Here is a data sheet for the gearheads that would be suitable
http://www.harmonicdrive.de/cms/uplo...omplete_en.pdf
Transmission accuracy is less than 2 arcmin variation and repeatability <+ or - 1 arcmin.
If you then have a 180 tooth worm these tiny variations are then even smaller. That is about one arc second at most variation in DEC or RA.
The other advantage is that about 30% of the teeth are always meshing. There is no non linear effect due to the elliptical wave generator.
Bert