Telescope Reviews: Mirror Choice for 14-16" Dob Project

FWIW ...

You certainly earn high marks for effort and homework. But it seems to me that you are being too much of a spreadsheet astronomer here, and trying to buy it "by the pound". Consider that producing a fine optic is almost as much art as science. It is not a commodity!

The thrill and satisfaction of quality is remembered long after the sting of high price. If you intend this to be a "lifetime" telescope take the pain now. There are certain opticians who maintain strong followings over time. The Fine Parabola club is not exclusive - you could grind a "0.95 Strehl" (whatever that is) mirror all by yourself. But if you prefer to buy vs. grind, I would stick with the more regarded names: Zambuto, Royce, Lockwood, Cowan. Forget the price for now. Aperture, length, and availability is what you need.

You could also watch the classifieds for older mirrors from other "regional" or "past" Masters of the Art - Alika Herring, Dick Wessling, Dan Joyce (and perhaps a dozen others whose names slip my mind at this moment). Research pays, and you seem inclined to it.

In terms of focal ratio we all have our preferences for different reasons. Personally, I have always favored the longer focal ratios at the cost of aperture. The reason is that a sphere is a very natural shape whereas the parabola is not and long mirrors are closer to spheres than shorter ones. The optician has to work (or force) the mirror to this shape, meaning that the overall smoothness probably will be less than the sphere. There is a measurement for this called Relative Transverse Error (RTA). It is a measure of smoothness of a surface (and the primary measure that Carl Zambuto uses). If you get a chance, check the back issues of Sky & Telescope circa February 1976 for more information on this (also known as the Milles-LaCroix measurement). In the words of my favorite visual observer (the late) Walter Scott Houston "Light that does not go into the image goes elsewhere to brighten the field. The result is a washed out image, and near the limit of visibility it may mean difference between seeing or not seeing the subject." - Deep Sky Wonders, January 1966. This is the essence of that elusive term "contrast".

This does not necessarily mean that a fast mirror can not perform as a slower one - a great optician can still get a great result on a fast mirror, it just takes more time (and money). You could probably do it yourself if you have a couple of hundred extra hours to spare making the mirror.

The best advice of all time comes (again) from Walter Scott Houston: “I had an even better view of the galaxy with the 12 inch Porter Turrent Telescope at Stellafane in Vermont. Its f/17 design is particularly free of scattered light that would otherwise reduce the contrast between the galaxy’s subtle features. Based on this, I occasionally tell people to design their optimum telescope by first determining the longest tube length they can manage and then the largest mirror that they can afford!” - Deep Sky Wonders, October 1988