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Several possible sources of errors - not just "cone error":
a) RA & dec axes are not perpendicular,
b) dec and the optical axis of the scope are not perpendicular,
c) the motors are stalling or the pulses are being miscounted (missing pulses) on either or both axes - these mounts use pulse counting of the drives to measure position, and they do miss,
d) backlas, especially on the dec axis,
e) flexure of the mount and tube assembly, in several places;
f) atmospheric refraction (I often wonder whether these mounts compensate for it) which can be half a degree near the horizon.
All mounts suffer from all of the above to some degree. If you have a scope larger than a 4" on a Vixen dovetail the first suspect I would guess is flexure right there.
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