CGEM Drift Alignment Procedure
Posted on 29. Oct, 2009 by Ray Shore in Astrophotography News, Astrophotography Tutorials
This procedure covers the steps I take to perform a drift alignment on my Celestron CGEM telescope. Although the All-Star Aligment routine works really well to get the CGEM closely aligned with the celestial pole, I have found that the alignment needs to be refined further to be suitable for astrophotography. Note that this polar alignment procedure is written for my setup: CGEM mount, 9.25″ Celestron SCT, 1.25″ Diagonal, and 12mm Illuminated reticule eyepiece. The basic concept still applies to other variations of telescopes, but you may have to move your telescope in the opposite direction for azimuth or altitude adjustments. In this case, just make sure you make note of the adjustments you make for future reference! I have documented the drift align method as I use it. I hope it is helpful for you as well.
Accessories:
- Orion 12mm Illuminated reticule eyepiece
- 1.25″ Diagonal (for comfortable viewing!)
Part I: Initial Setup
These steps are performed without the optical tube on. This makes it easy to rotate the mount/tripod combination for rough alignment with the celestial pole! If you have already done the All-Star alignment routine, then these steps are already complete. You can skip to Part II below. Refer to my article, Celestron CGEM Review- Part 3 for more information on the All-Star polar alignment routine.
1. Set the CGEM to the latitude for your location. Latitude for major cities in the US can be found on my other site here. This is done by adjusting the front and rear latitude adjustment knobs and dialing in the latitude on the latitude scale. Reference various parts of the CGEM below.
2. Rotate the CGEM (by spinning the entire mount/tripod) until the polar axis points toward the celestial pole. The celestial pole in my case is Polaris since I’m in the northern hemisphere. Tip: stand behind the telescope and eyeball the alignment. If necessary, rotate the CGEM further until it appears to be lined up with the celestial pole.
3. Level the CGEM using the built-in bubble level.
4. Unlock the declination lock and turn the telescope mounting platform 90 degrees from the home position (where the declination index marks are aligned). This opens the hole in the CGEM’s polar axis for viewing “through” the mount. Note that an optional polar finderscope can be inserted in this hole. Remove the front and rear finderscope hole covers and look through the hole for Polaris. Use the azimuth and latitude adjustment knobs on the CGEM to center Polaris.
Part II: Alignment on a Southern Star (Azimuth)
1. Find a bright star near the intersection of the meridian and celestial equator.
2. Using the CGEM’s hand controller, push the upper and lower direction keys and adjust the eyepiece in the holder (loosen the thumbscrew slightly) until the star moves parallel to one of the illuminated lines. Be sure to tighten the eyepiece thumbscrew when the adjustment is complete.
3. Now center the star on the vertical line using the hand controller.
4. Watch the star to see which direction it drifts away from the vertical line. If the star drifts to the right of the line, turn the azimuth control knob on the west side of the CGEM clockwise. If the star drifts to the left of the line, adjust the azimuth knob on the east side of CGEM clockwise. Continue making the appropriate adjustments until the star remains on the line for several minutes.
Part III: Alignment on a Western Star (Altitude)
1. Find a bright star near the western horizon close to the celestial equator.
2. Using the hand controller, push the upper and lower direction keys and adjust the eyepiece in the holder (loosen the thumbscrew slightly) until the star moves parallel to one of the illuminated lines. Be sure to tighten the eyepiece thumbscrew when the adjustment is complete.
3. Now center the star on the vertical line using the hand controller.
4. Watch the star to see which direction it drifts away from the vertical line. If the star drifts to the right of the line, turn the rear latitude adjustment knob (altitude knob) on CGEM counter-clockwise. If the star drifts to the left of the line, adjust the latitude adjustment knob (altitude knob) on the rear of CGEM clockwise. Continue making the appropriate adjustments until the star remains on the line for several minutes.
Repeat the steps in Part II & III above until the star stays centered on the line for several minutes. Perform this same procedure when the telescope is moved or when there is too much drift during astrophotography.
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
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Ray Shore
29. Oct, 2009
Please let me know how my CGEM drift alignment procedure works for you. I look forward to your feedback.
Thanks!
