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GEM Geometry ???s

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#1 Mopman


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Posted 31 January 2010 - 05:05 PM

I have been reading about polar alignment, cone error, and piggy back camera mounts, and now I am confused :confused: If one has a GEM that is polar aligned, and a OTA (which is also polar aligned) with a piggy back camera mount on top of the OTA, will the camera be polar aligned regardless of where the camera is pointed relative to the aligned OTA? Must the camera be aligned to the OTA in both axis, or only parallel to the RA axis in order to be polar aligned? Thanks! :bow: Gary

#2 SanDiegoPaul



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Posted 31 January 2010 - 07:31 PM

~anything~ that is attached to ~whatever the tracking device is~ (i.e. GEM Mount) is as well polar-aligned as the mount is.

#3 Eddgie


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Posted 31 January 2010 - 08:29 PM

SanDiegoPaul has answered the question, but has perhaps not addressed your confusion.

The confusion you are having is probably around cone error. Cone error will manifest itself as a POINTING error. If the mount has cone error, when you align the mount and do an alignment on either the or west side of the mount, if there is cone error, the indexes will only be accurate for that side of the mount (east or west). If you are using setting circles or if you are using a Go-To scope, when you slew the telescope to the opposite side of the mount and use the setting circles (or the Go-To), the cone error will result in the scope slighthly missing the target.

The early Go-to scopes and digital setting circles did not have any method for compensating for this error, so the telescope had to be shimmed to bring the optical axis into alignment with the RA axis.

Otherwise, even with cone error, once the scope is polar aligned an a target is in the eyepiece, it will not drift from the field. You might have trouble FINDING the object though. lol.

#4 Mopman


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Posted 31 January 2010 - 10:15 PM

Thanks SanDiegoPaul and Eddgie! Yes, I did not understand cone error was about goto accuracy and not tracking error. Thanks for the help! Gary

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