Guiding Issues
A number of guiding issues occured during the session.  These are listed below in order of significance (with the most significant listed first)

• Incorrect selection of default Dec Guide Mode
  (Polar Alignment adjustment has caused the required default Dec Mode to swap. Incorrect  Dec Guide mode results in uncorrected guiding or backlash issues when the Dec Guide Mode is auto-changed)
   
• Dec Guiding ineffective at controlling or correcting star drift over certain extended sections
  (telescope balance issue ?)
   
• Large jump in Dec from a single Dec guide pulse (e.g. position jumps from +2 to -8)
  (excessive reaction from scope's antibacklash compensation, with indications that backlash in one direction is different to the other direction for the very same telescope position).
   
• Impossible guiding for certain targets
  (cloud, strong wind vibration,  poor seeing)
   
• Unusual spikey movements during unidirectional Dec guiding
  (wind vibration ?)
   
• Unexpected jumps in Dec
  (possible cable hang-up ?, possible association with large focuser movements ?)
   

Examples

The various guiding issues are illustrated by the following examples.
( Red Line = Dec,  Blue Line = Ra, Black line = Star SNR, Green Line = Focus Changes, Orange Line = Star Lost Events)

1) Incorrect initial guide direction & non-effective Dec guiding (Guide Run 3, M31)

- North Guide Mode auto-selected instead of the required South Mode (fixed in AstroMain 3.47.3)
- Drift 18:18:00 to 18:20:30 (Dec Mode should have been changed during this settling phase)
- Dec Mode changed at 18:20:40.  Guide error gradually reduced.
- Drift 18:22 to 18:26. South Guiding not effective at holding or correcting star drift.
- Drift 18:29:30  to 18:33:30. South Guiding not effective at holding or correcting drift (is this a telescope balance issue ?)
 

2)  Incorrect initial guide direction & non-effective Dec guiding (Guide Run 4, Z And)

- North Guide Mode auto-selected instead of the required South Mode (fixed in AstroMain 3.47.3)
- Drift 18:38:00 to 18:41 (Dec Mode should have been changed earlier ?)
- Dec Mode changed to South at 18:41 (anti-backlash either not applied by scope or not an issue)
- Dec Error c. 2" from 18:41:00 to 18:45:30. Guiding stops further drift but doesn't correct the 2" error (is this a telescope balance issue)
 

3) Incorrect initial guide direction, excessive antibacklash (Guide Run 8, 132P/Helin-Roman-Alu)

- North Guide Mode auto-selected at start instead of the required South Mode (fixed in AstroMain 3.47.3)
- Dec Guide Mode changed to South at 19:24:27.  Single Dec guide step moves scope from +3" to -19" .
  Probably due to excessive anti-backlash compensation by scop    (fix attempted with reduction in antibacklash setting from 100% to 90%)
- Dec Guide Mode changed to North at 19:26.    Guide error quickly corrected (no issue with anti-backlash overcompensation)
- Dec gradually drifting (17:29 to 17:32).   Guide mode should have been changed to South at earlier point ?
- Dec Guide Mode changed to South at 19:32.   Guide error quickly corrected (no issue with anti-backlash overcompensation)
 

4)  Incorrect initial guide direction &  non-effective Dec guiding  (Guide Run 15, NGC 9)

- South Guide Mode auto-selected at start instead of the required North Mode (fixed in AstroMain 3.47.3)
- Dec Jump at 20:59:10 (possibly related to focuser change)
- Dec Guide Mode changed to North at 21:02:40.  Guide error gradually corrected (no issue with anti-backlash overcompensation)
- Guiding ok 21:04 to 21:08 despite passing cloud
- Lost star events at 21:08:20 & 21:09:30 effects guiding (cloud)
- Clear sky but Dec gradually drifting (21:10 to 21:19).  Northward Dec guiding not effective at holding/correcting position (is this a telescope balance issue ?)
 

