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Old 12-12-2011, 07:11 AM
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gregbradley
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Interesting thread. I have seen that effect more pronounced with sensors with a smaller well depth than with cameras with a larger well depth.

I assumed it was a case of oversaturated pixels dumping nearby but I think it is probably more the fact that there is more scatter with brighter stars and the light accumulating in nearby pixels shows that scatter more when its a bright star. Accordingly if you use shorter exposures it will have the effect of tightening up the stars but also at the expense of losing some detail in the faint areas of your image.

CCD chips have a cover slip standardly (Kodak). These can create small halos and reflections. That would add to it. Additionally I don't know how good the antireflection coatings are on these cover slips. On top of that most astro chips have a microlens layer that focuses the light to the side of the pixel that has the higher QE. So you've got a few things going on there.

Its a similar phenomena with blue filters versus other colours. Blue is often more bloated. I have read that this is because blue is more scattered. Seems to work as a theory.

Reading that article you linked it mentions anitblooming is less efficient on one side of the pixel than the other and that various antiblooming methods have different characteristics and efficiencies. The implication there is that perhaps they don't work 100% in every scenario so perhaps there still is some overflow to nearby pixels. It does not clarify that point too much.

Also I wonder if this comes into it:

http://www.paquettefamily.ca/astro/star_study/

In that colours are not registered with the same FWHM usually. Blue being worst. Sensors are often listed with QE for each colour channel and often are not equal in QE. It can vary by 10-20% more or less QE. Could that effect of bloated stars also be they were bright and bluish stars?
Greg.

Last edited by gregbradley; 12-12-2011 at 07:34 AM.
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