Re: [APML] RegiStar question

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

Hi Phil,

If the original images have very low contrast, I would
suggest to strech it a bit in PhotoShop first.

Stacking is to increase the S/N by averaging images.
However, because the pixel values are digitized, there
is always some "digital noise."  For example, if pixel
values can only be either 1 or 2, you never get 1.5 no
matter how you average these pixel values.  In other
words, there is always a +/-1 error (noise) associated 
with digitized pixel values.

This is especially a problem if you are working with
8bit/channel modes.  There are only 256 digitized 
levels. If the image contrast is low (<<256), no 
matter how you stack images, your S/N will never be
high than a few.  Stacking will only give you dead 
flat grey, but not fine details.

To avoid this, one way is to increase the contrast 
before stacking.  Another way is to use 16 bit/channel
mode.  16 bit/channel will give you 65536 digitized
levels.  The +/-1 digital noise will be much smaller
comparing to the full 65536 dynamic range.

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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