5)  Impossible guiding due to wind conditions (Guide Run 7, C/2017 K2 (PANSTARRS) )

- Wind / Cloud - Guiding impossible under these conditions
 

6) Unexpected jumps in dec,  unusual spikey movements during uni-directional guiding (Guide Run 1, NGC 1569)

- North Guide Mode auto-selected instead of the required South Mode (fixed in AstroMain 3.47.3)
- Dec Jump at 17:30:10 might be associated with focuser change, however the distinct movement first one way and then other is possibly due to a cable
- Over-adjustment in Dec position at 17:30:45, but might be due to seeing / wind
- Dec Jump at 17:32:40 might be associated with focuser change
- Dec Jump at 17:34 but unclear why
- Drift 17:34 to 17:36 . Guide Manager waiting for frame to finish before changing Dec Guide Mode
- Changed to South Dec Mode at 17:36:10.  Takes around 40s to be effective
- Very Spiky Guiding between 17:37 and 17:47 (+/1 to +/-2 arc sec spikes shown in Dec). 
  Guide Star S/N is good and uiding was uni-directional
   It is believed that spikes are a seeing or wind vibration effect ?. Wind from WSW.
- South Jogs not effective in bring guide star back to lock position between 17:47 and 17:49
 

 7) Jumps in dec / lost star events, otherwise good Dec guiding (Guide Run 16, M110)

- AstroMain 3.37.3 installed just before Guide Run 16
- North Guide Mode auto-selected at start which is currently the appropriate choice for West of Meridian.
- Some Dec Spike at 21:26 to 21:30  (probably due to wind / cloud ? , several lost star events)
- Generally good guiding 21:31 to 21:43
- Percentage  <1" Total Error  : 57%   ( <1" Ra Error: 68%,  <1" Dec Error: 91%)

Actions

  Default Guide Mode. Following apparent overadjustment to Polar Alignment in the preceeding S936 session the default guiding directions used for telescope positions either side of the Meridian need to be swapped over
(Done 2021-11-24, AstroMain 3.37.3)

  Excessive Anti-Backlash Compensation ?.  Examine cases where there is evidence of excessive antibacklash compensation with indications that the Dec backlash in one direction is different to the backlash in the opposite direction. 
 (Done 2021-11-26, See Investigation - Excessive Backlash Compensation (2021-11-26) )

  Dec Guide Mode Changes.  If change in Dec Guide Mode is called for then its best to change during Settling/Focusing Stage rather than wait to the end of focusing which places the chnage immediately before the start of imaging.  Also need to react quickly with a second Guide Mode change if there has been a clear overreaction of anti-backlash compensation. 
(To do 2021-11-24, AstroMain 3.37.3)

Update 2021-12-08 .  
Reducing the amount of Dec backlash compensation by the scope hinders the ability to correctly centre the telescope on target coordinates.

Based on the guiding experience from recent sessions it would seem preferrable that the number of Dec Guide Mode changes is minimised as the outcome of a reversal in Dec Guiding direction is unpredictable.   When the guide star is clearly on the 'wrong' side of the N/S zero error line (for the 'best/expected' guide mode) , than rather than changing the Dec Guide Mode to bring the star back to zero error (and then change the Dec Guide Mode again to correct star drift) it may be more prudent to simply change the lock position during either the initial settling/focusing phase or during a pause between frames, that resets the guide error at 0.

 

Issue:  The scope sometime produces excessive (8-18") movements in Dec.

At first sight looks like excessive backlash compensation,  but retrograde Dec motion may also be involved

Whilst the normal advice is to turn off anti-backlash compensation in the scope and use PHD2's own software controlled anti-backlash compensation the amount of Dec backlash displayed by the observatory's LX200 scope is large (and variable) ranging from 2700 to 4000ms (at guide rate of x0.90 sidereal) and unidirectional Dec guiding has to be used.   The Scope's (Anti) Backlash compensation is used to quickly compensate for any necessary backlash when making initial selection of Dec Guide Mode where it involves a change from scope's last direction of dec movement.  PHD2's own Backlash Compensation is disabled for guiding runs.

Description : The issue was first noticed in session S932 (2021-11-09).  See LX200 - Dec Guide Anomalies (SSE Sky) and is illustrated by the example below.   In this case there is apparent overreaction by the scope's anti-backlash system f which has moved the scope's Dec position from +3" to -5". Early time (first 10s or so) guiding isn't plotted, but logs reveal that there was a similar excessive anti-backlash movement in the first few seconds of guiding but in the opposite direction to the later event.



