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Celestron CGEM - mechanical problems

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#1 pcaker

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Posted 27 March 2013 - 02:01 PM

My CGEM mount is showing at least 2 and possibly 3 apparent quality control problems, which I describe below:

1. If the declination axis pointer is set to the initial basic position before starting alignment, so that the 2 pointers line up, the wedge mount above is rotated anti-clockwise at least 2 to 3 degrees from being parallel to the r.a. axis. Apparently, one or both of the pointers are seriously out of position. I have assumed that it is more important for the mount to be parallel than for the pointers to line up, so that is what I try to achieve.

2. If I line up the axes pointers, see above, then tighten the rotation lock bolt, the mount forcibly rotates another 1 to 2 degrees during the tightening. I am not strong enough to prevent this happening, and I am not weak. This, I suspect, is more of a design problem than quality control. The tightening bolt clamp design is just too crude, transmitting rotational force to the collar instead of solely radial clamping force. Therefore, I have to anticipate this tightening mount rotation and set the pointers initially out of alignment. In fact, I have to allow for the problem described in (I.) above, plus the tightening forced offset rotation of problem (2.).

Therefore, if it really is critical to get the wedge mount initially perfectly parallel to the r.a. axis, Celestron sure don’t make it easy.

3. The manual altitude adjustment bolt is unable to increase the mount’s altitude by itself, and requires major additional force applied to the mount to move it (the opposite bolt is of course backed off). The altitude bearing is just too tight. This does not matter too much when setting the location latitude, as at the time the mount is not set up precisely and the jerking around of the whole mount and tripod does not matter too much. However, for polar alignment, where precise careful manual adjustment of the altitude is required without disturbing other alignments, it is a fatal problem.

In practice, after doing a 2 star alignment plus 4 calibration stars, the goto accuracy is good to very good, and for visual use more than adequate.

For astrophotography, I have found the drift method of polar alignment to be too time consuming - on top of the time for basic calibration, plus the time consuming effects of the problems described above, and I am going to try the iterative method. However, neither method is possible if I cannot solve problem (3).

Any comments, insights or advice would be appreciated.

#2 rmollise

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Posted 27 March 2013 - 02:36 PM

1. The important thing is that you start from the same position each time.

2. See above.

3. There is no declination bolt. There is an altitude bolt and an azimuth bolt. Do you mean right/left or up/down? If the latter, make sure the bolt that holds the head to the tripod is not too tight until you are polar aligned. If the former, do the adjustment before you put scope/weights on mount.

If you don't want to drift, why aren't you using the AllStar polar alignment?

#3 Nitpick

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Posted 27 March 2013 - 07:44 PM

I don't know about the other points but re: #1, when the declination pointers are lined up on my CGEM, the clamp head is rotated about five degrees counter-clockwise (when looking north) from vertical. I didn't (and don't) think that's an issue.

But I could be wrong.

#4 pcaker

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Posted 27 March 2013 - 08:10 PM

Thanks for your comments.
I mean the up & down or altitude movement which I thought I had clarified by my reference to setting the location latitude with it. However, I have edited my text to refer to altitude instead.
I had thought the All-Star polar alignment was a drift method, but yes I should be trying it out preferentially. After the All-Star polar alignment, the undoing of the previous alignment, and re-doing it, without losing your place on the menu, is something that has not gone well for me.
The stiff altitude bearing seems to me to remain as a problem for the All-Star method.

#5 Stew57

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Posted 27 March 2013 - 08:48 PM

Rod is right make sure that the front bolt is completely loose. It is a only a stop, so when you finish aligning just snug it up. If set correctly you can raise the mount head via the shaft some. The CW keep the mount in contact with the front stop bolt. If it is still tight something is wrong, bad threads maybe?






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