Even amateur planetary observers and photographers need big aperture telescopes because when it comes to resolution, size does matter. A larger scope will resolve smaller planetary details. Every amateur astronomer also functions on a budget. Apochromatic refractors larger than 8” are almost impossible to fabricate because the telescope industry depends on camera companies like Canon and Sigma to supply special dispersion glass blanks to grind and 8” is the upper limit for even their exotic telephoto lenses. f/5 also appears to be the fastest limit for good color correction and with an 8” objective would yield a 1 meter long optical tube. So from a cost and portability perspective, large telescopes have to be reflecting telescopes.
The traditional favourite for many decades has been the Celestron 14” Schmidt Cassegrain, compact enough to travel with and to be lifted and mounted by a single person. Along with a convenient eyepiece location there are few alternate designs that also offer serious aperture.
The only issue with the cassegrain design is the central obstruction of the secondary mirror. This causes energy to be diverted to the first diffraction ring and reduces image contrast which can result in real loss of resolution.
As a result, astronomers have been experimenting with designs that tilt the primary out of the line of sight of the secondary mirror so that there is no longer any obstruction. This unfortunately introduces coma and astigmatism into the image which can only be corrected with custom ground refractive elements making construction likely beyond the ability of most amateur astronomers.
To better locate the eyepiece, the optical path is folded with a optical flat at some point before the corrective optics. Axial astigmatism can be corrected by changing the tilt of the lenses. The front lens is tilted counterclockwise and the rear lens clockwise and the production of axial lateral color can be minimized by slightly decentering the rear lens. Although almost any focal length can be used, 500 mm is better than 200 mm because the shorter focal length lenses require greater tilting and more separation and more axial color production. The color produced is less than the difference between achromatic and apochromatic refractors so may be perfectly tolerable. But you can eliminate it all together by using lenses of unequal radii and a slower Newtonian mirror. It’s just easier to buy a matching pair of PCX and PCV lenses premade, but you may need to regrind one of them to get superior color correction.
Since most commercially available lenses are rarely larger than 2″ in diameter, this limits the primary mirror to no larger than 10″ diameter to prevent vignetting.
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