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Declination Error

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We have had a Paramount MX+ running with The SkyX at our local observatory for about 2 years now, and recently we have started to have declination errors of around 8 deg when pointing after homing from startup.   Initially I thought it may have been the model which had been corrupted in some way because of the error that was reported by The SkyX when we tried to sync to an object, but changing to an identical PC we have set up as a backup for our main PC we got the same result, although not immediately as it worked fine for a couple of weeks before exhibiting the same declination error of about 8 degrees.

One of our members who has experienced the problem most feels there may be some correlation with the direction the mount approaches the target position from, or whether it flips on its way to getting there, but we are currently uncertain of the best approach to identifying the cause of the problem so felt it worth raising a query here to see if the symptoms were known to others.

 

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What exact version and build of TheSkyX is being used with the mount? And is this on Windows?

Are you using a TPoint model? If so, have you tried performing a recalibration run? If your mount is fixed, rather than portable, you can use the Recalibrate Permanently Mounted Telescope option.

When the mount is homed, the telescope should be pointing to zero degrees declination.

Brian S. Rickard
Software Bisque, Inc.

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Hi Brian,

 

I will check the version and build when I can next get to the observatory, but it was an update I installed direct from your site approximately 8 weeks ago and is running on Windows 7 with all latest updates installed.

 

Yes we are using a TPoint model but we had been avoiding a recalibration run on the basis that it had been working fine for 2 years, and still works fine sometimes.   The mount is permanent on a concrete pier in our observatory   

 

I will also check that the home position is in fact 0 degrees declination

 

Leith

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I have just been advised by one of our members the "The home position showed as Declination +00 28' the other night"

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The SkyX is version 10.5.0 build 10305 running under Windows 7 Pro with SP1

 

 

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Sorry, was off for the holiday, so just getting back to you. Has there been any new development in the meantime?

New Plymouth Astronomical Society:
I have just been advised by one of our members the "The home position showed as Declination +00 28' the other night"

Well that's a lot less than an 8° discrepancy, were it to be the "smoking gun", but just how significant the 28' is depends upon how it was determined. Was the mount homed and then the declination of the centre of the telescope's field measured (perhaps by doing an Image Link)? If so (or something equivalent), then might it be that the mount's polar axis is off by a similar amount? Or might the OTA's axis not be copasetic with the mount? Or a combination of these?

It might be good to perform a new full TPoint calibration run (you can keep the previous one as a backup, should restoring it prove desirable). If you still have the pointing problem after that, sharing your TPoint data (.TPTX) file might allow us to glimpse some further insight into what might be going on with the OTA/mount assemblage.

New Plymouth Astronomical Society:
The SkyX is version 10.5.0 build 10305 running under Windows 7 Pro with SP1

I don't think it will cure your symptom, but you might give serious consideration to updating your copy of TheSkyX Professional Edition for Windows to the Latest Update (as this is presently build 10305, you can skip it, but I include it for future readers), and—optionally—the latest Daily Build (presently build 11177). Just be aware that you'll need to re-define your mount's Park position after updating, as the method of storing this information changed with an intervening build.

Brian S. Rickard
Software Bisque, Inc.

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The only new development so far is the following detailed description of the problem we are experiencing, as told by one of our most experienced members, which might help in identifying the cause ...

 

The following is the procedure which I have used when using the Pukaka (the name we have given to this particular dome and telescope) Telescope.

1.       The computer is booted up normally.

2.       The telescope is switched on normally.

3.       The telescope is then set to the Home position, either by double clicking the button on the hand paddle, or by using the ‘Home Position’ in the Start-up Menu.

4.       The telescope is the told to go to a synchronisation star, using Sky X

5.       At this point, the telescope often (but not always) sets off from the position of the synchronisation star by ~ 6-8 degrees.

Initially the telescope was then driven to the star, judging by eye, and driving in both RA and Dec. Upon attempting to synchronisation once the star was in the centre of the field, and error message appeared, stating that synchronisation was not possible as the error was too great.

During my investigations, the telescope was normally set to Achernar in the South-eastern sky, but on another occasion I used Fomalhaut in the North-west. At this point I discovered that it the telescope was set to a star on the Western side of the mounting, the telescope invariably had an offset error to the North of the star, but if the telescope was set to a star on the Eastern side of the mounting, the telescope invariably set to the South of the star.

This showed that the telescope was invariably offset rotationally around the Declination axis in a ‘clockwise’ direction.

What was not originally noted was whether the error was in both axes. However, during one run, I discovered that it had set correctly in Right Ascension, and the error was totally in Declination. Upon further investigation, once this had been noted, the telescope was driven to the star purely using the Declination adjustment, and synchronisation was achieved each time. On further investigation it appears as if this true in all cases, and that the error is purely a Declination one.

