Did you mean Mark? You're arguing my case here.
Greg.
Quote:
Originally Posted by Wavytone
Greg nice try but you've missed two subtle points.
Firstly, the reason smaller aperture scopes often cope better with turbulent seeing is that the dimensions of the microthermals are typically of the order of 15-40 cm. In average seeing a 10cm scope will show discernible diffraction rings and an Airy disk in conditions where a 20 cm or larger scope will just show a fuzzy blob. The atmospheric effects are easily seen if you defocus on a bright star. In daytime I've flown through this kind of turbulence at heights up to 3000 metres on my paraglider, you can feel the turbulence on your face and it makes for a bumpy ride, and it's also one of the reasons no-one likes to fly paragliders over urban areas.
The seeing on most nights at Observatory Hill is so poor that it does imply a large aperture scope is going to be unsatisfactory. A site survey to assess this quantitatively would be constructive, before making any decisions to commit to any telescope.
Secondly, the reason refractors perform so well is that having no central obstruction at all and with good optics, they will deliver a textbook diffraction pattern with the maximum energy concentrated in the central Airy disk (making it brighter) and with comparatively less energy spread into the diffraction rings. Any scope with an obstructed light path (Newtonians, cassegrains etc) has a less intense central Airy Disk, and more light spreading into the rings. The effect is quite noticeable on a side-by-side comparison of two scopes working at the same magnification, and in particular this has a lot to do with why a smaller refractor often matches a rather larger reflector What's more it is dependent on transmitted power, which is image intensity squared, so what you think of as a relatively small obstruction has a more significant effect on the perceived intensity of the central peak than you evidently think - and also says lot about why a smaller refractor can indeed match a larger aperture reflector. There is plenty of optical theory to back this up, FWIW. For example, while an 8" SCT may provide a significantly brighter image on an extended object, most 8" SCT's cannot out-resolve a good 6" APO on double stars or the bright planets.
The only designs that will outperform refractors are the all-reflecting unobstructed designs such as the schiefspieglers, with the advantage of perfect achromatism. However they're not appropriate in this application for many reasons.
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