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Old 12-09-2010, 03:04 PM
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rat156
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Join Date: Aug 2005
Location: Melbourne
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Hi Paul,

I'm imaging at 0.76"/pixel, also seriously oversampled. FL of 1854mm and pixel size of 6.8um. I also suffer from somewhat bloated stars, so I'd like to get to the bottom of this.

I still don't understand the link between sampling and bloat. Just so that we make sure we're on the same page, by bloat I mean inflated stars, not blooming, which I mentioned because you mentioned well depth. This particularly affects the brighter stars.

So far you've just said that oversampling results in bloat, I'm struggling to understand why. I always though it to be a direct consequence of imaging at long FL, rather than image resolution. For mine, the causes of bloat are there for everyone, whether they are doing widefield or narrow, it's just much more noticeable in the narrow filed image because of image scale as displayed on screen. AFAIK the star's apparent size depends on the brightness, mainly because of the photon flux. Take a perfect scope and perfect seeing and the star's light will fall on one pixel, it's a point source. The fact that it doesn't is due to many things, chiefly our atmosphere, tracking errors and the diffraction limit of the scope. Once the image is produced it is faithfully recorded by the CCD. Now, as the star's light is wobbling around on the CCD, each pixel will record the number of photons that hit it, assuming a time based probability function spreading from the theoretical point source, the brighter a star (higher number of photons/second) the more highly populated the outer regions of the probability function, or for us, bigger stars.

So I can see that high resolution imaging can result in bloating of stars, but due to the FL of the imaging scope more than the pixel size of the CCD, unless you're seriously undersampled. Long FL leads to the familiar problems of battling seeing, mount tracking etc. all of which get significantly harder the longer the FL.

That's my take on it anyway.

Cheers
Stuart
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