Setting Circle Telescopes

The first telescope I bought, 25 years ago, was a setting circle telescope. It was a Meade 8” LX and was built about 1983. This scope folded the light in mirrors twice and then through a diagonal. The light entered the front of the telescope through a plate of glass, bounced off a curved primary mirror at the back end of the scope, which focused the light on a curved secondary mirror in the center of the glass plate. After reflecting off the secondary mirror, the light passed through a central hole in the primary mirror, reflected off a diagonal and then into an eyepiece.

This type of design is known as an SCT, short for Schmidt Cassegrain Telescope. Another design, the Maksutov Cassegrain has a more radically ground corrector on which the secondary mirror is mounted. The Celestron NexStar 4se scope that I have written about in another blog is a Maksutov scope.

What I really want to get around to talking about here is the mounting that my scope had, because that has something to do with learning about the sky. But first I should mention a couple other mountings that scopes I had played with as a young teen were fitted with.

The table-top refractor my parents brought home had a swivel and a fork. This type of scope is still sold in grocery stores just before Christmas. Such scopes are bought by the thousands and most of them are carefully packed in the back of a kid’s closet until they move away for college. They are cheap. They are hard or impossible to use for any real astronomy purpose. They are the equivalent of the $20 guitars which are sold in the grocery stores next to the telescopes. They are guaranteed to frustrate the kids who receive them. But I digress… The swivel and fork mounting is formally called an altitude/azimuth or alt/az mounting. Such a mounting allows the telescope to be swung in a circular direction to every part of the horizon (azimuth) and to be raised to some portion of the sky (altitude).

The second scope I got to play with had a “german-equitorial” mounting. This was a much more complex mounting, and I had some idea of how to use it back when I was in high school. (The problem with that scope was not so much the mounting as the optics.) To set up an equatorial mounted scope, the direction north needs to be known in azimuth, as well as the altitude of the north celestial pole. Just like the alt/az mounting, which rotates in two planes and can point in any direction, the equatorial mounting can move in two planes and point in any direction. The difference between the two mounting systems is that in an alt/az mount, the pivot rotates around a vertical pole, as though it came from the center of the earth and aimed toward the highest point overhead (the zenith). An equatorial mount has that pivot rotating around the line that connects the north celestial pole and the south celestial pole – and that is a line that is very close to the line formed when I point a laser pointer at the north star. Rotating around that line, the 360 degrees of rotation are measured NOT in degrees, but in hours!

I know. Those astronomers are crazy. But there is a method in their madness. Describing the circle in 24 hours makes sense, because the sky actually moves around that circle in … 24 hours. We call it a day. Before you get all excited, let me deflate your expectations some. The 24 hours in the sky have almost nothing to do with the time on your watch either where you are or at Greenwich or anywhere. An hour and minute of Right Ascension (RA) simply marks out a north/south line which starts at the north celestial pole and ends at the south celestial pole. All the stars on a single RA line stay there, month after month and year after year, for all practical purposes. It is like knowing the longitude of a city on the earth.

Since RA is like longitude, there has to be something like latitude to give an address for a star. That second number is declination (dec) and it is measured in degrees north and south of the “celestial equator” which is the circle equidistant from the two celestial poles.

So do I have your head spinning yet? It was a big concept for a 16 year old kid to figure out before the days of the internet. Back then we had encyclopedias for that purpose, and I am sure that I spent time with the Encyclopedia Britannica and a lot of pacing to figure this all out.

Well, it was one thing to see how it works, and another to get it to work. Like I mentioned, the first thing one needs to do is to aim the pivot axis toward the north star. (Actually a little off from the north star, but for a 16 year old the north star was pretty close.) The required pointing north and adjusting a joint to the latitude of Cincinnati, Ohio – about 39 degrees. That accomplished, a kind of magic could now operate. When I pointed the scope at a star, and the star began to move west out of the field of view, all I needed to do was to very slowly turn a gear that drove the RA adjustment in order to follow the star.

I saved that for the last – maybe to surprise some of you. The only reason to have an equatorial mounting is that wonderful advantage. If a scope has an alt/az mounting, the observer needs to constantly adjust both the altitude and the azimuth as the object moves through the heavens (well… as the earth rotates). If you have ever been to a star party and the scope host needed to readjust the scope to point at an object you experienced this. But with an equatorial mount, only one axis, RA needs to be adjusted. Dec for the star stays the same all night, all month, and all year.

Fast forward to my first purchased scope, the pretty Meade 8 inch SCT. It had an equatorial mounting and a super cool new feature. The RA adjustment had a clock driven motor that kept an object in the center of the field. Now that was the height of cool! It required that I set up the scope near an electric outlet or next to my car, but I was free from having to constantly correct the RA adjustment. Maybe I would even be able to take a photograph as the scope body stayed locked to a particular part of the sky.

In the end, that scope was both a great piece of optics, with lots of light gathering power (4 times as much as my 4” computerized scope) but it also taught me the celestial mechanics of how the stars move in the sky and why.

When I got interested in astronomy in the summer of 2008, I took that Meade telescope out of the closet, had a little work done on the secondary mirror (it had begun to lose its coating) and still use the scope occasionally. I was able to find dozens of Messier objects in it once the computerized scope showed me what I was looking for. I will probably have it forever. It is practical and it has sentimental value.

But my SCT is not my favorite telescope and it is not what I would recommend that a new astronomer buy. That story will need to wait for another blog entry.