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why my celestron goto mount as‐gt not precise?

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#1 ahmedalaghbarey


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Posted 15 September 2020 - 01:58 PM

When I try to find an object it’s way out of view. My best guess is about 1°. The telescope is 200/1000 and I have a 40mm 40° ocular, which should give me a FOV of about 1°35′. Some objects I can’t even see it with the finder scope, which has a slightly larger FOV. But it seems to depend on the object, for example last night it was possible to find Mars with the finder scope. It was right at the edge.
I did repeat alignment multiple times but always got about the same bad results.

Possible problems that I can think of:

I messed up data entry before 3 star alignment. I use GPS from my phone. I’m in Switzerland and set the zone to “Zone 1”, which is GMT+1, right? Time was certainly correct. Daylight saving was activated. I just checked again and everything seems fine.I messed up alignment somehow. The index marks are there and I moved both. I did align the German mount to Polaris before I started with the goto alignment. When I did the actual alignment I could actually find 3 stars to align. It already seemed imprecise there, but with an app on the smartphone I could find them. So this should be fine. Os it normal that it was already rather imprecise there?The data on the mount is outdated. This thing looks like it’s from the last millennia and it probably is. The NexStar hand control has a RJ9 connection. I can’t see the exact version on it. But it has a large sticker warning me to never target Polaris (but it’s on the list of named stars). “INFO” doesn’t show me any info about the device. Should I buy a new hand control? I think NexStar+ is compatible with the mount and comes with USB.The motors are bad. How likely is that and how could I test this? I would like to start at Polaris, then find something that is way south, and then go back to Polaris. But the label tells me to never do this. And I’m not the guy who would cross the streams of unlicensed nuclear accelerators, so I won’t do this either. What would be the closest object to Polaris that is still save? Yildun? Epsilon? Zeta? Kochab? Pharkad?My expectations are way too high and this is just normal. That would mean this was a complete waste of money. Sure, it can track objects or rather rotate against the rotation of the earth and that seems to work well enough but then a motorised mount for just that would have been enough.
The mount is a “Celestron AS-GT mount”. I think it’s a C8-SGT. That’s on the manual but this manual is also for C5-s and C9.25-s. I use a different telescope than the one it was originally sold with. It’s a bit larger. But as I understand the Goto uses the data from the alignment and then does some calculations to make sure this doesn’t matter.
The powerbank I use is from Celestron, so there should be no problem with that.
I bought it second hand I couldn’t update the firmware. I just used it as it was.

#2 Stelios


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Posted 15 September 2020 - 02:21 PM

Moving to Mounts.


It's completely normal to not have the initial alignment stars be centered. But once you find these stars, you now need to *precisely* center them. You should use a reticle eyepiece for that, and the mount controls, taking care to ALWAYS FINISH WITH THE UP and/or RIGHT arrows. That means that if you overshoot, go back a long way the other way and approach again with the UP and RIGHT arrows. This is very important for backlash compensation reasons.


The more perfect the centering is, the more perfect subsequent GoTo's will be. For excellent GoTo's, you should use 2+4 calibration (2 alignment stars *and* four calibration stars, centering all).


The mount (depending on version) could be up to 25 years old (with aluminum tripod legs) or 20 years old (with steel tripod legs). 


The part about not targeting Polaris is for alignment. Once you have aligned, you can most definitely slew the scope to Polaris, a pleasant double star. The reason not to use Polaris for alignment is that near the pole stars move very little in RA, so the effect of your adjustments is harder to compute. 


There's a cable that converts RJ12 to DB9 and another that converts DB9 to USB, so you don't need to buy a new handcontrol to just connect the mount to USB. 


The CG-5 (AS-GT) mount is a fine mount, and if you have a little patience with it, it will repay you well. 

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#3 sg6



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Posted 15 September 2020 - 02:25 PM

Well sounds/reads right. The problem is in the detail.

First the filed is 1.6 degrees, agree with that, but it is really +/- 0.8 degrees.

If a target is 1 degree off from the center it is out of view. The world is sneaky like that.


The mount is polar aligned - how level is it? Basically the more accurate the better.

Where did the scope start from? Polaris or what/where.


For the goto alignment the software has to assume so initial position, and being software it assummes that it is absolutely perfect. So if you start 1 degrees out the first star is 1 degree out, you then center it and the software works out how bad you did it.


So yes usually the first star will be out of view. At least in the main scope.


What does the manual say about alignment?

One was and maybe still is: Goto and center the first star in the finder, Press Enter, center the star in the main scope, Press Enter. Then off to the secoond alignment star. So 2 presses of Enter per alignment star. Just wonder if you have one that does that and you are not doing the full set of requirements.


Unsure of timezone - GMT+1, it is the most likely but some doubts as the EU covers 2 zones now and uses 1 time. But GMT+1 is the most probable. The catch to me is the scope has no idea of political demarkations.


