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Very bad tracking with CPWI

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#1 Stephanpls


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Posted 27 February 2021 - 11:20 AM

My telescope is a Celestron Nexstar 127 SLT.


I connect my laptop to the mount via hand control. Alignment with CPWI is easy, I like it this tool.
Then, after slewing to a planet I notice that tracking is bad. After half an hour the scope points about half a degree away from that planet! Quite a nuisance.

Tracking rate is sidereal.


I thought it might be a power problem.


But .... if I do an alignment the old way, that is by using the hand control only, there is no problem at all. After alignment, and slewing to an object the scope tracks the object very well.


What may be the cause of this?

#2 mlord



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Posted 27 February 2021 - 03:42 PM

When aligning, do you finish all final movements to reference stars with UP/RIGHT motions in both CPWI and the HC?

#3 archer1960


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Posted 27 February 2021 - 10:01 PM

It sounds to me like your polar alignment is off. A Star alignment gives you goto accuracy, but not tracking.

#4 Stephanpls


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Posted 28 February 2021 - 02:57 AM

Useful answers! I'm still a newbie in this matter. I'll try it if we have clear skies again.

#5 mclewis1


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Posted 28 February 2021 - 10:39 AM

If Stephan is using his SLT in it's original stock configuration (in Alt Az mode) then polar alignment doesn't enter into the picture.


First I certainly would double check the power source for the mount. Power problems can show up as all sorts of strange and non sensical behaviors. Is this a fairly new SLT scope? What are the firmware levels (from the Version command in the utilities menu)?


Tracking accuracy on a Celestron Alt Az goto mount is dependent on how accurate the initial alignment is. A more accurate initial alignment the better the tracking will be because the mount mimics the RA sidereal motion from simultaneous motions of both the Alt and Az axis'. Those motions vary in different areas of the sky. There can be areas of the sky that a mount doesn't track well in. If you are comparing the goto and tracking accuracy you should be using objects in the same part of the sky. In general, if the goto accuracy is good then the tracking accuracy should be as well.


So first a good initial two star alignment is preferred. Good goto accuracy is the measure of a good initial alignment. As Mlord mentioned using consistent up/right motions for the final centering is important. 


Stephan is the tracking (and/or goto accuracy) different between DSO (stars, etc.) compared to solar system objects? If so this is usually because of a problem with local time/location settings. Solar system object positions and motions are calculated from those settings (because they are constantly changing). The static positions of the DSOs are calculated once from the initial alignment. Since CPWI gets it's local time from Windows it is usually very accurate, but location data could be an issue (depending on what location data is used - manually entered or from Windows). I would carefully go through the user data settings (time, date, location, DST setting, and time zone) in CPWI to see if they match those used in your physical hand controller.


Another potential issue affecting tracking is physical backlash. This would however normally also affect the tracking and gotos of all the objects (both DSO and solar system) so I doubt this is your issue. It might however be good to just check this between the physical hand controller and CPWI ... even though the settings for this compensation are supposed to be stored on the motor controller board in the mount and are therefore common no matter which hand controller is used for the initial alignment. https://www.nexstars...ds/Backlash.htm

#6 speedster


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Posted 28 February 2021 - 05:11 PM

CPWI is doing the same math the hand controller is doing although it can go much further by allowing many more points in the sky model.  The minor difference you are seeing in tracking could be from HC alignment stars being in one part of the sky and CPWI alignment stars being in another part of the sky.  Could also be centering stars at low power as opposed to using a high power reticle.  Half a degree off in an hour is really not at all bad for that setup. 

#7 Noah4x4



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Posted 01 March 2021 - 02:03 AM

A few suggestions.

CPWI asks you to start with your OTA "roughly" aimed North. I discovered that it was best to always use a compass, especially as Polaris isn't visible from my rear garden. The less accurate this initial starting 'Home' (North) position, the larger my GoTo and tracking errors. This is quite logical on an Alt-Az mount that you don't Polar Align (unless on wedge). Levelling your mount might also contribute. .

In a similar vein, I discovered how poor I was at judging the centre of an eyepiece when performing alignments and calibration (StarSense) procedures. I now use a 12mm cross hair reticle illuminated eyepiece. However, the defocus star until you can see a doughnut technique also works. Aligning the doughnut with the centre of the eyepiece is easier than a tiny star. Similar with computer screen. Use the reticle (target) overlays in Sharpcap or ASILive.

Lastly, judge your tracking by reference to stars, not planets. Mars is notorious for tracking due to its retrograde motion.

After taking these measures (and adding additional alignment Stars with StarSense), my GoTos and tracking are awesome. But quality of mount is inevitably material. Accuracy inevitably reflects price.

Edited by Noah4x4, 01 March 2021 - 02:10 AM.

#8 Stephanpls


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Posted 03 April 2021 - 12:16 PM

Thanks for all comments and tips!


After some time we had clear skies again and the same problem persists.

About my scope, I purchased it nov 2018.

About power, I use a Celestron power tank since the purchase. I charge it regularly and the led is always green. Just for a power check, I connected to a/c "from the wall" one time and there is no difference. 

About alignment accuracy, last time the overall RMS error according to CPWI was about 20" (arcsec, for three alignment stars), IMHO this is quite good. Despite that, the tracking error is about one degree per hour, for planets and stars. The distance to Polaris does not affect the error (closer to Polaris the tracking logically is slower because the stars make smaller arcs around it in the same time), I keep getting this one degree error per hour.

About goto results: these are good. 


I keep investigating. How could I check if the alt and az tracking motors are ok?

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