Quote:
Originally Posted by Paul Haese
I am not sure the sampling rate will be what you expect with short focal length and small pixels. Think of it like this; that with a longer focal length you will get more data for an object over more pixels. I recommend that using larger pixels with a longer focal length. Just like planetary imaging if you don't use a barlow or powermate you can only capture so much detail and can only resample to a certain extent. You will never get the same sort of detail that you would with a longer focal length scope. Just check out any galaxy which is reasonably distant (not the close ones) and go looking for images of that object via google. Then check out the scopes used. It will be uncommon to see short focal length scope capturing detailed images of those galaxies.
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thanks Paul. I know that long fl = hi res is the reigning philosophy, but I think that the new small pixel sensors with high QE open up an alternative way of getting the same results. The same sort of thing starting to happen in the planetary imaging field where pixels are getting smaller. We only use f25 because we have 5.6micron pixels. the next generation of sensors with around about 3 micron pixels will only require f15 or so to get the same pixel scale and if we get to 2 microns, you will not need a Barlow at all. It is the same in DSO imaging. That is not to say that the new approach is any better in principle, just that it opens up an alternative way of doing things. At least that is the way I understand it. Interesting times. Regards Ray
Quote:
Originally Posted by Peter.M
I have been thinking about this recently, I too will be looking at a galaxy rig later in the year and I wonder if the reason these newtonians do so well is they can get away with a small (relatively) obstruction. Punching some numbers into newt I can make a 8 inch f8 newt with a 9% obstruction (by area) with a 100% zone large enough for my kaf8300 to be 200mm outside the tube, plenty of room for an AO unit. Not sure if I would need a corrector at f8 though. I always wonder if this would be better than a rc8.
If I dont bother with the extra room for an AO unit, and put in 55mm for camera instead of the 200mm I can get away with a 50mm secondary which is a 6% by area obstruction.
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Hi Peter. FWIW, my understanding is that the resolution of a 200mm scope is so much better than the average atmosphere (maybe 2 arc sec or worse), that things like obstruction have little effect on DSO imaging results - the atmosphere dominates to the extent that even systems like the Riccardi Honders work OK with extreme obstruction. I haven't done enough work to be sure of this yet, but I think that the only thing that can make a mess of things is bad SA - but even that can be acceptable as is shown by the number of successful systems using the MPCC, which is reported to probably introduce about 0.7 wave of SA.
Suggest that a 200 f8 system with the 8300 might be slight overkill, giving about 0.7 arcsec per pixel. f6 might be better, giving >0.9 arc sec per pix and nearly doubling the sensitivity in the process. would be a nice system at either focal ratio.
regard Ray