This article applies to those who have a telescope with a German equatorial mount. Most GEM’s have setting circles that can be calibrated with the celestial coordinates, and if you learn how to properly read and use them, then it helps guide your telescope to the general direction of what you’re looking for!
Equatorial Mounts are designed to compensate for the Earth’s rotation by having one rotational axis parallel to the axis of Earth’s rotation. When set up correctly, the only manual adjustment one has to make is a simple twist of a slow motion control to get the object back in view.
As stated in Using the Celestial Coordinates, Right Ascension is the equivalent of Earth’s longitude, and Declination is latitude.
A German Equatorial Mount
Most amateur telescopes with an equatorial mount are the German Equatorial Mount, or GEM. The structure of the mount is in the shape of a T, with the lower bar being the RA axis and the upper bar being the Dec. axis. The telescope itself is placed above the upper bar. with a counter weight placed below it. This keeps the telescope balanced, making it much less of a hassle when letting go of the tube after loosening the locks.
It needs to be polar aligned first!
The lower bar also serves as your polar axis, and depending on the hemisphere you’re observing from, it needs to be aligned with your celestial pole. Northern hemisphere observers have the privilege of using Polaris as a pole star.
Make the R.A. axis aligned to your celestial pole accurate as possible, either using a compass or using your pole stars to find true north/south.
The angle of the lower bar serves as your latitude marker, and there should be controls and a right triangle with the numbers 0-90. As an example, if your latitude is 34° north or south of the equator, then you should set the latitude marker to 34.° This angle is super important for later.
Some GEM’s have polar finder scopes, or even just an opening where you can look through and see if your polar axis is directly lined up with Polaris. The closer you are in alignment the more accurate the setting circles get.
About the Setting Circles Themselves
On the mount, there are two setting circles. Take note of which are movable, and which are stationary. The stationary setting circles have a pointer that moves, while the movable circles usually have a stationary pointer.
The top one under the upper bar is your Declination. You can see that the numbers on it are 0-90-0 on each side. Declination coordinates range from +90° at the north celestial pole, to 0° at the celestial equator, to -90° for the south celestial pole.
If your telescope is polar aligned, the Dec. should be pointed to +/-90 by default.
The setting circle that’s on the lower bar is the Right Ascension. As RA Coordinates are measured in hours and minutes, the numbers range from 0-24 hours with minute increments in between them.
The setting circles for RA often have two sets of numbers that are going in opposite directions. Unless otherwise stated in your instruction manual, northern hemisphere observers use the bottom set of numbers!
Calibrating the Setting Circles
It is recommended to use a bright naked eye star that can be found with ease. Then you adjust the setting circles to match the coordinates of that bright naked eye star.
So as an example, we are going to find M 13. Vega is a good candidate as an alignment star, so we will use that. Here are the following coordinates.
Vega – RA 18h 36m 56s | Dec +38° 47′ 1″
M 13 – RA 16h 41m 41s | Dec +36° 27′ 35″
Setting the Declination
By default, the telescope should be pointed due north (or south in southern hemisphere). If it is pointed due north, then the pointer should already be set to 90. If your Dec. circle is movable and the pointer isn’t on 90, move the circle until it is and then lock it.
Now move your telescope to Vega and center it. If it is done correctly, you should see the arrow pointing at or near +38°. If it isn’t, then either the telescope isn’t properly polar aligned, or the circles didn’t properly move when you moved the scope. After getting it to +°38, and at 47′, you should be two thirds of the way between +38 and +39.
Setting the Right Ascension
With the Dec. set, then get the RA setting circles in the right spot. Using the bottom set of numbers, move the circle until the arrow is pointed at 18h, and then using the increment lines between 18h and 19h, get it as close to 36m as possible.
If you have both setting circles on the correct coordinates for Vega, then you are now calibrated and are ready to go to M 13.
Moving it to Your Target
It can be done in any order, but start with one coordinate, make sure it’s correct on the dial, lock it, and then move the other until the object comes into view.
Remember to use as wide of an eyepiece as possible to account for being off by a degree or two. A square degree covers a small patch of the sky (the equivalent of 4 minutes in RA), and a 10 minute difference in RA is 2.5 degrees across. Take into account for how wide of a field your eyepiece covers.
Declination– in this case of finding M 13 from Vega, M 13 is 2 degrees south of Vega in Declination, so make sure when you slew the telescope from 38 to 36, your tube is moving south, not north! Your Dec. reading should now read +36°, and at 27′ it should be close to half way between +36 and +37.
Right Ascension – The hour order of RA coordinates are in the same east to west direction as Earth’s rotation, but remember that if you’re dealing with RA coordinates in the 23’s, the coordinates for objects east of it reset back to 0.
Since Vega is at 18h and M 13 is at 16h, that means you need to move it west. Move the telescope until the arrow is pointed at 16h, and using the increment lines between 16 and 17, as close to 41m as possible.
At this point, if you did it correctly, you should be able to see the star cluster come into view!
If you don’t see what you’re looking for, it could go back to poor polar alignment, to errors in moving the setting circles during calibration, to errors in reading them, or you may have confused the RA hour angles between north and south due to the two sets of numbers.
Remember that you can always goback to your calibration star, reset the circles, and try again!
Using the setting circles may be a long task, but when practicing them and getting them done right, it makes finding any deep sky object much easier! You’ll find many things you couldn’t otherwise find by simple eyeballing techniques!