Celestron CGEM Review- Part 3

Posted on 06. Oct, 2009 by Ray Shore in Astrophotography News, Equipment Reviews

Part 3 of my review covers polar alignment of the CGEM. I actually used two different methods. First, I just had to try the new “All-Star” polar alignment routine that Celestron has been touting. Then, I fine-tuned it with a standard drift align.

To get set up, the polar axis of the CGEM must be pointed close to the celestial pole. Since I’m in the northern hemisphere, I pointed it toward Polaris, the North Star. The first step though was to set the latitude scale of the CGEM. This is a simple adjustment to the latitude adjustment knobs.

Click to Enlarge Photo of CGEM

Basically, I adjusted the angle to correspond to my latitude here in Kansas. Here is a handy tool on my other site to find the latitude for various cities in the U.S.

Once the CGEM’s latitude is set, then it’s a matter of aiming the polar axis of the CGEM toward the celestial pole. I can see Polaris from my backyard so I first eye-balled the alignment by adjusting the CGEM so the RA axis was aimed at Polaris (as shown below).

Next, I leveled the mount with the handy bubble level that is built in. Then I removed the polar finderscope covers and adjusted the azimuth and latitude adjustment knobs until Polaris was centered. Note that I do not have a polar finderscope; I just centered it the best I could through the hole where a finderscope can be inserted. This made it close enough to perform the All-Star polar alignment.

All-Star Alignment of the CGEM

Before starting the All-Star alignment routine, a two-star aligment must be performed first. I covered this in Part 2 of my CGEM Review. This has become a fairly simple task since I’ve done it a few times.

After the two-star alignment is complete, then the All-Star alignment can be initiated. First, a star must be selected from the “Named Star” database list in the hand controller. When a star is selected, the CGEM will automatically slew to the star. When the command is given to align the mount, the CGEM actually slews a short distance away from the star then it slews back. The star must be centered in the finderscope first and then in the eyepiece. It is helpful to use a reticle eyepiece to get the star centered exactly in the middle of the eyepiece. The one I use specifically is the
Orion 12.5mm Illuminated Reticle Eyepiece. Once the align button is pressed, the CGEM will sync on this star then slew to the position that it ”should be” if it actually was polar aligned. Then it’s just a matter of adjusting the latitude and azimuth knobs to center the star in the eyepiece. It is important here not to center the star by slewing with the hand controller. The “mount” must be adjusted to be properly aligned with Polaris.

Tips for Polar Aligning the CGEM

During the All-Star alignment routine, the hand controller will warn you not to choose a star near the meridian or one that is on the eastern or western horizon. I of course chose my first star on the meridian and I could not get it centered in the eyepiece! After moving it to a star between the meridian and the western horizon, it was easy to center the star.
Use a reticle eyepiece to center the star exactly. The following is a good choice:
Orion 12.5mm Illuminated Reticle Eyepiece.
If the star is not visible in the eyepiece during mount adjustment (i.e., adjustment of the latitude and azimuth knobs), use the viewfinder for a wider field of view and center it there first. I figured this out my first time after working like mad to get the star to show up in the eyepiece. The light bulb finally came on and I used the viewfinder first. Then the star was visible in the eyepiece!

All-Star Polar Alignment Result

I found that this new polar alignment routine did a pretty good job for a quick and close alignment. To test the accuracy, I put the telescope on Jupiter and tested the tracking. It held really well in lower power eyepieces like 30mm and 25mm. But Jupiter started to slowly drift out of the field of view with higher powers such as 10mm and 5mm. This may get better once I get more experience with the All-Star polar routine though. For my first trial, I thought it worked well to get the CGEM closely aligned!

CGEM Drift Align

As mentioned at the beginning of my review, I fine tuned the polar alignment with a traditional drift align. Basically, I found a star in the southern part of the sky (near the meridian) and did the first part of the alignment which invoved adjustments to the azimuth knobs until the star stopped drifting. Again, I utilized my 12mm reticle eyepiece.

Then, I pointed the telescope at a star close to the western horizon and adjusted out the drift with the altitude adjustment knobs.

I may write up a detailed procedure on how to perform a drift alignment on the CGEM (especially if I get a lot of requests for it!).

Drift Align Result

My test on Jupiter was a sucess! The CGEM kept Jupiter centered quite nicely in higher power eyepieces for several minutes. I think the CGEM will work very well for tracking during astrophotography. I hope to be getting to this stage very soon. I think the Canon Digital Rebel needs to get back to work now!

If you have any questions or comments, please post them in the “Leave a Reply” section below. Also, I have added a survey below to find out if this review was helpful to you.