This leaves me with the possibility that the errors caused by driving the telescope in both axes to correct the position prior to synchronisation, confused the system resulting in the error message, and that just correcting in Declination only is a solution to the problem. However, it does not explain why the error occurred in the first place or, why the error is so large. The actual location of the ‘Home Position’ shows that it sets to the correct location in Hour angle, but a minor error of ~28’ occurs in Declination. An approximate half degree error in the Home Position is of no real consequence, but it is after initial homing, and then setting to a synchronisation star that the problem occurs, apparently randomly.

It does not have the error every time, and if it synchronises correctly the first time there is no further error in setting on that particular night. The error appears randomly, as it will appear on one night, but works perfectly the following session; or vice versa, regardless of whether the telescope has been sitting for several days without use.

 

I have not yet investigated whether boot-up, or shutdown procedures by different observers have any influence on the offset errors, but it seems unlikely since there is really only one way to do it.

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New Plymouth Astronomical Society:
5.       At this point, the telescope often (but not always) sets off from the position of the synchronisation star by ~ 6-8 degrees.
Something's definitely amiss (most likely an incorrect synchronization).

  1. What is the exact version and build of TheSkyX?  (TheSkyX 10.5.0 is strongly recommended.)
  2. From TheSkyX, click Telescope > TPoint Add On. 
  3. On the Model tab, what are the values of IH and ID?  Or, can you share the TPoint Settings file (.tptx)?

Daniel R. Bisque

Software Bisque, Inc.

862 Brickyard Circle

Golden, Colorado 80403-8058

Office Hours: 8 a.m.-5 p.m. Monday-Friday GMT-7:00

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To reiterate:

New Plymouth Astronomical Society:
The SkyX is version 10.5.0 build 10305 running under Windows 7 Pro with SP1

Brian S. Rickard
Software Bisque, Inc.

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The SkyX is version 10.5.0 build 10305

I will take a copy of the TPoint Settings file in the next day or so and post here


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Here is the current T-Point Settings File ...

 

*** Note : I've deleted the content of the tPoint file that I did have in this post as I had incorrectly attached it ... see my later posts for the correctly attached files

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New Plymouth Astronomical Society:
Here is the current T-Point Settings File ...
Please read this article for details how to share files here.  (You've somehow posted the binary format of the TPoint Settings file, which cannot be easily opened on this end.)

 

Daniel R. Bisque

Software Bisque, Inc.

862 Brickyard Circle

Golden, Colorado 80403-8058

Office Hours: 8 a.m.-5 p.m. Monday-Friday GMT-7:00

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My apologies ,,, It should be attached correctly now

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After the last series of declination errors, I had re-imaged the PC to a complete copy of the hard drive to a time when there were no issues, and it ran well until recently when it began exhibiting the exact same problem again ... Attached is the tPoint model file following the return of the error ... I replaced just that one file with a known "good" copy as previously messaged to you here but with no improvement ... I restored the PC to a "previous known good restore point" but with no improvement ... My next step will be to reimage the hard drive again tomorrow, because that has worked previously 

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New Plymouth Astronomical Society:
After the last series of declination errors,
Thanks for the TPoint settings.

  • IH is 4,973 arcseconds (that is, the difference between the position of the star and the control system in RA is about 1.4 degrees). 
  • ID is 19,187 arcseconds (that is, the difference between the position of the star and the control system in Dec is about 5 degrees). 

This means the telescope is pointing almost 5.3 degrees away from where the control system thinks it should be. Any ideas why?  Is an accurate time base being used (IH only)?  How is the OTA attached to the Versa-Plate?

From here, please try these "baby steps":

  1. Install the latest daily build (standard practice to ensure we're on the same page).
  2. From TheSkyX, restore the default Paramount MX+ settings: a) From TheSkyX, on the Telescope window, click Tools > Bisque TCS.  b) On the Parameters tab, click the Restore Defaults command from the Commands pop-up menu. c) Click the Save All Parameters To Mount command on the Commands pop-up menu. d) Click the Copy Parameters (From This Window To Clipboard) command on the Commands pop-up menu.  e) Launch Notepad, click Edit > Paste, save this text file, and share it here.  This will help us ensure there's nothing wrong control-system configuration wise.
  3. After backing up the existing TPoint model (so it can be restored later), from TheSkyX, click Telescope > TPoint Add On, and click the New command from the TPoint Add On Settings pop-up menu.  This clears the existing model.
  4. See "Labeling the Home Position on the Sky Chart" in the Paramount GEM User Guide.  After labeling the home position on the sky chart, turn off the mount, turn it on again, and issue the Find Home command.  Do the telescope cross hairs return to the labeled home position?  If not, what is the approximate angular distance between the cross hairs and the labeled home position?  (Use the Angular Separation tool to measure it).

We'll go from here.

Daniel R. Bisque

Software Bisque, Inc.

862 Brickyard Circle

Golden, Colorado 80403-8058

Office Hours: 8 a.m.-5 p.m. Monday-Friday GMT-7:00

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