Finding North in a Telescope

One of the Astronomical League observing programs I am working on is the double star club. It is not very difficult to find most of the stars, especially in a dark sky. And once the star is found, 120X is usually plenty to resolve the pair of stars. But the instructions for the observing program say that each double star needs to be sketched. The two points of light with relative distance and the celestial direction are the required elements for each drawing.

When I started thinking about how to find the celestial direction, I went through a long process of thinking about where the north south lines would be in the part of the sky I was looking at. These lines can be seen on planetarium programs if I set the program to show them. Then I needed to be able to remember how my scope turns the light upside down and backwards.

It was not really complicated, but I wanted to make sure I was doing this right before finding a hundred stars and drawing them. So I took the chicken way out. I posted a note on my local astronomy club and on an observing yahoo group and asked if I was going about this right.

I got a great set of answers back. Boy, am I glad that I asked the question.

It turns out that for a non-tracking scope like my dob, finding the celestial direction is very simple. After centering a star at a reasonable magnification like I mentioned above, all I need to do is to wait a minute or two and watch the direction the star tries to head out of my eyepiece. That direction is west. Of course it is. No matter where in the sky I am looking, the celestial globe rotates directly west all the time! Then to find north, all I need to do is to rotate counter-clockwise 90 degrees for my reversing telescope optics and that is north.

Now, I understand it is a little more complicated for a tracking telescope like a clock driven or a computerized scope. Turning off tracking is not all that practical when the plan is to observe a couple dozen stars in a night. Who wants to reset the tracking all those times? So for a wedge mounted scope, a practical method mentioned in the answers I got was to rock the fork slightly northward while watching in the eyepiece. If the tube is moved slightly northward, the star in the eyepiece moves southward. OK, so a direction is found. If the scope has a diagonal eyepiece holder, the view through the scope is probably right-side up and backwards. So if north is known, then west is 90 degrees counterclockwise from north.

For a computer driven go-to scope with alt-azimuth mounting, good luck. I think I’d go back to my complex way of looking through the scope and knowing which way north is to be found in that part of the sky where the star of interest is found.

I guess that is another good reason why a new amateur astronomer like me should be glad to have a manual dob as my primary scope.