There have been a number of cases since then and they typically occur when the scope is pointing to SSE region of the sky.
In the following example from the S937 session there is a dramatic jump in Dec position when the Dec Guide Mode is changed to South following a single 'small' (3" x 85%) guide pulse.  Dec error curve has exceeded the graph range, but logs reveal that Dec position changed from +3" to -18".   The small guide pulse (reversing the last dec movement made to scope during centering) is assumed to have triggered the scope's Backlash Compensation (Dec Drive recently Trained and Anti-Backlash set at 100%).   When the Dec Mode is changed to North following Frame 1, the Backlash Compensation for the reverse Gear movement is evidently not excessive and several large guide pulses are still required to return the guide star to its lock position (The issue may or may not have been exposed here because the wrong Dec Guide Mode was initially selected)

Analysis

Guide Run 8 (132P/Helin-Roman-Alu)  :  
When Dec Guide Mode was changed to South the Excess Motion was 18.5"  corresponding to 1360 ms (at x0.90 guide rate).
When Dec Guide Mode was changed to North   27.4" of Dec Guiding was required to correct the position, corresponding to 2020 ms (at x0.90 guide rate).     This suggests a 3380ms difference in backlash depending on the direction of movement reversal.   This is an almost impossible to handle.    A lower anti-backlash setting that prevents excessive compensation in one direction would lead to guiding taking an inordinately long time to correct movements.

Before Dec Drive was 'retrained'  the antibacklash setting was reduced from 130% to 80% to 20%  but this failed to show a demonstratable reduction in excessive compensation. This led to the decision to retrain the RA/Dec drives (See Ra/Dec Drive Training (2021-11-22) ) and the underlying backlash was remeasured using PHD2 Guiding Assistent. See Dec Backlash Measurements (2021-11-21)  and Residual Dec Backlash Measurements (2021-11-23).

With scope pointing at Az/Alt position of earlier TT Ari guide run, Guiding Assistent was unable to demonstrate lower Dec Backlash in this region of the sky, and could not explain why the backlash compensation would have been excessive for these SSE locations.  It should be noted that Guiding Assistent only measures the Backlash when changing from North Guiding to South Guiding - it never measures the backlash when changing from South Guiding to North Guiding.

Examining Guide Run 8 case in more detail :
- Scope moving northwards during first part of slew to Target
   (last part of slew saw the scope moving in RA only, but it might have flicked south in final stage)
- Locate 1,  Pointing Error -1.76'  N,   1.76'  North Jog commanded,  scope moved 2.41' North in response !
  (excess motion equivalent to 2880ms at x0.90 guide rate)
- Locate 2,  Pointing Error +0.65' N,   0.65' South Jog commanded,   scope moved 0.12' North in response !
  (retrograde motion equivalent to 530 to 3410ms at x0.90 guide rate)
- Locate 3, Pointing Error +0.77' N,   0.77' South Jog commanded, scope moved 0.76' North in response !
  (retrograde motion equivalent to  3360 to 6780ms at x0.90 guide rate)
- Locate 4, Pointing Error +1.53' N,   1.53' South Jog commanded,  little or no scope movement in response !

- Star Drifted South 3",  2.55" South Guide commanded,  Scope Moves south by 21"  (0.35')
- Frame 1 taken. Star at 186.3 px
 - North Guides commanded, equivalent to a 17" movement  (0.28')
- Frame 2 taken. Star at 172.9 px, equivalent to a 18.8" (0.31')  movement to North since Frame 1

Prior to commanding 2.55" South Guide the scope had undergone 3 successive commands to Jog South so there seems to be no reason for the scope to have applied backlash compensation at this point - unless the scope was keeping track of actual scope movement (which was to north even though commands were to move south, in which case the scope might have felt there was a reversal of direction and applied backlash compensation.   Prehaps the 'excessive' motion to the south was represented release of stored energy from the 3 failed attempts to jog the scope south (a total South jog of 2.95'  which resulted in a motion of around 0.9' north).

