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Old 03-08-2011, 01:38 PM
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troypiggo (Troy)
Bust Duster

troypiggo is offline
Join Date: Nov 2008
Location: Brisbane, Australia
Posts: 4,837
FITS, bit depths, signed/unsigned, let's talk

Trying to get my head around bit depths and flicking my FITS files between different software. I have a basic understanding of what it all means, I know the higher bit depth, the better for stretching and maintaining data. I understand there's no point converting an 8 bit image to 16 bit because you don't gain anything, and I understand that dropping a 16 bit to 8 bit could result in data loss if you try to stretch afterwards etc.

My understanding is that my CCD camera (QSI583, KAF-8300 sensor) captures 16 bit data, and I assume that's unsigned (0-65535). So that's what my individual subs will be.

Different software may handle FITS files with different bit depths internally, I think MaxIm and PixInsight are 32 bit (unsigned?). Apparently once you stack your 16 bit images, you can increase the bit depth, presumably because you now have much higher signal to noise ratio?

It seems that the documentation in both MaxIm and PixInsight recommend once stacked you could/should save as 32 bit IEEE floating point FITS, particularly for big stretches etc.

Then for output to, say, Photoshop, you'd save as 16 bit (signed/unsigned?) TIFF because that's the maximum it can handle. And after that, saving for web it'd go to 8 bit JPG for example.

So, am I on the money above? What bit depths do you operate at? Do you find 16 is enough, or is 32 worth it? 32 bit signed, unsigned integer, or IEEE floating point? Benefits of one over the other? I notice PI does 64 bit IEEE as well, anyone go that high?
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