SUNSET: 
Why do Schmidt-Cassegrain telescopes have focal ratios of f/10?
Why do most if not all of the Schmidt-Cassegrain telescope I see advertised have focal ratios of f/10? During the past 20 years or so, amateurs have favored shorter tubes for both refractors and reflectors, even if it meant paying for more costly optics. Portability is the chief motivation -…
Can you adjust a f/7.7 spherical mirror to act like a f/8.0 parabolidal mirror by racking the focus farther out?
My 114-millimeter (4.5-inch) Newtonian reflector came with an f/7.7 spherical mirror. I can purchase a 114-mm f/8.0 paraboloidal mirror. Can I simply adjust for the 34 mm of added focal length by racking the focuser farther out, or should I extend the main tube? Either way, is the upgrade worth…
How can an astrophoto shot through a refractor have diffraction spikes on bright stars?
In your Gallery department (S&TSeptember 2004, page 144), you had a nice image of the star Pollux showing diffraction spikes. That suggests it was taken with a Newtonian reflector, but the accompanying note says a Takahashi refractor was used. Where did the spikes come from? Many imaging enthusiasts like the…
What's the difference between two Plössl eyepieces?
I bought exactly the same Plössl eyepiece that a buddy has, but mine doesn’t work as well as his. Did I get a lemon? That’s possible, but very unlikely. Our guess is that your telescope has a short focal ratio, perhaps f/5 or less, while your friend may have an…
Is software available to analyze optical designs of telescopes?
Does software exist, preferably downloadable from the Internet, that I can use to analyze original optical designs of telescopes? Yes. The easiest way to find what’s out there is with a fine-tuned Google search. Right now, if you enter “optical design software” as the search terms, you get a few…
How can eyepieces offer a telescope's widest true field?
Some eyepieces are being advertised as offering the widest possible true field in any telescope. How is this determined? The amount of sky shown by an eyepiece is governed by the diameter of its field stop, the ring that defines the edge of the visible field. On a given telescope,…
