Quote:
Originally Posted by Satchmo
Alpal : The modification to the airy pattern caused by central obstruction or spherical aberration are not quite the same although parallel points can be found at similar Strehl ratios . When you combine spherical aberration and central obstruction the effects are more complex and I would think under different kinds of seeing , it is not so easy to model the outcome. Conversations I have had with Peter Read from SDM who often compares telescopes of known quality together visually , is that better optics seem to fair much better in some kinds of slower period inclement seeing than marginal ones- which in the immediate instance seems counter intuitive. I think it just comes from having more headroom for transverse error in a geometric analysis
I looked at this with Aberrator a while back and I dont think it is a simple as just allowing spherical aberration effect to be as large as the central obstruction degradation and assuming it will just mask it. In geometric terms your ray bundle is already 3 times the diameter of the airy disc with an unobstructed system.
I think getting the best images possible is achieved by trying to keep all aberrations to a minimum . You are going to have seeing , optical mounting problems , mis-collimation and and local thermal issues all in the `wobbly stack ' from star to image plane, even assuming perfect optics . But I don't think letting optical aberrations loose to the theoretical limits of your obstruction will always yield maximum quality - I think it is prudent to try and build some headroom into all aspects of the telescope.
Professional instruments with large obstructions and sites with no better than 1 or 0.5 arc second could probably specify their optics to a number of waves if they were to let aberrations slide to equal the best case blur spot for a given seeing condition , but they generally try to achieve better than 1/20 wave because they can and it is a form of future proofing.
Most of the best imaging around is done by amateurs who have paid attention to every aspect of their setup to try and keep their signal to noise as good as possible . They are generally not taken with cheap optics , cheap mounts etc . Obviously chasing or paying for 1/100 wave optics would be a hollow pursuit , but I think good headroom means optics of 1/8 wave front or better where the geometrical ray bundle meets at about half the diameter of the theretical airy disc. It would be hard to pinpoint exactly how all the improvements are made , but I'm guessing its the some total of a lot of subtle gains in every aspect of the instrument .The anecdotal evidence I have seen from imagers using known quality optics suggest to me that quality it is not an empty thing to strive for in the pursuit of fine images. Headroom in optical quality is important too.
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What is the program that allows you to compare the obstruction
with the theoretical Dawes limit or Rayleigh criterion?