Clear Skies! Ray Shore

CGEM Review Part 1

CGEM Review Part 2

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Tags: celestron review, CGEM, CGEM review

25 Responses to “Celestron CGEM Review- Part 3”

Jeff Turner

10. Oct, 2009

Ray,

Enjoying reading your reviews keep them coming. A detailed alignment procedure would be great! My new CGEM arrived last week. Waiting for my edge800hd ota to arrive and will be hopping into imaging as soon as possible. I have already changed out the plastic knobs for the aluminum orange anodized. Initial indoor testing of the mount so far has been flawless. Waiting on several cables to arrive to test out my computer hook up. Reviews on imaging and setup with computer would be greatly appreciated!

Clear Skies

J. Turner
Ray Shore

10. Oct, 2009

Hi Jeff,

Thanks for your feedback! Nice that you chose the edge800hd. That is an interesting OTA. I thought seriously about getting that instead of my 9.25″ standard SCT, but couldn’t wait until next year for that size to be available. Would be good to hear how your edge800hd is working out for you.

I’ll continue to work on more reviews of my CGEM. I just got a USB to RS232 cable the other day so I can operate the CGEM with my laptop. I haven’t had a chance to try it yet though. But I’ll be sure to post my results along with a tutorial for the drift align.

I wonder if most people are finding that a drift align is a neccessary follow-up to refine the All-Star alignment when the CGEM is used for astrophotography? Will have to check into this.

Congrats on receiving your CGEM. You’re going to like it!

Thanks,
Ray Shore
Jeff Turner

12. Oct, 2009

Ray,

Thanks I will keep you updated with my experiences with new setup. I have included the link to machine shop which provides the new orange anodized knob set for the CGEM. I received the knobs within two days of ordering very quick and efficient service. It took me about 5-10 minutes to replace all plastic knobs on my new mount and looks great with celestron orange anodizing.

http://www.admaccessories.com/Miscellaneous_CGEM.htm
Ray Shore

12. Oct, 2009

Thanks Jeff for posting that link. Those orange anodized knobs look awesome on the CGEM. Looking forward to your updates!

I need to look into a saddle through ADM so I can mount my ED80.
Jim Pecorella

19. Oct, 2009

Thanks for your reviews. I have been looking for a new telescope mount for my Meade 2080 GEM SCT. I have read a lot of reviews concerning telescope mounts and concluded that the CGEM will probably work best for me. Your articles convinced me that is the way to go. Thanks.

Please add another article regarding the drift alignment procedure.
Ray Shore

20. Oct, 2009

Jim,

I’m glad my CGEM reviews are useful to you. One of the reasons I wrote them is because I had troubles finding reviews on them when I was trying to decide.

I’ll be sure to write a tutorial on drift alignment.

Thanks,
Ray
Steve Wright

02. Nov, 2009

Ray,

I, too, have found your CGEM journey to be very useful. I’m seriously considering matching the CGEM up with a f4.1 Borg 101ED. Your comments are supporting this plan, but I would love to hear more (and I’ll vote for the drift align tutorial, too!).

Cheers,

Steve
Ray Shore

02. Nov, 2009

Steve,

The Borg and CGEM would be an interesting setup. If you go with this combo, would love to hear how it is working out for you. DSLR imaging in mind?

I still need to mount my ED80 on the CGEM and try some astrophotography with the Canon Digital Rebel.

By the way, I just posted my tutorial for the CGEM drift align: http://www.astrophotography-tonight.com/cgem-drift-alignment-procedure. I hope it is helpful!

Thanks,
Ray
Steve

03. Nov, 2009

Ray,

Shortly after I posted my note I found your tutorial (man, your fast!). But seriously, I think it’s a valuable contribution; simple, straightforward. I’ll try it out!.

I currently have a wedge-mounted CPC800 that I’ve been using with Hyperstar. I like it a lot, and have had some reasonable success with my initial forays into astrophotography (I’m using a QHY8; my Nikon D40X is not really applicable). I figure to use the CPC800 on the CGEM, mounting the BORG as a guidescope, giving me the option to use either as the imaging scope with the other serving as guide. At least, that’s the plan.

I’m in the final phases of building a small observatory so I can keep the kit set up. rather than breaking it down every night. The roof should be up within a month or so (work is slowing things down…).

I’m rambling. Again, thanks again for your contributions. I’ll check back from time-to-time to gauge your progress.

Cheers,

Steve
Ray Shore

04. Nov, 2009

Steve,

In order to use your CPC800 as a guidescope, will you have to use a focal reducer? I’m thinking about something similar where I piggyback my ED80 to my 9.25″ SCT on the CGEM. Then I could use either as a guidescope or imaging scope.

