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Old 28-06-2015, 08:07 PM
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Atmos (Colin)
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Join Date: Aug 2011
Location: Melbourne
Posts: 6,291
As a picture is worth a thousand words, I thought I would get out the trusty white board and draw a picture as to what is actually happening. When under-sampling a star could take up an entire pixel irrespective of the rest of the background sky that is also falling onto that single pixel. This is the very centre square. The other squares around that are that same pixel as it is being dithered.
There are two other pixels to the NW and SE that also interact with that star, so these will also have the star taking up the entire pixel, albeit with differing amounts of flux to the centre one due to containing more sky background.
To the further NW, SW and NE the pixels don't contain that star so they put boundaries on the actual size of that.

Dithering/drizzling helps sharpen up and add "resolution" to an image by reducing some of the incorrect aspects of the image, this is the apparent size of everything within any single frame (this is the definition of over-sampling).

DISCLAMER:

I'll be honest, I never even knew that something like this worked until yesterday when it was brought up. It was something that I was thinking about testing when I had some clear nights with my new "old" CCD that I got a few weeks back. Weather hasn't really even allowed me First Light yet!! Terrible!
Anyway, 7.4 micron pixels on a 900mm FL give about 1.7 arcsec/pixel which is over-sampling a bit more than I would personally like.
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