Ray Shore
Jeff T
29. Oct, 2009
Ray,
Great job in keeping the drift alignment simple. Thanks for the guide, appreciated!
Brian Coniglio
30. Oct, 2009
Ray,
Your reviews are helping a lot, especially considering they are about the only ones available for the CGEM.
I was wondering if you considered the GPS module for the mount. It’s supposed to make the polar alignment even simpler.
Ray Shore
30. Oct, 2009
Jeff,
Thanks for the feedback!
Ray
Ray Shore
30. Oct, 2009
Brian,
Glad to hear that my reviews are helpful. I hope to provide more information as I get more time with my CGEM.
I hadn’t really thought about the GPS module. I wonder if the drift align would be necessary with it? If I find some user information on it, I’ll be sure to pass it on.
Thanks,
Ray
Jeff T
30. Oct, 2009
Ray,
I am still waiting to test out my new mount. I have been working alot of hours and one of scopes are at a friends house in his observatory. Between his hours and mine it has been difficult to time things out to grab the scope. The weather here in Western Ma, has also been horrible. I am planning on hopping into some quick widefield imaging right away. Will post how I make out. Hopefully, this weekend I can pick up my scope. I am looking foward to trying out your procedure. Keep the reviews coming it is hard to find good reviews out there and practical stuff for this mount.
Ray Shore
30. Oct, 2009
Jeff,
Hopefully the weather cooperates for you this weekend! I’ve been having troubles getting time to use mine as well. Lots of projects going! I can’t wait to hear how my procedure works out for you. Let us know how it goes. I’ll try to provide more reviews as I get more use on the CGEM. I need to do something on astrophotography.
Thanks,
Ray
robert campbell
09. Nov, 2009
Hi Ray,
Looks like one of the best set of directions I’ve seen for polar aligning a GEM. I have a SkyWatcher EQ-6 mount which is their name in Canada/ while in the US, they are known as Orion Atlas.
I believe they make the same mount as the Celestron. Anyway, I’ve been struggling with the instructions given in the accompanying SkyWatcher manual, I think yours will do the trick nicely.
Now . . . if it would only stop raining.
Thanks again,
Rob.
q
Ray Shore
10. Nov, 2009
Hey Rob,
The Orion Atlas was my second choice when I bought the CGEM. It looks like a very solid mount for the money. I hear that the Atlas and CGEM are made by the same company- Synta.
Nice to hear that my drift align procedure will work well for you. I always have to refer back to this myself because it’s hard to remember which way to adjust the mount according to the direction that the star drifts. Your feedback is well appreciated! Thanks, Ray Shore
Jeffry Turner
16. Nov, 2009
Ray,
I finally had a chance last week to setup, it was a last minute setup decided while I was on my way home from work. I had to run up to a friends house to pickup my scope so I could get it mounted.
It was dark when I began but overall things went very smooth. I was not prepared when it came to equipment. I have been working on some home renovations and my equipment is located between 5 various rooms in the house. I was able to align very quickly but could not find my higher power eyepieces they are around hiding somewhere. The alignment process went smooth and I was setup and aligned in under 15 minutes. Since I could not find where I placed my eyepiece set I did some visual observing with my 35 panoptic and 20mm nagler which were in another case through my meade 80mm ed 5000 apo…..very nice views. I was very impressed with the mount from my initial quick setup. I am hoping some of my projects at house will wind down soon so that I can get some more time in and get reorgnized. I have a week off coming up later this week hoping I may get a break in weather to set up for some imaging looks to be rain beginning of that time off. I followed your guides without any issues, best I have seen yet. Keep up the articles I look forward to them.
J. Turner
Ray Shore
17. Nov, 2009
Jeff,
I’ll bet you can’t wait to get your project done and get reorganized! I had a night awhile back that was all messed up! When I first got going, I saw an awesome shadow transit occuring on Jupiter. I wanted to image it so I hurried to get everything set up. Of course I had to trip over the power cord to my CGEM unplugging it. Then my computer was giving me problems and by the time I got my camera set up everything was soaking wet with dew. So I had to use hair dryer on the corrector plate. By the time I could actually start imaging, the shadow transit was over
.
