Re: Astrophysics GTO 3600 V2, pointing shifts 2 minutes of time in right ascension

Mark Foster

Hi Roberto,

I have a 3600 running ACP as well.  A few things to consider:

  1. With a decent sized scope and mount, it's not unusual for big slews to be several arc-min off, even with the best pointing model.  But several arc-min should still be well within your camera FOV (you mentioned 18x18?) and well within PinPoint’s ability to quickly plate solve and perform a pointing correction. Properly configured, ACP will automatically do pointing corrections and center the target after a slew as long as PinPoint and ACP are configured properly (e.g. pointing exposure time, bin, filter, default plate scale, catalog).  Make sure pointing corrections are enabled in ACP, and that PinPoint is working correctly and able to plate solve.  The ACP documentation shows how to test all of this.  This way, ACP will center your targets and get good shots even with sloppy initial slews as long as everything else its working.
  2. For really bad slews (the 30 arc-min you mention), ACP can also do pointing corrections, but make sure the all sky plate solving feature is properly configured and tested.  If you have poor internet connectivity from the ACP host PC, you can install a local copy of the allsky plate solver and database, and configure ACP to use the local copy of the astrometry server for plate solves PinPoint isn’t able to solve for.  The ACP documentation also explains how to do this.  This way, you’ll still be able to shoot targets while diagnosing the root cause of the slewing problem.
  3. With a large scope and counterweights, the gear mesh on the 3600 will need to be regularly adjusted to avoid gear chatter which can contribute to slewing and guiding issues.
  4. Check the gear lube on the RA and Dec axis gears.  AP now recommends, I believe, AeroShell 33MS Grease.
  5. Check your Y-cable on the mount.  Mine developed a flakey connection and then later the RA axis died altogether, even though it was safely run though the inside of the mount.  It does happen.
  6. I’d also check the RA and Dec mount clutches to make sure they haven’t come loose and there’s no slipping
  7. If you have a keypad attached to the CP3 or CP4, make sure it’s set to not auto-connect, as this can override the mount time and location set by the PC and send slews off
  8. If you have an older CP3, check the control chip rev.  You might be overdue for an upgrade, and the older revs may not have automatically had the precision encoders enabled by default (mine wasn’t).
  9. Make sure you’re running the latest AP ASCOM driver, of course, and that the driver is set to sync mount time and location to the PC.  Also double check the GPS location coordinates in ACP and the AP ASCOM driver.
  10. Install a real NTP client (e.g. Meinberg) on the PC, one that will auto adjust the PC clock time to keep in time sync.  If there’s not a decent stratum 1 or 2 time server on the local network, invest in a cheap GPS receiver (they plug in with USB) or a GPS NTP time server (Time Machines by CSS work great and are very inexpensive) via ethernet and sync your PC time to that.
  11. Make sure you only turn off mount power after the mount is parked (AP says you shouldn’t have to do this, but I get the most reliable pointing if I do.).  This might take some testing with ACP, as ACP tends to auto unpark the mount when it finds it parked, so make sure your ACP shutdown script does leave it in a parked state before powering down.  The AP driver settings should be set to restore last position on power up.
  12. Check your scope balance.  That’s easiest with an amp meter in the DC power path to the mount to measure the load when tracking or slewing.  Powerwerx have a DC inline power analyzer for $50 that works with the Powerpole system used by AP now.  Bad balance can adversely affect the gear mesh and wear over time compared to keeping it balanced.
  13. And lastly, check your pier and all mounting hardware…

Good luck!

R, Mark

On Aug 23, 2017, at 7:00 PM, Roberto Avilés A. rlaviles@... [ap-gto] <ap-gto@...> wrote:

Oh!, Thank you Sir! I have been very concerned because I have spent amounts of time and been unable to find what is going on. Sometimes we have perfect nights, and next night, all is lost. I cannot 'blema' the mount, it might be the SW (ACP) but they have their arguments.... any suggestion or idea will be very helpful, thanks in advance for your time.


On Wed, Aug 23, 2017 at 7:53 PM, Howard howard@... [ap-gto] <ap-gto@...> wrote:

Hi Roberto,


I am copying this to my normal work account to give a more full answer.  This situation is a bit too complex and unique for the user’s group, although they are certainly always welcome to respond.


Mag. 7 Skies!


Howard Hedlund

Astro-Physics, Inc.

Phone: 815-282-1513

Please include this e-mail with your response.


P Consider the environment before printing this e-mail.



From: ap-gto@... [mailto:ap-gto@...] 
Sent: Monday, August 21, 2017 4:21 PM
To: ap-gto@...
Subject: [ap-gto] Astrophysics GTO 3600 V2, pointing shifts 2 minutes of time in right ascension



I have been remotely operating an Astrophysics 3600 GTO V2 mount (plus a an Officina Stellare RC500, using ACP + MaxIm DL and related SW.) In last weeks I have found the pointing fails almost consistently in sending the telescope 2 minutes of time shifted in Right Ascension (meaning more than 30 arc-minutes, well beyond the size of our CCD.) I have started pointing models from scratch more than once and, at the beginning, for the fist 2 or 3 nights it behaves well but then it starts shifting in RA and the ACP+MaxImDiffLimited Pinpoint, continuously adds points to the pointing model so the model is constructed on not less than, say 25 points, may be 86 or more.  Also, there is a 2-3 arc-minute shift in Declination which is acceptable for our 18*18 arc-minutes CCD.

I asked AstroPhysics but I have not received any answer about how to access/read the encoders, so to know if the telescope is going or not to the place I command (so far the answer is nones but we do not know why.) 

This may be a timing issue as long as the shift is more notorious in Right Ascension. The telescope is remotely operated using TeamViewer and sometimes we lost connection but it recovers easily. I use to synch -daily- the PC clock against the 'authorized' UTC clock (the telescope is installed in Cerro Tololo, Chile) and I have not found nothing strange there. We knew there was a mechanic issue, the scope have some jumps, back and forth but those are in both RA and DEC and not real big, are sort of a very slow drift. Also, we know the scope is sensitive to 'high' winds (beyond 16 mph i.e. 25 km/h, and the image is blurred, moved but that is mostly a Declination issue and we have some control using the dome as a way to 'slow' the dominant North wind (ours is a 2 halves dome.) 

This is something 'different', example, given this RA issue, last night observations are 100% useless.

I will appreciate if someone have a clue, tip, hint, idea, suggestion....Thank you very much in advance.

Roberto Aviles A ---- rlaviles@...

p.s. may be the subject looks redundant but my point is a 2-3 arc-minutes shifts in declination or right ascension, is not that bad but, a 2 to 3 minutes of TIME shift in Right Ascension is a tragedy.


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