Quote:
Originally Posted by gregbradley
Great article. Thanks for posting Rick and thanks to Richard for the detailed write up.
The airy disc rings showing up on some brighter stars is kind of what I thought was going on. So when you have a higher QE cameras those outer rings of the airy disc can show up giving wider stars. The detection limit concept of wider stars when they are fully bright to expose the whole airy disc and get it above the read noise is quite a visible phenomenom when processing images from some cameras.
Also there is another question mark over how well antiblooming works on all chips.
So from this we get what? - with high QE small well low read noise sensors it makes more sense to do shorter subs and keep the airy disc outer rings below the detection limit to get tighter stars. Use longer exposures with sensors with deeper wells and higher read noise.
Also fast scopes get tighter stars. An interesting one. ASA Newts always seemed to me to have the smallest stars. Newts in general seem to have tighter stars than other systems. I guess they are often fast compared to other scopes.
Greg.
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it's aperture that determines how tight the stars are Greg, not FNo - FNo is important though if you have fixed pixel size and focal length (then a faster scope will give you a bigger aperture for the same focal length). The FSQ106 for example produces relatively fat bright stars, even though it is fairly fast - because it has a small aperture.
When you consider the effects of seeing, the Airy disk rings are filled in and you get basically a smoothly varying profile with most apertures. You also get a fatter core region, where seeing determines what resolution you get, not diffraction.
The bottom end star skirt size will depend on the degree of stretching - you will get bigger stars with more nearly saturated cores if you stretch more. If you need to expose deep and stretch hard to get faint stuff, you will get a lower detection limit and larger stars again. You get the same result regardless of QE or well depth, provided you choose appropriate sub lengths. However, if your small wells are from small pixels, the stars (and everything else) will be larger because you will have more of the smaller pixels under each one.