Anyway, I’m glad to hear that my drift alignment procedure worked for you. I appreciate the feedback! Ray Shore
algady
03. Dec, 2009
Hi all,
Thank you for this great review, I just need to understand something, when I used your drift align procedure what is next? I mean will the object be centred for long time? how long?
On the other hand, do you think it’s worth to get? please advice me.
Please forgive me because of my poor English.
Again thank you very much sir.
algady
Ray Shore
03. Dec, 2009
Hi Algady,
Using my drift align procedure should keep the object centered in the field of view for several minutes. If you keep fine tuning the alignment, your CGEM should track on an object long enough for unguided astrophotography. Given that you are using a short focal length system for your imaging anyway.
Thanks,
Ray Shore
algady
03. Dec, 2009
Hi Ray
If I understand you correct, that means those several minutes are enough for good exposure time ?
In general, dose that mean that CGEM is NOT accurate enough? if yes is that an issue? or that is normal in most mounts.
finally, what did you mean by unguided astrophotography?
Thanks Mr Ray for your patience.
Regards
Ray Shore
03. Dec, 2009
If the CGEM tracks on the star for several minutes, you should be able to get some decent shots; especially if you are imaging with a short focal length telescope. The longer the focal length, the more accurate the mount needs to be. At a certain point, you may have to use some sort of guiding (usually with a separate camera & scope) on a guide star to track accurately. This is the nature of astrophotography and is normal with most mounts. Unguided astrophotography is where you don’t use a seperate camera & scope to guide. You rely on the mount itself to track accurately. If you are new to astrophotography, I highly recommend doing a lot of research before deciding on a particular telescope. There are a lot of things to consider
. That being said, I have seen some great astro-images from the CGEM.
Ray Shore
xiaodong
23. Feb, 2010
Hi, Ray:
I have the same CGEM and bought the Polar finderscope [this finderscope can also be used for CG-4 mount & CG-5 Advanced mounts. this sentence is said in its manual]. But I am so confused with this manual.
After I setup the finderscope, look through it and can see the pattern: Big Dipper is at 12:00; Cassiopeia is at about 6:00. In its manual, it lets me rotate the mount until one of the pattern matches the real constellation, (http://www.celestron.com/c3/product.php?CatID=16&ProdID=526) please see this address and this is the manual.
If I do as the manual, i.e., in 22nd Nov., Big Dipper is at 6:00 position in the actual sky, I have to rotate the CGEM 180 degree to match the actual constellation. But this makes the OTA touch the tripod,. ….
Ray, now I am so confused about the discription in the finderscope’s manual. Please give me help~~~~~
Looking forward to hearing from you~~~~
Please send a letter to my E-MAIL. lidong2221@sohu.com
Thank you!
Conor
19. Jun, 2010
Hi Ray
First of all, excellent guide. I’ve recently bought a CGEM and put my Celestron 10″ Newtonian on it. I’m something of a novice to astrophotography but an intermediate astronomer and this is the first goto/equatorial mount I’ve owned. I’m still getting used to the many concepts involved in astrophotography. I’ve noticed as you’ve mentioned that good tracking doesn’t last very long and trailing is creeping into my images, even after 30 seconds, as is the maximum with my current DSLR. My question is what to do at this point. Is the CGEM Sync feature sufficient to return good tracking or would you recommend switching off the mount and starting the whole two star/polar alignment process all over again?
Thank you in advance for taking the time to answer my question. I’ll be pointing people to this guide!
Ray Shore
19. Jun, 2010
Hi Conor,
I don’t normally start the polar alignment over. I do a pretty good drift alignment then I go to autoguiding which I talk about here: http://www.astrophotography-tonight.com/orion-starshoot-autoguider-review/. This made a big difference for me. I would say until you get into autoguiding though to just follow the procedure for the All-Star alignment and then do the drift alignment to refine it further. Also, make sure that you balance your mount in both axis for the area of sky that you are imaging in. This should help a lot.
Pat
20. Jun, 2010
H Ray,
Reading your tutotial without a ‘scope or a star to look at, would you please confirm:
When corredting the stars drift, I take it that I would be bringing the star back to the centre line each time. Am I correct? ( I have read elsewhere that to reduce the Azimuth drift I need to move the star FURTHER AWAY from the centerline, the re-center with the hand box before watching for drift again.)
