Astronomy Benalla
Reports - 2011
Astronomy Benalla  General Meeting Presentations - Wednesday 16th March 2011 Popular Visual Telescope Designs.      Presenter: Patrick Watson The discussion was generally limited to the more popular telescopes designed for visual use. These were grouped as Refractors which use lenses, and Reflectors which use mirrors. There followed a brief explanation of refraction and also certain image aberrations which, for the coverage of the presentation, were limited to: chromatic aberrations which appear only when a system contains refractive elements, and spherical aberration which is a monochromatic aberration and may occur in both refracting and reflecting telescopes. Refractors. A brief description was given of the design differences between an achromat  and  an apochromat, and included an overview of the photo sensitive receptors of the human eye and its response to various colours. Designs were shown that corrected for (suppressed) colour fringes noticeable in spherical lenses, by the use of two (achromat) and three (apochromat) lenses of various shapes and materials. The refractor is also referred to as a dioptric. Reflectors. These included the Newtonian, Cassegrain, Richey-Chrétien and, for some variety, two interesting obstruction-free designs, the Herschelian and Schiefspiegler – rarely seen in most amateur groups. A brief return to school days for a review of conic sections showed how varying the curvature of mirrors could correct certain image aberrations. The reflector is also referred to as a catoptric. The session concluded with a look at compound telescope designs, that is, those that use both refracting and reflecting elements, referred to as catadioptrics.   These included: Schmidt-Cassegrain.    A Cassegrain with a Schmidt corrector. Maksutov-Cassegrain.  A Cassegrain with a Maksutov corrector. Schmidt-Newtonian.     A Newtonian with a Schmidt corrector. Maksutov-Newtonian.   A Newtonian with a Maksutov corrector.
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