Questar telescopes are exquisitely built optical and mechanical gems. The basic design of the scope is little changed from the 1950s, when this particular sample was built. Photo by Gary Seronik
By Gary Seronik
It’s funny how things tend to go in cycles. Having a reputation as an equipment guy, I’m asked about scopes quite often. As it happens, last week I received two e-mails asking essentially the same thing. An old friend had just acquired a 3.5-inch Questar and wanted to know the telescope’s actual focal length. Another acquaintance recently bought his first Schmidt-Cassegrain telescope (SCT) and was curious about the scope’s focal ratio. The answers weren’t as simple as you might expect. To understand why, you have to consider the inner workings of these scopes.
At its heart, a Questar is a high-quality 3.5-inch Maksutov-Cassegrain, or “Mak” for short. Meade’s popular ETX is optically very similar. My friend wrote, “What is the actual focal length of a Questar? I see different numbers everywhere (e.g. 1,280mm, 1,300mm and others) and I’m a little confused.” In truth, all those numbers could be correct, even for a single telescope. Why does it even matter? Simply because the focal length of the scope determines the magnification a given eyepiece will provide, and also the photographic image scale.
For Maks and SCTs, the focal length (and, consequently, the f/ratio) varies depending on the spacing between the primary and secondary mirrors. Both scopes are “catadioptrics” (or CATs, as compound instruments like these are known) that usually focus by moving the primary mirror back and forth along a central baffle tube. As a result, the distance between the primary and secondary mirrors changes a little each time you set the focus to accommodate a different eyepiece, or attach a camera. Changing the mirror spacing alters the scope’s focal length (and, consequently, the f/ratio) because a CAT’s secondary mirror isn’t simply an optical flat like those found inside Newtonian reflectors. Instead, Maks and SCTs have convex secondaries that amplify the focal length of the primary mirror. For example, a typical SCT has a focal ratio of f/10, which is achieved with a f/2 primary mirror and a secondary mirror that amplifies 5´.
When you focus the scope and move the primary mirror closer to the secondary, the focal length of the scope increases. Conversely, when your increase the separation between the mirrors, the focal length decreases. To what extent depends on how much you change the mirror separation. So, for these CATS, there is no single focal length that’s always correct. But for practical purposes, you can usually rely on the manufacturer’s stated value.