Retrograde Dec Motion ?
This behaviour may not be due to excessive backlash compensation and might represent some form of Retrograde Dec Motion.  This is something that LX200GPS scopes sometimes show.  It's unlikely that this relates to a sudden deterioration of scope's Dec bearing, but may relate in some way to the recent installation of a New Dec Motor Unit (see  New Dec Motor Unit (2021-10-12) ). Prehaps the motor isn't seated correctly with respect to the main Dec Gear.   This seems unlikely since the Drive sounds perfectly ok / smooth when slewing (certainly better than the old Dec Motor Unit). 

A couple of charts from backlash tests using PHD2 on 2021-11-21 seemm to show indications of retrograde motion, where at certain telescope positions the scope seems to move north after changing over to 'South' guiding :

Useful Links:
Doc G Dec Axis Notes :
http://www.skymtn.com/mapug-astronomy/MAPUG/DecAxis.htm

Repairing the most common problems on the LX200 Dec/RA drive. 
 http://www.skymtn.com/mapug-astronomy/ccdastro/decfix.htm

 

From DocG Dec Drive Notes from MAPUG  http://www.skymtn.com/mapug-astronomy/ragreiner/lx200repair.html  :

"..The instructions indicate that some (back)lash is to be expected when changing direction; doing North to South reversals. The manual says that values of 2 to 4 seconds are normal. Since the declination drive speed in normal guiding mode is 15 arc seconds per second of time, one can correct for the delay by entering a number into the computer to correct for the (back)lash. Nominally, the number entered is 15 times the number of seconds of delay. This number is entered once and need not be changed. The number entered clearly corresponds to the number of arc seconds of mechanical lash in the declination drive. Technically the lash should be entirely in the gear reduction train and should be quite symmetrical. Since the maximum lash that can be corrected is 99 arc seconds, the actual delay time must be less than 6.6 seconds.

Many users have found this (backlash) correction does not always work. Often, users have found much larger delays and delays that depend upon the position of the declination axis, the direction of the reversal, the loading on the telescope and many other elements. Hysteresis, dead zones, of up to 15 seconds have been reported .... If the delay were in the motor/reduction gear train as expected, it should not vary much since the "winding up" of the gear train is similar in either direction. Loading effects on the declination axis are not strongly reflected back into the gear train because of the almost unidirectional transfer of forces through the worm gear. Typically it is not possible, with a low pitch worm gear, which this is, to turn the worm at all with any amount of torque on the main gear. Breaking of the gear would likely take place first. However, loading of the main gear, as by unbalance of the optical tube will greatly increase friction between the main gear and the worm. Thus with an unbalanced optical tube, considerably greater drive force through the reduction gearing is necessary.

The 146 mm diameter declination gear should provide good pointing accuracy (but) it might be noted that the same size drive is used on the 8", 10" and 12" LX200s. So while the gear is adequate for the two smaller telescopes it (the large Dec Gear) is somewhat marginal for the 12".

Many users have discovered that end play in the worm gear mounting contributes to the reversal delay. This is certainly an important effect. With the given characteristics of the gears, a quick calculation shows that 1 arc second of motion of the telescope tube corresponds to only 0.355E-3 mm of axial motion of the worm. This is a required tolerance that is incredibly tight. Thus end play in the worm drive must be eliminated as completely as possible. The worm must be "snug" in its bearings and the entire drive platform must be snug in its pivot mount. Adjustments are provided for in the Meade mount via an end screw on the worm shaft bearing and a screw on the platform mount bearing that can be adjusted. Both should be tightened enough to eliminate all possible end play.

There is however another source of play between the worm and the main gear. This is the radial motion of the worm with respect to the main gear. For some reason, the worm in this design is on a "floating" platform which allows for motion of the worm radial to the axis of the declination drive. It is hard to understand why this "floating" action is as large as it is. No other, of about a dozen worm/gear drives I have inspected, has an action that allows for the large motion that this one does. If one carefully measures the "float" action one finds that the worm can move as much as 0.5 mm radially. The amount of (floating) motion depends upon the direction of reversal and also on the accuracy of balance of the telescope about the declination axis. If the full "float" motion of the bearing platform is allowed, it results in 0.08 mm motion of the main gear edge which is 220 arc seconds of motion of the telescope tube. This is a motion ratio for the worm to main gear surface of 7:1 which seems a bit large for this type of drive.