Glad to hear my CGEM drift align procedure is a valuable contribution

Thanks,

Ray
Spliney

15. Jan, 2010

Thanks (again ) for these articles so far, Ray. Also I am glad you decided to write a tutorial on the CGEM drift alignment! Thumbs up!

Robert
Ray Shore

16. Jan, 2010

Robert,

Thanks again for the feedback! More articles to come.

Ray Shore
John

22. Mar, 2010

Ray,

PLEASE post something on the CGEM drift alignment. I’m pulling my hair out trying to figure this out. I even have PHD and do autoguiding and cannot keep any pics over 60 seconds due to drift

Please, please, please!!!

John Preston
Rochester, MN
Ray Shore

22. Mar, 2010

Hi John,

I’ve got it covered on CGEM drift alignment: http://www.astrophotography-tonight.com/cgem-drift-alignment-procedure. Please let me know how it works.

Thanks,
Ray Shore
jose

25. Apr, 2010

Hi Ray,

Can the CGEM mount work a lower latitudes (10º degrees)? The GT mount has 2 tricks: 1) remove front latitude adjustment knob; 2) rotate mount 180º.

In GGEM mount, the front latitude adjustment knob doesn’t seem to coming into contact with the R.A. motor assembly. The mount is already rotate 180º (GT mount has 1 leg north and CGEM has 2 legs north).

Celestron support team says that I can use GT mount tricks but I can’t see that.

thanks,
Jose
Ray Shore

25. Apr, 2010

Hello Jose,

At first I thought that it would work at 10º degrees latitude but the user guide says that the CGEM altitude can be adjusted from 15 to 70 degrees.

Regarding the legs, the alignment peg on the CGEM tripod can be turned to the other side so that there is one tripod leg facing north. This is how mine is setup because it fit in my observatory better.

Not sure how you get the CGEM to work at 10º degrees latitude though. Can you explain further what the mount trick is all about?

Thanks,

Ray Shore
Jose

25. Apr, 2010

Hi Ray,

The trick is for the GT-5 mount. 1) Put 1 leg to south in order the counterweight bar doesn’t touch a leg (CGEM has 1 leg to south). 2) Remove the front latitude adjustment screw in order the r.a. mount doesn’t make contact putting a lower latitude.

Thanks,
Jose
Ray Shoe

26. Apr, 2010

Jose,

So according to Celestron, the CGEM can be lowered to 10º by removing the front latitude adjustment screw completely. This is good to know.

Thanks,

Ray Shore
Jose

27. Apr, 2010

Hi Ray,

This is a good news. Thanks Ray.

Celestron wrote me today “Our Alt-Az Goto mounts can be wedged for latitudes less than 10°”. That is not a contradiction with your answer because they are talking about “less than 10°” and not “exactly 10°”. I feel like I’m talking with a lawyer when Celestron answers my questions.

Thanks
Jose
Ray Shore

27. Apr, 2010

Jose,

I don’t know that the CGEM can be lowered to 10° unless it is by the trick you mentioned. I have not verified this myself. But the Alt-Az mounts are different than the CGEM which is equatorially mounted. Why did Celestron mention the Alt-Az mounts I wonder?

Thanks,

Ray Shore
Ed

01. May, 2010

Ray,

I wonder if after the All Star Polar Alignment you went back and did the two star alignment as suggested in the Celestron Manual. I guess that would not have made any difference, except to the Go To performance. Correct me if I am wrong, but I think you are right – the two star alignment affects Go To performance, and the Polar Alignment affects drift after you line up your target.
Ray Shore

02. May, 2010

Hello Ed,

Correct- the two star alignment does affect goto performance. And if you do at least one calibration star afterwards, your goto performance gets much better. Also correct on the polar alignment- the better your alignment, the less drift you will have.

Thanks,

Ray Shore
Jon

06. Jun, 2010

Ray,

What weight would you feel safe using on the CGEM for CCD imaging – the main scope, off-axis guide scope with autoguider, and the CCD camera itself?
Jeffry Turner

12. Jun, 2010

Jon,

I have approximately 35lbs on my setup and that is about the max I would push for imaging. Ideally i would want to be just a few lbs lighter, but so far my setup is working out well, the key is balancing it very well. Depending upon which portion of sky I am on I usually have to adjust the balance a bit. I try to image only one portion of sky at a time.

Regards,

J. Turner
http://www.daltonskygazer
Jeffry Turner

12. Jun, 2010

Jon,

I have approximately 35lbs on my setup and that is about the max I would push for imaging. Ideally i would want to be just a few lbs lighter, but so far my setup is working out well, the key is balancing it very well. Depending upon which portion of sky I am on I usually have to adjust the balance a bit. I try to image only one portion of sky at a time.

Regards,

J. Turner
http://www.daltonskygazer.com