Alistair,
It is very unusual to use anti-backlash gears in those transmissions.. they are not really needed. Much better and cheaper approach is the use of timing belts and pulleys.
http://www.iceinspace.com.au/forum/s...ht=timing+belt
Planetary gearboxes are used where the high torques are involved (power tools, for example).. such conditions do not exist in telescope reduction train (unless you want extremely fast slewing, if you want to jump from object to object in a matter of seconds - I am guessing this requirement would be highly unlikely).
As for general design directions, why don't you have a look at how EQ6 is made? There are plenty of websites that discuss dismantling, cleaning and hyper-tuning of this mount, there you can find a lot of interesting design details...For example:
http://www.astro-baby.com/EQ6%20rebu...rip%20down.htm
http://ejcruz.smugmug.com/Photograph...85975881_wfgb7
http://www.bloomingstars.com/vega/ar...tmare/english/