Surprises

I need to be able to speak knowledgably about the history of telescope design in a week or two, so I brushed up on the details of Gregorian reflectors and I am glad that I did.  They use a parabolic primary mirror and an ellipsoid concave secondary mirror on the far side of the primary's focal point, reflecting the image back through a hole in the primary mirror.

This actually predates Newton's design.  I was so enamored of it at 13 that what turned out to be my current 3" f/4.5 Newtonian started out as two 3" and two 1" Pyrex blanks.  It is always amazing the fantasies of energy and ability that 13-year-olds have.  

Note that along with figuring the paraboloid shape of the primary, you need to cut a hole through the primary (before you start grinding) and figure the ellipsoidal secondary.  The paraboloid process is well understood and enjoys the benefit of the 19th century mathematician Foucault's development of the Foucault tester.  How to figure the ellipsoid would likely have turned into an exercise in frustration, at 13.

The surprise was when I searched for Gregorian telescopes for sale.  While some large professional telescopes use Gregorian design, when I went searching for ones for sale, most of what I found were 18th century antiques, a few identified as "decorative."  This is unsurprising.  Until after the Civil War, reflecting telescope mirrors were speculum, a copper-tin alloy that required frequent repolishing.  An American father and son, frustrated by this need for repolishing, developed the current system of silvering glass.  (Of course, now we use aluminum.)