[Barrykgerdes]

Here are some practicalities of measurement

1. Using a spirit level with merlins method on a 12" telescope
The readout precision of the spirit level will be about .5mm over
the 500mm of the measurment centres. I can't read the bubble
any better than this. You could be lucky and your read error
could improve your accuracy or unlucky and reduce your accuracy.
This will equate at .5mm /500mm as about 3 arc minutes of
orthogonal error. This would in effect make most gotos within a 26mm eyepiece

2. Using the dial guage the precision improves greatly. You should
be able to measure with precision about .01 mm with an accuracy
of about .03 mm. over 600mm. This will equate to .03/600 or
about 10 arc seconds. This coupled with the acuracy of the OTA
being within 30 arc seconds with a visual alignment on the TV aerial
Will theoretically put your Gotos inside +/- 1 arc minute cover the
range.

Quite an improvement!

A thing to remember also is that orthogonality is less of a problem in ALT/AZM telescopes than in a polar mounted telescope. Getting a true polar alignment is virtually impossible if the orthogonallity is more that a few arc seconds out when using the standard procedures. This is why a drift alignment always seems to give different results than the manual described method.

Barry