by John Menke, August 2001. Reprinted with permission.
To perform accurate pointing on a German equatorial mount one must measure and correct for any orthogonality error. The plate holding the scope is attached to the dec axis. If the optical axis of the scope on the plate is not perpendicular (orthogonal) to the dec axis, then the scope will generally be pointing to the east (or west) of the correct position. This is not a problem by itself as it can be calibrated out. However, when the scope flips at the meridian, the scope will now point west (or east) with twice the error. A pointing error of 5 a-min can arise from an optical axis tilted by 2.5 a-min, which can be caused by slight optics misalignment, or by a mounting error of .015 inch in a 10 inch mounting span. It is easy to correct orthogonality error on a GOTO scope.
1. Align the mount reasonably well using your standard method.
2. Use TheSky or other program to select a star near the celestial equator and just E of the meridian.
3. Slew the scope to the star and center the star in your eyepiece or CCD. This calibrates the mount.
4. Select a star near the celestial equator just west of the meridian. Write down the coordinates of the star.
5. Slew the scope to the star and center the star in your eyepiece or CCD.
6. Use your scope control program (or scope hand control) to get the apparent position for the star.
7. Calculate the difference between the RA readings, and express the difference in seconds of RA. This is the orthogonality error times two. To convert seconds of RA into arc minutes, divide by 4 then by 2 to get the orthognality error. (You can also calculate the difference between the dec readings which gives the error times 2 of the polar axis altitude.)
8. You will now insert shims or otherwise adjust the tilt of your optical axis on the scope support plate. If the support points are 10 inches apart, a shim .015 thick will correct an error of 2.5 a-min. You may use proportions to get your needed thickness, or use the formula shim=(error)*(spacing)/1666. Soda cans have a thickness of about .004, cat food cans about .008 inch. Insert the necessary shim thickness under one end of the scope supports then redo the measurements.
9. When the orthogonality error is within your tolerance, realign your mount.