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Question Large declination backlash reported by PHD2 guiding assistant

  • Schaefer
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25 Mar 2022 07:56 #904 by Schaefer
Hi,

I'm using an m-zero mount and I'm always trying to improve my guiding. It is not completely off but in multi-hour sessions my overall RMS is always slightly above 1". Temporarily I see values down to 0.8" but typically it tends towards 1"
I'm using a 188mm (focal length) guidescope and meanwhile an ASI290mm (before I had the 120mm but thought the smaller pixels of the 290mm might improve things - but I didn't see a big difference yet).

What puzzles me the most, however, are the results of the PHD2 guiding assistant regarding my Dec backlash. It reports a severe backlash in the range of 3000-4000ms which I guess should not happen with this mount. Note that I already have relatively high guiding rates configured, in Dec even more than in RA (Dec: 0.75, RA: 0.55).

I attach the screenshots of the guiding assistant. Is that the expected behavior or can I do anything to improve it? What values do other users get here?

Stephan
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25 Mar 2022 08:14 #905 by stephan
Hi!

I See similar results on my M-Uno mount. I am using a QHY MiniGuideScope, which has 130 mmm focal length, and the ASI290mm. I also tried Stellarste yesterday night. The internal guider ran into problems calibrating the DEC (timeout) and issued an error message saying something like you either have mount problems or large backlash. I don't expect that backlash is an issue but some software setting seems to be wrong.

Cheers
Stephan

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  • Schaefer
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28 Mar 2022 09:34 #909 by Schaefer
Hi,

I would be really interested in an official statement to my findings above. The m-zero (as all other Avalon mounts) is advertised with having zero backlash, so this result from PHD should alert everyone.
The graph (and the measurements) clearly indicate that the mount takes a while until it is able to revert its direction in the Dec axis. How can that be?

While experimenting I also tried setting the exact same guiding speed in both axis (I know the Avalon recommended PHD settings use different speeds). As a result, PHD calibration ends with a warning that Ra speed is much too high which might be caused by large declination backlash (this is suggested in the PHD error message).

So, is there any explanation why I see those issues? Is something wrong with my mount? What results are expected by the PHD guiding assistant, especially with regards to Dec backlash?

Regards,
Stephan

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  • Schaefer
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28 Mar 2022 10:18 #910 by Schaefer
This is the part from the PHD log with the exact error message that occurs when calibrating with the same guiding rates in both axis.

Alert: Advisory: Calibration completed but RA and Dec rates vary by an unexpected amount (often caused by large Dec backlash)

