Quote:
Originally Posted by Slawomir
Interesting read Greg, thank you for sharing. I am still however uncertain about what all of that means in practice for those of us imaging with less than perfect seeing. Have I understood correctly, that for less than perfect seeing refractors could perform better for the given aperture, but since people own reflectors that are usually significantly larger in aperture, the difference is kind of nulled or could even be reversed?
Yes that's how I read it. But the point of the article was in response to Roland making an earlier comment about how the refractor cuts through the seeing better. Most refractors are 175mm (in this case this was the refractor being discussed) or less and most reflectors are 200mm or larger.
As for star bloating, wouldn't small wells also mean smaller pixels leading to lower sensitivity? Perhaps I simplify things a bit but I think that stretching images makes pixels around bright stars brighter anyway thus possibly leading to bloated stars?
The Sony chips are higher sensitivity than the Kodak ones. Lower sensitivity due to small pixels is dependent on the scope aperture and focal length. If you go below .66 arc secs/pixel you could be said to be oversampling and lowering overall sensitivity by spreading the collected light over too many pixels.
Having said that, I think what you are saying about stars bloating is very true when comparing images acquired with cameras with varying well depths but at the same resolution in arcseconds per pixel.
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Yes as a general rule I notice refractor images tending to have smaller star sizes (with exceptions) than reflectors although I am often amazed at how small stars look in some Newtonian images. Especially the ASA type scopes. I take it that is more from the excellent corrector they use more so than the Newtonian scope.
The bloated stars I am referring to are what Roland is saying here. With a reflector perhaps 50% of the energy is in the outer airy disc not the central bright dot of the star. Now if you have small wells and high QE like the KAF8300 and the Sony ICX694,814, 894 sensors then those outer airy discs are going to overexpose quicker than with a refractor where less of the energy is in the outer airy discs.
Quote:
Originally Posted by Shiraz
hi Greg.
In Australian conditions, the star shape and size for all but smaller refractors is determined almost entirely by the atmospheric seeing. With anything above about 6 inches aperture, you will never get anywhere near the optics diffraction pattern. ie the stars from my 250f4 and your CDK17 will be exactly the same shape and size as those from the AAT in typical conditions. It woud be nice to have seeing of 0.5 arcsec, but I tend to jump up and down if the seeing gets below 2 arcsec and I think that even the best Australian sites rarely get down to 1 arcsec.
I am not altogether sure what you mean by bloat, but if you mean that the stars look bigger, the two main reasons for this are: 1. that the pixels are small (eg the stars from your Trius will be 4x as big as those from your 16803) 2. you may be stretching the image to get to deep features (you see more of the star skirts as you go deeper - ie they look bigger).
If you are managing to saturate the stars excessively, then the simple answer is to expose for shorter subs - not just a bit shorter, but a lot. The optimum sub exposure varies almost with the square of the read noise - at the same aperture and pixel scale, you only need subs about 1/4 as long with your Trius as with your 16803. There is a tendency to overexpose with the new cameras, because that is how we had to do it with the older Kodak ones - those cameras have relatively high read noise, so you need long subs to overcome it - and then of course you need deep wells to handle the extra signal you get from a long sub. If appropriate subs are chosen, the results from both classes of camera should be identical.
Edit: as Slawomir said - I posted before reading his summary
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Yes Ray you may be right there. I need to experiment with exposure length more. I am now settled on just using the 2 scopes - the AP RHA and the CDK with or without the .66X reducer. I see little difference between a 5 minute sub and a 10 minute sub (usually tighter stars with 5 minute subs due to less tracking errors) in terms of brightness, noise and detail. A bit more in the 10 min. I have been using 5 minute subs with the RHA and 10-20 minute subs for narrowband on the RHA (usually 10).
For example I imaged Corona Australis 5 minute subs and I noticed the main stars were overexposed. 5 minutes. Wow.
I am also thinking maybe 2 sets of subs like you do for M42. One for the bright areas and one for the general field and combine them in layers using Photoshop.
On the CDK with reducer 5 minute subs are usual and sometimes 10minute ones for fainter objects. That at 17 inches aperture and F4.4 with 77% QE so its pretty fast.
Greg.