The forces upon the worm that push it partly out of engagement with the main gear are caused by the friction in the declination bearings plus the forces due to unbalance of the telescope tube. This seems to be one source of the varying delay in reversal operations. It is also the source of retrograde motion. After evaluating numerous operations of the drive with different unbalance loads, it became clear that the amount of "float" is large, irregular and not necessarily repeatable. Motion of the bearing platform was measured with a precision dial indicator and varied from 0.025 mm with the tube well balanced to the full 0.5 mm with a substantial unbalance. In terms of tube motion this amounts to about 11 arc seconds .

There is also a time factor involved with the resettling of the "floating" platform to its new stable position. In addition, the platform takes on a different position when the tube is being driven compared to what it takes when it is allowed to rest. This settling of the platform position after ending a motion cycle causes the tube to drift of the order of 2 to 20 arc seconds. The drive mechanism seems to sort of relax after being exercised

The computer based correction scheme would work with constant mechanical relaxation, it (the correction scheme) does not work well when the relaxation is variable and erratic. As well as being dependent upon the reversal direction, the "float" and relaxation motion was much smaller for a telescope tube that is perfectly balanced about the declination axis because then only the friction forces and acceleration forces must be overcome.

There is a small spring under the floating bearing platform that presses the worm against the main gear. If this spring is strong enough, it can keep the worm pressed properly against the main gear as long as the unbalance is small and the forces required to move the telescope tube are small. As the unbalance gets larger, the spring no longer maintains good contact between the worm and the main gear. The concept that the telescope tube should be kept unbalanced to keep the drive wound up in one direction is not valid in the case of the worm design. Unbalance only increases friction in the drive and requires greater drive force. Adding unbalance generally will not help nor work consistently even if the end play and pivot play have been "tweaked" out.

Unfortunately making the spring much stronger than the original causes the force and thus friction between the worm and the main gear to become too large and the drive binds.

Why is the worm on a "floating" platform at all. One reason would be to keep the worm, on its floating platform and via a spring, to be held in optimum contact with the main gear. Another would be to allow for slight run out of the main gear.... In the case of the gear measured the run out was 0.1 mm. If main gear tolerances are typically 0.1 mm, there seems to be no reason for a "float" of 0.5 mm. In fact, there is an adjustable stop on the floating platform that limits the disengagement of the worm to the 0.5 mm observed. It seems that this adjustment could be tightened up to limit the "float" to be not more than required for the main gear run out. Reduction of the allowed worm platform motion was tried and does reduce the looseness of the drive linkage and the maximum slack allowed. To do this, the motion limiting screw needs to be raised toward the bottom of the platform. It was possible to tighten this tolerance until only 0.03 "float" remained on one drive and 0.05 on the other. This caused significant improvement in the total slackness within the drive systems. The retrograde motion was reduced but not eliminated.

.. another strange motion of the declination pointing mechanism was observed. When the motion was reversed in either direction a small retrograde motion remained. This was finally traced to the mounting between the bearing platform plate and the gear train housing on which the motor is mounted. Unbelievably, the entire drive train/motor housing is attached to the worm bearing housing with four small bolts and a thick rubber ring or gasket (actually a small "O" ring.) Thus the whole reduction gear train housing can move with respect to the worm bearing and when it does it allows the worm to rotate with it. The amount of motion on one drive was 0.5 degrees rotation of the worm. On the other it was 0.2 degrees. This corresponds to an angular motion of the telescope tube of 10 or 4 arc seconds. Before the motor drive train can move the worm any amount, the rubber gasket must go from clockwise to counter clockwise compression limits. (or vice versa for a change in the opposite direction.) This working of the rubber gasket is undoubtedly complex and may cause jerky motion of the worm often observed during reversals. First retrograde and then correct motion is sometimes observed. It is not at all clear exactly why this strange phenomenon takes place. It was however, observed to be repeatable over many reversal cycles. It must be related to the use of a rubber coupling element in the drive chain. It is a weird hysteresis phenomenon which would not take place in a linear system.

