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Aiming a Telescope

Posted on 02/18/2008

When I described, probably a little too rapturously, a recent night I spent calibrating the Margaret Gilbert 20-inch telescope at the observatory, my listening colleague remarked wryly, "It sounds as though you and your Valentine had a lovely night together!" Touché. But if you want to know the sort of thing that stirs geek souls, read on.

The 20-inch telescope does not have a wide field of view. Even with a "low-power" eyepiece, it is essentially a really big telephoto lens, and you can only see a small patch of sky through it. This is a good thing if you are trying to see fine detail on a small object, or a big one-the moon is absolutely spectacular through this scope. This is not a good thing if the scope is not aimed properly, and you don't really know what you are looking at.

The telescope is aimed with the use of planetarium software that shows us a simulated sky. This software is linked to the mount that aims and moves the telescope. The general procedure is to get the software and the hardware talking to each other, find on a computer screen the object you want to see, click on it, and tell the telescope to go there. We mechanically aligned the telescope mount as well as we could, to within a degree or so of where it should be. But when your high-magnification view is less than ¼ of a degree across, this isn't really good enough.

Another software package to the rescue! This package considers all the myriad ways error can be introduced into this process, from misalignment of the telescope mount in either of two axes, to flexing of the telescope itself as it shifts position. But one must train it by centering a star in your view and telling the computer that NOW you are aiming precisely at that star.

We had done some of this in a somewhat haphazard way late last year. Some of our data was good, some was not, and we simply could not get our pointing error down to the levels it should be, with an average (OK, actually root-mean-square for you math folks who are still with us) error in the tens of arcseconds (60 arcseconds equals one arcminute; 60 arcminutes equals one degree). Our pointing error was 1800 arcseconds-the width of the full moon! Not good.

So I junked that data set and started over. On Valentine's night. By myself.

My procedure was to locate a bright star, aim the telescope (roughly) in that direction, and use a wide-angle eyepiece to find the star. Center it in that eyepiece, then switch to a medium power. Center it in that one, then switch to a high-power eyepiece with crosshairs (no kidding). Put what is now a shimmering blob of light dead center on the crosshairs, inform the software, and move on to the next bright star. Forty or so of these spread across the sky, and the software can make the corrections necessary to aim the scope properly.

The first star I picked was Capella, the eleventh brightest star in the sky, noticeably yellow in color. I got it centered (I thought), thinking to myself that it didn't seem as bright as it should in such a large scope. But there were some thin high clouds that night, so I told myself that was blocking some of the light. On to Aldebaran, a reddish star. Same story-the color was OK, but it just didn't seem bright enough. Oh, well.

Rigel, the blue-white beacon at the lower right of Orion, was next. I hunted and hunted, manipulating the joystick that moves the telescope, peering through the eyepiece. Where was it? All of a sudden, a dazzling blue-white searchlight moved into the field: oh, yes, that was Rigel, all right. And those earlier identifications? Flat wrong. Now I know what to look for.

I was able to map only five more stars that night, moving very slowly and methodically. One does not want to drop a $300 eyepiece through careless haste. By the time a coyote howl reverberated through the dome, the combination of fatigue and the chills induced by that howl convinced me to pack it up for the night. But I must tell you that the bond between observer and telescope grew a little stronger that night. She (yes, she is definitely a she) seemed to be telling me that it was about time I learned how to treat her properly! In my defense, I responded that I had not dealt with such a high-class lady telescope before. We'll be back, and she will be properly aimed, and we will move toward public access. Stay with us!

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