Announcements and News
Cloudy Nights Announcements
Discussion of CN Articles, Reviews, and Reports
General Observing and Astronomy
ATM, Optics and DIY Forum
Cats & Casses
Video and Electronically Assisted Astronomy
Astrophotography and Sketching
Beginning and Intermediate Imaging
DSLR & Digital Camera Astro Imaging & Processing
CCD Imaging & Processing
Solar System Imaging & Processing
Astro Imaging/Sketching Contest
Solar System Observing
Deep Sky Observing
Double Star Observing
Solar Observing and Imaging
Variable Star Observing and Radio Astronomy
Telescope Specific Forums
Celestron NexStar, NexStar GPS, CPC
Astronomy Shows & Conferences
Astro Art, Websites & Other Media
Astronomy Clubs and Star Parties
Astronomy Software & Computers
Books, Star Charts & Stellar Media
Science! Astronomy & Space Exploration, and Others
The Off Topic Observatory
Off Topic Observatory Games
Cloudy Days & Microscopes
Cameras & Photography
Swap and Shop
S&S Misc Astro
S&S Information for New Members
CN Feedback and Vendor Announcements
Vendor and Group Announcements
International Year of Astronomy 2009 - Forums are now closed.
IYA Archive Info
Articles and Columns
/ Celestron CGEM - mechanical pr...
Celestron CGEM - mechanical problems
March 27, 2013 3:01 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.
March 27, 2013 3:36 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?
Uncle Rod Uncle Rod's Astroblog:
March 27, 2013 8:44 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.
March 27, 2013 9:10 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.
March 27, 2013 9:48 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?
Mark ETX-80 Messier project
ETX80 article on modifications
/ Celestron CGEM - mechanical pr...
Back To Top
Like This Topic
Blue & Grey
Remove This Ad
CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.
I don't want no stinking ads, but I want to help Cloudy Nights!!!!