________________________
>Astro-Photo mailing list
>Astro-Photo@seds.org
>http://seds.org/mailman/listinfo/astro-photo


_______________________________________________
Astro-Photo mailing list
Astro-Photo@seds.org
http://seds.org/mailman/listinfo/astro-photo