Re: [ATM] Field derotator or equatorial?

["Mel Bartels" <mbartels@bbastrodesigns.com>]

First of all, an equatorial mount is a mechanical 4 axis coordinate 
transformer, where the 2 terrestrial axes are usually of limited motion.  A 
3 axis mount is actually easier, all things otherwise being equal.

For guiding a 3 axis, you have several choices.  A popular choice is to let 
software derotate the images for you by stacking.  I call this a virtual 3 
axis scope.  If you want off-axis guiding at a target star that is truly 
off-axis from the subject matter, then look at Fletcher, Shaw, and Kelly's 
work.  Rusty Fletcher is at http://www.afountain.org/astronomy/

The challenge really comes down to whether you want to roll your own system. 
It can be quite satisfying and educational.  The complexity of melding 
software and mounting is a constant in a 3 axis mount, and the structural 
complexity and demands of an equatorial grows exponentially with increasing 
size.  At some pivot point, they are roughly equal in challenge.  Smaller 
than that, the equatorial is easier overall, and larger than that, the 3 
axis is easiest.  Scopes in the 20+ inches range are largely 3 axis, while 
scopes smaller than 12 inches are largely 4 axis equatorials.

Mel Bartels 

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