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Old 12-09-2010, 11:19 AM
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rat156
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Join Date: Aug 2005
Location: Melbourne
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OK, but why does sampling result in bloat?

The way I see it, or more correctly my camera sees it, is that if a star is 4 pixels FWHM and I change the camera so the pixels are twice as big, the the star will be 2 pixels FWHM.

Now for my system the 4 equates to about 3" FWHM, which for a 5-10 minute exposure from Melbourne is about as good as it gets, seeingwise. Yeah, sure we get some nights with 2" seeing, but over a longish exposure and 1-2" PEC, you'll get around 3" FWHM.

So if I swap my camera out for one with bigger pixels, the arc-second per pixel goes up (to around 1.5), but the star will still be 3" FWHM, unless the pixels are so big so that the star image only falls on one pixel, this happens with widefield imaging and results in square stars.

Have a look at the stars in the images below, both are from the same imaging run on the same night. The Ha is unbinned, the red is binned 2x2. Are the star profiles that much different? Once you blow them up so you can see each pixel, you can see that the oversampled image contains more profile information for each star, but overall the stars are the same size.

Cheers
Stuart
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