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GEM question

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#1 treaves


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Posted 10 May 2010 - 10:27 AM

When a GEM is centered in both axis, if you were to look through the 'scope, should it point (basically) at Polaris?

I have my CGE Pro on a pier, and asked the cement contractor to alight the pier bolts in a particular fashion; when he did a poor job of. So when I do a two star alignment, the two stars the mount points me too are very much off from where the 'scope is aimed. Once I have added additional stars, the GOTO gets close to the object, but not on it. When I tell the 'scope to go to Polaris, the 'scope is very much left of center.

So before I tear all of this down, and try to get the 'scope to point at Polaris when in the switch position (both axis centered), I just wanted confirmation.

#2 TxStars



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Posted 10 May 2010 - 11:34 AM

Polaris is slightly off center of the celestial sphere so if the Dec axis is pointing north the scope maight not have Polaris in the field of view depending on it's focal length, but it would be pointed in the same general area of the sky.
If you have the polar scope in the mount you would first roughly aim it at Polaris.
Then you adjust the altitude and azimuth to get Polaris in the correct spot in the polar scopes reticle.
Next you would want to do a drift alignment to get it exact.

#3 Skylook123  Happy Birthday!



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Posted 10 May 2010 - 11:35 AM

Whether you should see Polaris when you are in your Home position depends on the field of view in the eyepiece. The North Celestial Pole differs from Polaris by about 3/4 of a degree. If your mount is set perfectly (level, counterweights down, azimuth zero, DEC = 90, latitude correct), you may not see Polaris in the eyepiece. Its path around the NCP is a circle 3/4 degree in radius/1.5 degree in diameter.

With my 10" SCT, focal length = 2540 mm, my lowest magnification eyepiece is 51X, about 1 degree FOV, so if it is exactly pointed at the NCP to start, Polaris is 1/4 degree outside the field. A marked finder scope can give you an answer, though; if you know Polaris' position at the time you are looking for it, then the finder (even a Telrad) can give you a hint how close you are. IF the other parameters are correct! Level is not important for the polar positioning, just the first alignment star GOTO. The rest, though, (latitude, azimuth = 0, DEC = 90) are essential to setting or diagnosing the polar alignment.

#4 Fred1



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Posted 10 May 2010 - 11:30 PM

Perhaps I'm looking at this issue too simply. The CGE Pro has no accommodation for a Polar Alignment Scope. Celestron purposely designed it that way because the Polar Alignment program in the Hand Control is so accurate. I suggest you leave the pier alone for now and try following the Polar Alignment program in the HC. More details are available in the manual, at least it is for the CGEM. If followed correctly it should resolve your GoTo issues, as well as tracking.

#5 Chris Rowland

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Posted 11 May 2010 - 07:19 AM

The polar align menu (available by pressing the Align button) has a Display Alignment option that will show the polar align error in Altitude and Azimuth.

It depends on how good your alignment is of course but after 2 align and 2 or 3 calib stars it should be good enough to see how much you need to move your mount.


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