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

[Jerry Lodriguss <jerryapml@astropix.com>]

>>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).


Hi Wei-Hao,

I would have thought that you would need to use the same film to model the 
film's reciprocity failure...

Also, wouldn't you need a flat field for each exposure length for the same 
reason?




>>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.


Why not create a synthetic flat field from the original image then?



Jerry


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