# Dome geometry for Slaving - "off center" guide scope

#1

I’m reviewing my dome/mount geometry to verify and refine settings for slaving the dome opening to my mount/scope . Wondering about how I should handle the “GEM Offset” value considering that I have a guide scope that is mounted “off center” from the main OTA. Here is a (crude) drawing to illustrate:

If there was no guide scope, the GEM Offset would be “x”. If the guide scope was mounted directly on top of the OTA, I would expect it to be a distance from Mount Pivot Point (MPP) to a point along the line between a and b (1/2 way between outer edge of guide scope and bottom edge of OTA). However, given the assymetrical nature of the guidescope, how should this be handled?

DaveNL

#2

Hi Dave,

Perhaps someone may come back to you with a nice mathematical formula…but while you are waiting on that, why not just experiment.

How critical this is, really depends on how wide your shutter opening is. Why not pick a number, say ‘x’ plus the radius of the main scope (x + 5­­½") and send the scope to 179° & pointing low. Has the guider still got a clear view out of the shutter opening. If no, then perhaps you need to increase the number. If 1st test was OK, then send the scope low on the other side at 181° and check again. Adjust the numbers for the situation as you see it.

Basically you just have to find the number that allows the main scope & guider scope have a clear view at worst case scenario either side of the pier. Doing it via simple testing won’t take too long…and maybe better than getting the brain overheated

#3

Hi Kinch,
Thanks for the response. The opening is narrow enough that in combination with the guidescope being tossed from one side to another (either side of meridian looking south) that it is critical.

I’m already having trouble from experimentation side of things. I suspect there are other issues at play, which is why I was trying to remove this particular variable.

I would be interested to see the formula’s used - I wonder if this is published somewhere?

Dave

#4

Hi Dave,

No idea…I did mine by trial & error…just tweaking things as required. Maybe someone else has some other way of ‘nailing’ it and will chirp in here.

(Just of note: the dome manufacturer may note somewhere what the max diameter scope should be used for that particular shutter opening. For you that means scope + guider. As long as you are within that limit…then you will get it eventually.) Good luck with that.

#5

Hi Dave,

No idea…I did mine by trial & error…just tweaking things as required.
Maybe someone else has some other way of ‘nailing’ it and will chirp in
here.

(Just of note: the dome manufacturer may note somewhere what the max
diameter scope should be used for that particular shutter opening. For you
that means scope + guider. As long as you are within that limit…then you
will get it eventually.) Good luck with that.

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