Need to get the finder and scope aligned, makes life easy and if you need 2 Enters could be essential.


Scopes sounds old so I doubt another handset will operate on it.

It will need a good power supply, if you can find it in the menu, and if it there, could be a good idea to reduce the Max Slew rate to 7(ish).


Bit surprised there is a 3 star alignment. The third one is for "Cone angle" and on the older mounts that was a problem. The required accurancy was too high and usually alignmnet failed. Try 2 star.


As said the detail is likely the problem, it all needs to be reasonably accurate.

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#4 Stelios


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Posted 15 September 2020 - 04:23 PM

Get the mount reasonably level. The actual level of the mount doesn't affect GoTo at *all*, what it affects is the difficulty of doing accurate *polar* (not star) alignment, especially for astrophotography.


The finderscope has a field of view much wider than one degree. Each star should be first centered in the finderscope, and *then* in the reticle eyepiece. But EVEN IF the star is out of the finderscope FOV (can happen) what you can do is put one eye in the finderscope and look through *while keeping the other eye open*. You will see the star with the open eye that is not looking in the finder, and then you can slew the mount closer with the hand control until it *is* in the finder. 


The MOUNT should be pointing (roughly) at Polaris. You do that by sighting through the mount hole (or using a polar scope, or using something like Sharpcap Pro if you're doing astrophotography). The MOUNT and the SCOPE do *NOT* point at the same place when the mount is at the index marks (a common misapprehension). It is the *mount* axis that matters. The scope must be at the index marks. If, after star alignment, you do slew to Polaris, you will see that the scope is angled quite a bit. 


The cone error is a problem on every mount in this class. It *must* be fixed for accurate Goto's, and that's what 2+2 or 2+4 alignment achieves. 


I had two CG-5's, one of which I used for over 13 years. When I learned to follow GoTo instructions exactly, every object would be close to center of a 10mm EP on a C9.25". As night progresses this changes and you need to add additional calibration stars to recover the accuracy. 

#5 RTLR 12


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Posted 15 September 2020 - 08:28 PM

Any one but me notice that he did a 3 star alignment? What is the version of your operating system in the hand controller?


#6 Tom Stock

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Posted 15 September 2020 - 10:27 PM

Time zone in Switzerland (GMT+2)


Daylight savings time ended on the 15th



Start by using a small bubble level on the tripod head to make sure it's level. This will make the initial alignment gotos more accurate.


Is your polar alignment scope aligned correctly with the RA axis?  When you rotate the mount the cross hairs should stay centered on an object.


Are you rotating the RA axis to the correct position (aligned with ursa major or minor) and then centering polaris in the little circle, or are you centering polaris in the cross hairs? It should be in the circle, not the cross hairs.


Make sure the time AND DATE is correct. I once had the wrong month or year and got very strange results.


Make sure your OTA is aligned parallel with the dovetail bar... otherwise you have have cone error. I use shims between dovetail bar and OTA.


It's normal for the alignment stars to be a bit off, but after alignment gotos should be very close.

Edited by Tom Stock, 15 September 2020 - 10:33 PM.

#7 Cooksterman


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Posted 22 September 2020 - 09:53 PM

I have very similar issues with my CGEM DX.  This week we have had great skys.  Three nights in a row I've  seen 1 degree issues.  Tuesday night I was out 3. Factory reset the Nexremote + and solved that issue.  Had the same issues a few weeks ago.  I calibrated the polar scope...very tedious and requires a great deal of patience.  Got it done though.  Next I Factory Reset the Nexremote + to assure that there were no errors or corrupted data points.  Installed the I realize that Stellarium introduces about a 2 degree error in scope management. I have been using CPWI 2.3.4 as a result. Assured that the scope was perfectly balanced with both bubbles centered.  Polar Aligned with Ursa Major in the proper assention.  The delta was consistant for 3 nights of viewing. Perhaps someone has a good solution?  

#8 skywolf856


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Posted 23 September 2020 - 08:21 PM

When you do your star alignment do you use Uncle Rod's method of Up & Right on the final telescope moves to eliminate any backlash?

#9 JGlover



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Posted 23 September 2020 - 11:55 PM

You mentioned you used three stars for alignment. As Stan said, that is not usual for an ASGT, it sounds like possibly you have the wrong controller, perhaps an AltAz controller for a Nexstar GT mount. If I recall on the bottom of your contoller, inside near the RJ9 port, the cpntroller should be marked GEM. That will have the correct firmware for the ASGT mount. If it is marked at GT, NXS  or something other than GEM, it is probably and altazimuth hand controller.


I just purchased a new AVX a few months ago and it arrived with a dead HC. A quick call to Celestron and new one was sent out. I immeadiately noticed it was wrong as the lignment options were wrong...my first choice was SLT. I flipped over the controller and they sent me the incorrect NXS version. I ran a formware update and installed the GEM formware and everything was fine. So check your controller to see if it is the correct firmware for a GEM mount.

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