It is very tricky to get at the rubber "O" ring. The entire gear drive assembly has to be dismantled. This operation is full of traps and should not be attempted unless you are ready to replace a broken motor/gear drive assembly in the case that you ruin it. The drive is assembled from the inside out and at several points items are glued into place and press fitted. It is exceedingly difficult to take apart....

Additionally, In both drive trains, it was found that the gear at the end of the worm shaft was not tight. In one case 5 degrees and in the other 3 degrees of looseness was found. This accounts for most of the remaining looseness and consequent hysteresis in the gear reduction system. ...

... when requesting reversal of declination motion, there is a total windup in the gears, worm and main gear of 45 arc seconds. This seems like a lot of windup in the gear train ... . The system as adjusted is now very tight mechanically, but still very smooth running. Since this wind up is symmetrical and consistent in amount, it can now be compensated for by the declination lash compensation. The compensation entered into the computer simply causes the drive motor to windup the required amount in the desired direction so that mechanical lash is absorbed and the forces applied are just enough to start motion of the declination axis.

In addition to tightening the looseness in the declination drive, it is useful to reduce forces due to unbalance and acceleration. The first is done by balancing the telescope tube carefully. The second can be reduced by reducing the slewing speed to less than 8, the default value. ..  As a compromise, a setting of 4 might be used

Forward Plan
1) Reset Dec Motor Unit, and reduce the free play in Dec Motor Tension Spring by adjusting the limit screw.

2) Rebalance the scope in Dec, going for either perfect balance (ideal) or very slightly nose heavy.  
Scope currently has issue with moving south at low torque, and a tail heavy imbalance might be causing this.

3) Try with Dec Anti-Backlash setting of 80%  (instead of 90% / 100%), to a) reduce the probability of excessive movement occuring and b) reduce the size of excessive movement if it still occurs.

4) Try with PHD2 Backlash Compensation instead (even though Uni-Directional Guiding is still the planned base method of guiding), turning off scopes' Anti-Backlash compensation and compares results with 1).

 

Retrograde Motion
This describes a drive (usually the Dec drive) that moves a bit in the opposite direction selected before resuming in the correct direction. Autoguiders are confused by retrograde motion. In effect, a small oscillation is set up as the autoguider program attempts to correct for retrograde anomalies. Guider software has great difficulty when this problem exists . This motion should not be confused with backlash which is do to a normal mechanical problem with the gearing system and which can be corrected with the backlash setting provided in the LX200 software.

Notes from "Declination Drive Adjustments (without rebuild)"
(information supplied by Michael Hart, 13 May 1998, with editing and commentary by Doc G)
http://www.skymtn.com/mapug-astronomy/ragreiner/decdriveadjust.html

The worm carriage spring tension can be adjusted to minimize retrograde motion by REDUCING carriage spring tension just enough to lift the worm into contact with the worm wheel and NO MORE. The purpose of the carriage spring is to variably remove backlash in the Dec drive. A small amount of carriage float is necessary to handle any eccentricity of the worm gear. The amount of carriage float is adjusted by the carriage set screw stop. If all of the float is removed with the carriage stop set screw, it is likely the worm will bind against the worm wheel at some point. The idea is to remove excessive carriage travel, but still allow for a bit of worm wheel run out.

The carriage spring easily handles run out in the main gear and in the facing between the inside Dec clutch plate and the 1" optical tube shaft , but extra tension not needed causes retrograde motion.  On can selectively add or remove retrograde motion by increasing or decreasing carriage spring force. If the Dec drive is excessively noisy or retrograde motion is experienced, reduction of carriage spring force can help.

Note by Robert Preston
https://www.skymtn.com/mapug-astronomy/MAPUG/DecAxis.htm#anchor669042

... eliminated retrograde backlash completely simply by unscrewing the motor assembly slightly (two allen-head screws) and then tightening it down again, in almost the very same position it had before I loosened it. It seems that the allowable slop in the mounting holes of the motor assembly had allowed my motor assembly to be located in a position that caused retrograde...