What it is magnitude and meaning of cone error readings given by Celestron German equatorial mounts?
October 28, 2012
In the hand control menu under Scope Setup, Mount Settings, your Celestron GoTo German equatorial mount displays Cone Value, which is set when one or more calibration stars are added to an alignment.
The cone value or cone error feature attempts to compensate for errors caused by the telescope’s optical tube pointing in a different direction than the telescope's mechanical (polar) axis when the tube is set to point along (parallel to) the polar axis.
If the cone error is large and in certain directions, there are certain parts of the sky near the celestial pole that will be inaccessible to the mount. Shimming the tube will help this issue.
To help with shimming, the cone error magnitude is in degrees and the size of the saddle does matter. As a good starting point, for Celestron’s CGE with its 6.5 inch saddle, a shim the thickness of a dime corresponds to approximately 30 arc minutes. A positive value corresponds to the need to shim the back end of the tube. Experiment for yourself to see what works. It’s easy enough to figure out.