Second question- Does the Permanent PEC survive repeated set-up / breakdown /setup as I have to travel to dark-ish sites for my imaging?
Third question- Do you think that the software alignment (All Star) plus permanent PEC might give me 5 minute exposures without guiding?
As you will guess, I am spending more time aligning ahd PEC traininh with my HEQ5 Pro than I spend imaging.
I am hoping that the CGEM will give me 5 minute exposures before I have to tavkle guiding!
I’ll very much appreciate your comments.
Pat
Ray Shore
21. Jun, 2010
Hi Pat,
I believe what you are reading is true. When you make an adjustment for drift, the star moves away from the center. Then you bring it back with the hand controller to watch for more drift. The better you get the drift corrected, the longer it will take to drift away from the center.
I work with Jeff Turner at http://www.daltonskygazer.com on some astrophotography related projects. He has more experience with PEC on this scope than I do so I’ll add his comments about the subject:
“I do believe that nexremote can retain pec correction. The problem with pec correction is that these scopes use a worm gear. Every 7 or so minutes the worm goes through one full revolution, even which pec correction there will always be one spot that has a bounce on the graph for pec once every revolution. The pec will help tame the scope yes, but ultimately one would want to autoguide this mount. I think a well trained mount with superior polar alignment could get up to 3 min exposures in ideal conditions but not when one breaks down and sets up all the time”.
I’ve been using guiding so I didn’t get into pec. Jeff had some experience with it. I hope this helps!
Thanks,
Ray Shore
Jeff Turner
22. Jun, 2010
Pat,
You can save your settings using PEC TOOL.
Regards,
J. Turner
http://www.daltonskygazer.com
Pat
19. Jul, 2010
I have decided to bite the bullet and have ordered the bits to go guiding! (Unfortunately, no girl.) I am hoping that after the software polar align I can find a guide star, get the guider going, and start imaging. Hopefully, this will sidestep the drift alignment so that i can turn this time into imaging time.
Any comments? Tips on guiding?
I plan to use every chance, during the short summer nights, to get in some practise with the new gear.
Thanks for your help.
Pat
Kenn
13. Aug, 2010
I’ve been using a Fork Mount for the last 40 years and just switched over to the CGEM for my C-8 and C-11. I’m very familiar with the Drift Method, using a Fork Mount. I always had the drive motor on when I used the Drift Method for the Fork Mount, but when I just turn on the CGEM it does not ‘Track’. I can only assume you have your CGEM turned on and at least a two star alignment completed before you start your drift method, correct?? I ask Celestron the same question, but they answered by not answering the question. So my question is: Turn on the CGEM, then Do or Not Do a alignment using the CGEM Hand Control Method, then start the Drift Method?? You mention a Southern Star, I assume that means a Dec of less than 0, eg: -5 ??
Ray Shore
14. Aug, 2010
Kenn,
I have noticed the same thing. When I first turn on the CGEM, it does not track until I do a star alignment. My LX200 would track when I first turned it on. I will ask someone about this and let you know.
The southern star doesn’t necessarily need to be a Dec of less than 0. As long as it’s close to the meridian and celestial equator. Some use a Dec of around +20.
Thanks, Ray Shore
Ray Shore
14. Aug, 2010
Kenn,
From what I am hearing there are 3 options: must do the alignment, resume from hibernation if pier mounted, or do last alignment.
Ray Shore
Kenn
19. Aug, 2010
After a few go arounds with Celestron they finally answered the question: A drift alignment requires that the mount axis of rotation be precisely orthagonal with the celestial pole. The CGEM has a latitude scale but you should not rely on the reading of 34 degrees. Use a angle reader to verify that the vernier is accurately reading the altitude on your CGEM. Also use the All Star Polar Alignment feature that is available with the mount.
The CGEM requires at least a one star alignment to activate tracking.
Do a two star alignment , followed by the ASPA and then one or two calibration stars. After that try the drift method.
Thanks for being there.
Cheers, Kenn
Pat Forster
22. Aug, 2010
Thanks Ray! Do you do this drift align before or after 3 star align? Pat
AstroPhotography Tonight
22. Aug, 2010
Hi Pat,
You’re very welcome! I would do this after the 3 star alignment.
Thanks,
Ray Shore