Re: [APML] stacking & adding film images

[Wei-Hao Wang <wang@IfA.Hawaii.Edu>]

Hi Tony,

I think Carlos pretty much answered your question.  Below are my own comments.

Image formats such as TIFF, JPEG, or PSD are not designed for astronomical 
images.  They have very limited dynamical ranges: 256:1 (8bit/channel) or 
65536:1 (16bit/channel).  We can avoid saturation by averaging images instead of 
adding.  However, because of the rounding error Carlos mentioned, we still lose 
fine details that have brightness variation smaller than +/-1 after images are 
averaged.  This is what "limited DR" means.  We either lose the bright part due 
to saturation or lose the faint part due to rounding error.

If you want a much larger DR for your CCD images, FITS is the file format you 
should use.  With this format, you can almost do whatever you like (adding, 
averaging...) without saturation or losing details.  This file format allows 
double-precision floating points, but not just integers smaller than 65535.  
After you process your CCD images in FITS format, you can use some nonlinear 
function (log, or square root) to compress the dynamical range and then 
transform the images into 16bit TIFFs.   

I'd never used any amateur softwares for CCD images.  But I assume at least some 
of them can handle FITS format well.

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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