RASC Hamilton Pentax Astro Tracer

[Mark Kaye]

On 13-11-30 8:28, David Galbraith wrote:
> There's compression and then there's compression. "lossless 
> compression" changes file size without altering pixel-by-pixel raster 
> data. Lossy compression (like JPG) alters the bitmap data to save file 
> space. Some cameras can store data straight to TIFF files. No loss.

    I do not declare to be an expert on digital imaging.  I can only 
guess that the way a dark raw is compressed and data frames are 
compressed is enough different that an external dark subtraction is not 
quite as good as an internal.  I am only the messenger here.  I concede 
that doing your own dark subtraction is attractive given that we have 
limited actual dark sky time and that doing internal darks cuts one's 
imaging time in half.
    It could also be possibly because the dark frame done in the camera 
is done at pretty much exactly the same conditions, while a dark library 
is slightly different.  As originally stated, both Terry and Alan 
conclude that in camera dark subtraction is marginally better.  They do 
have the experience in this sort of thing to be taken seriously.  But 
imaging a moving object like a comet, where you have a very limited 
window for imaging, you are going to be better served by using a dark 
library.

-- 
    Clear Skies!
                MK

http://www3.sympatico.ca/mark.kaye/index.htm

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