Quote:
Originally Posted by ghsmith45
OK, I'll bite. How do you do it?
Geoff
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Normally done in daylight - assemble scope on mount so you can focus on an small, distant target (anything).
With the scope on one side of the mount, read the dec circle - call this D1.
Flip the scope over to the other side of the polar axis (equatorials) or rotate the fork 180 degrees (fork mounted SCT), centre on the same target and read the dec axis again - call this D2.
If D1 and D2 aren't equal the circle isn't reading true and the correct value = (D1+D2)/2. While the scope is still pointing at the target, adjust the circle to read the correct value.
There may also be another error in cheap mounts - if the circle is not concentric with the actual axis of rotation the circle may read true in some positions but not in others. There may be not much you can do about this other than be aware of it. Try a few other targets say 60 degrees apart and if they all agree to 0.5 degree or better the circle is fine. You can probably do this with digital circles too, though when getting down to minutes of arc expect to detect errors from non-concentricity.