Not sure if it is related to the original issue but it also indicates something unexpected. After I reduced the guide speed in RA until calibration completed without that error, the original issue reported by the guiding assistance did not go away though.
This is the part from the log where the guide rates are read from the mount (GetGuideRates returns the same value for both axis: 13.537 13.537 a-s/sec) and later the error message appears, complaining about different rates:
...
20:07:50.605 00.016 2240 Mount::SetCalibration (scope) -- xAngle=-54.6 yAngle=-145.5 xRate=4.971 yRate=3.999 bin=1 dec=19.6 pierSide=1 par=+/- rotAng=None
20:07:50.605 00.000 2240 Mount::SetCalibration (scope) -- sets m_xAngle=-54.6 m_yAngleError=-179.2
20:07:50.605 00.000 2240 ScopeASCOM::GetDeclination() returns 19.6
20:07:50.605 00.000 2240 ScopeASCOM::SideOfPier() returns 1
20:07:50.636 00.031 2240 ScopeASCOM::GetGuideRates returns 0 13.537 13.537 a-s/sec
20:07:50.636 00.000 2240 ScopeASCOM::SideOfPier() returns 1
20:07:50.651 00.015 2240 GetString("/profile/1/scope/calibration/timestamp", "") returns "03/25/22 20:07:50"
20:07:50.651 00.000 2240 GetDouble("/profile/1/scope/calibration/xRate", 1.000000) returns 0.004971
20:07:50.651 00.000 2240 GetDouble("/profile/1/scope/calibration/yRate", 1.000000) returns 0.003999
20:07:50.651 00.000 2240 GetInt("/profile/1/scope/calibration/binning", 1) returns 1
20:07:50.651 00.000 2240 GetDouble("/profile/1/scope/calibration/xAngle", 0.000000) returns -0.953655
20:07:50.651 00.000 2240 GetDouble("/profile/1/scope/calibration/yAngle", 0.000000) returns -2.539180
20:07:50.651 00.000 2240 GetDouble("/profile/1/scope/calibration/declination", 0.000000) returns 0.341761
20:07:50.651 00.000 2240 GetInt("/profile/1/scope/calibration/pierSide", -1) returns 1
20:07:50.651 00.000 2240 GetInt("/profile/1/scope/calibration/raGuideParity", 0) returns 1
20:07:50.651 00.000 2240 GetInt("/profile/1/scope/calibration/decGuideParity", 0) returns -1
20:07:50.667 00.016 2240 GetDouble("/profile/1/scope/calibration/rotatorAngle", -888.000000) returns -888.000000
20:07:50.667 00.000 2240 GetInt("/profile/1/scope/calibration/focal_length", 0) returns 188
20:07:50.667 00.000 2240 GetDouble("/profile/1/scope/calibration/image_scale", 1.000000) returns 3.181750
20:07:50.667 00.000 2240 GetDouble("/profile/1/scope/calibration/ra_guide_rate", -1.000000) returns 0.003760
20:07:50.667 00.000 2240 GetDouble("/profile/1/scope/calibration/dec_guide_rate", -1.000000) returns 0.003760
20:07:50.667 00.000 2240 GetDouble("/profile/1/scope/calibration/ortho_error", 0.000000) returns 0.844124
20:07:50.667 00.000 2240 GetInt("/profile/1/scope/calibration/ra_step_count", 0) returns 11
20:07:50.667 00.000 2240 GetInt("/profile/1/scope/calibration/dec_step_count", 0) returns 13
20:07:50.667 00.000 2240 GetDouble("/profile/1/scope/calibration/orig_binning", 1.000000) returns 1.000000
20:07:50.667 00.000 2240 GetInt("/profile/1/scope/calibration/last_issue", 0) returns 0
20:07:50.683 00.016 2240 GetString("/profile/1/scope/calibration/orig_timestamp", "Unknown") returns "03/25/22 20:07:50"
20:07:50.683 00.000 2240 GetInt("/profile/1/scope/calibration/orig_pierside", -1) returns 1
20:07:50.683 00.000 2240 GetString("/profile/1/scope/calibration/ra_steps", "") returns "{0.0 0.0}, {2.3 -2.4}, {3.5 -4.0}, {4.3 -5.4}, {6.1 -8.7}, {7.7 -10.2}, {8.8 -11.8}, {10.4 -13.7}, {11.7 -16.3}, {13.2 -17.6}, {14.0 -20.0}, {15.8 -22.3}, {15.8 -22.3}, {9.2 -13.4}, {3.2 -4.2}, {2.0 -1.3}"
20:07:50.683 00.000 2240 GetString("/profile/1/scope/calibration/dec_steps", "") returns "{0.0 0.0}, {1.5 1.3}, {2.6 2.5}, {5.0 3.2}, {6.1 4.6}, {8.0 5.7}, {9.4 7.1}, {11.5 8.5}, {13.5 9.5}, {14.8 10.3}, {16.3 11.8}, {18.4 12.6}, {20.0 14.1}, {21.4 14.7}, {21.4 14.7}, {19.0 13.0}, {12.8 7.9}, {6.6 4.7}"
20:07:50.698 00.015 2240 GetBoolean("/Confirm/1/CalWarning_Rates", 1) returns 1
20:07:50.698 00.000 2240 Alert: Advisory: Calibration completed but RA and Dec rates vary by an unexpected amount (often caused by large Dec backlash)
20:07:51.261 00.563 2240 Calibration alert details: Expected ratio at dec=19.6 is 0.942, actual is 1.243
20:07:51.261 00.000 2240 Status Line: Calibration complete

I'm happy to share the whole logfiles of course.

Stephan

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  • Stefano82
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30 Mar 2022 13:23 #911 by Stefano82
Avalon Instruments mounts are backlash free, what the PHD2 software calibration notice as a "backlash" it is the belts elasticity behaviour, that comes with the Belt Drive System.

This can happen during the calibration process specially in case of a certain unbalancement on the DEC, on the other hand, while the mount is in tracking, the motors move only in one directions and the corrections are performed by increasing or decreasing the speed.

More suggestions about the calibration can be found at the following link:

www.avalon-instruments.com/forum/welcome...near-the-equator#347

Best regards

Stefano
Avalon Instruments Team

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  • Schaefer
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15 Apr 2022 10:41 #925 by Schaefer
Hi Stefano,

thanks for your reply! However, I guess I need to bother you again, as I still do not really understand, why I see those results.

This can happen during the calibration process specially in case of a certain unbalancement on the DEC, on the other hand, while the mount is in tracking, the motors move only in one directions and the corrections are performed by increasing or decreasing the speed.

I would like to clarify again that I was using the Guiding Assistant and not the Calibration process. And I am talking about the Dec axis here, which should not move at all during tracking.

So you are saying that because of the belts the mount is backlash free but PHD measures a lot of backlash due to the belt's elasticity. That sounds contradictory to me. From what I understand, the Guiding Assistant does exactly the same as the normal guiding process (in terms of which commands are used to move the mount's axis). So it first guides the mount to the north and then back to the south and measures how long it takes until the star moves back. Without any backlash this should happen immediately, but in fact it took 4(!) seconds in my case (see my screenshot in the initial post). Just because of elasticity? That means whenever during real guiding PHD needs to revert the Dec direction it will take 4 seconds to do this?

In all your tests, did you ever encounter measurements like this or do you never use the Guiding Assistant?

What results do other forum members get when running the Guiding Assistant?

Please don't get me wrong, I really want to understand what I'm seeing here. In the end it is all about improving my guiding which I never got consistently below 1" for whatever reasons.

Best regards,
Stephan

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