[ATM] Illuminated field question

[Paul Bradley]

Hi,

Appologies to the mods if this isn't sufficiently ATM related, I thought 
this would be the best
place to find people who can answer my question.

I am considering using an X-Y finder like this: 
http://www.us-astro.com/XYF.htm to save the
hassle of guide scope adjustment rings. I'm thinking of using a meade etx 90 
maksutov as the
guide scope as with focal reduction it would double up nicely as a cheap 
wide field scope, well
cheap compared to an apo of that aperture anyway, plus it comes with a flip 
mirror which would
save a further few shekels.

My question is how big would the illuminated field on the etx be roughly, 
and at what point towards
the edge of the field would coma get bad enough to adversely affect the 
guiding accuracy / centroid
calculation. Essentially, I'm trying to find out if the x-y finder 
arrangement, with the guide scope rigidly
mounted to the main OTA, will give decent flexibility in finding a guide 
star a few degrees from the
imaging target, or would it be best to stick to normal adjustable guide 
scope rings.

If the x-y finder is a bad idea, I'd appreciate any pointers to ATM or 
commercial X-Y adjustable guide
scope rings, as the 3 screw type look like a real fiddle to adjust.

Thanks.

Paul