Adjustment
A polar alignment adjustment plan was executed which aimed to lower the Polar Axis of the Telescope Mount by 18 arc min.  This was the planned adjustment according to Proposed Polar Alignment Change (2021-10-19), which was designed to correct the majority of a 19.3 arc min elevation error indicated by recent TPoint Modelling in order to reduce the excessive star drift that has been difficult to reliably correct by Guiding, despite a nunber of attempts to improve guiding performance.  Aim was to leave a small residual error to utilise uni-directional Dec Guiding, since scope has a backlash of around 3000ms (at guide rate of x0.90 sidereal)

Polar Alignment Information from Mapping Run (2021-10-19) 179 data points.  Best TPoint Model 16 terms

Method
A suitable bright star near to Meridian/Dec 0 was positioned at the centre of the Cam1 view (LX200+ST10) in CCDSoft . Base line images were taken & plate solved.   At the same time Cam2 View (TS80+178MC) was shown in SharpCap as it has a larger field of view and could keep the star in sight  whilst the polar align chnage was being made.   The scope was commanded to jog 18.0' to the south.   A new image was taken and plate solved. This showed the scope had moved 16.83' to the south. 

Equatorial Wedge was then adjusted to raise it by 18' causing the telescope to move back to the bright star. A slight (1') overcorrection was accepted rather than reverse the movement on the equatorial wedge plate.  Wedge lock screw were tightened up and a final image was taken and plate solved. This showed that that Telescope Pointing had been raised by 17.7'  (which means that the Polar Axis had been lowered by 17.7').

Adjustment Measurement
Pointing prior to adjusting the wedge elevation
 - Plate Coordinates (RA)      23 55 08.521 (RA J2000)
 - Plate Coordinates (Dec)    +06 47 29.55 (Dec J2000)

Pointing after adjusting the wedge elevation
 - Plate Coordinates (RA)     23 55 08.096 (RA J2000)
 - Plate Coordinates (Dec)   +07 05 13.16 (Dec J2000)

The plate solutions show that the wedge adjustment has raised the telescope by 17.73' in south sky, which means that the Mount's Polar Axis has been lowered by 17.73'    (The RA positions show polar axis azimuth was unchanged by the procedure to lower the polar axis).

Post-Adjustment Actions
Following the adjustment to Polar Axis (and in recognition that current and forecast sky conditions were unsuitable for running new full sky mapping required for  generating a fresh TPoint model) the existing TPoint Base Model was modified to fix the ME term from -1158 to 0 as stop-gap solution.

Scope was then synced on new field position and a short 12 point mapping job performed.

Scope was then parked and then restarted to continue the session
(this step is required so that subsequent pointing errors in the session are around 0'  +/-  error ,  rather than lying at around 20' +/- error)

Guide runs from the remainder of the session (and the following session) provide information that allow a rough assessment of the impact of the Polar Alignment adjustement

Assessment of change
Guide runs following the Polar Alignment adjustment (Session S936 Part II and S937) show reduced rates of star drift as hoped for but the direction of Star Drift which had been southwards when west of meridan (and northwards when east of meridian) had unexpectedly swapped over. 

Analysis
The guide runs following the Polar Alignment adjustment suggests that the TPoint Model assessment of Polar Axis elevation error was excessive, and that an over- adjustment of Polar Axis elevation has been made.   Why might this be the case ?

The Polar Axis Elevation term (ME) varies according to the use of other terms such as Tube/Fork flexure, which are necessary for minimising the overall TPoint error, and this is the probable reason for the overestimated polar axis error.

Another possibility is that required change was to raise the telescope by 18' in altitude (= 18' in Dec when pointing due south),  but  the 17.73' rise in Dec position when scope was between Az 188 and Az 192' corresponded to >18' rise in altitude.

It's not possible to simply measure the altitude difference between the pre and post adjustment images,  although scope pointing was theoretically raised in altitude, during the time it took to make the adjustment the celestrial position descended in altitude by coincidentally the same amount due to increasing angle from the Meridian.

Pointing prior to adjusting the wedge elevation
 - Azim 188° 08' 02"   Alt 39 21' 06"     (39.35)

Pointing after adjusting the wedge elevation
 - Azim 192° 07' 08"   Alt 39° 21' 09"    (39.35)

Using TheSky6  the centre-time point  at Azim 190.1°, Alt 39.35° corresponds to Dec 07 02 24   (2.4)
A 18' rise in altitude would put the adjusted centre-time point  at Azim 190.1°, Alt 39.65° which corresponds to Dec 07 20 19   (20.31
which is a 17.9' rise in Dec.    (so a 17.73' rise in Dec would be equal to a 17.82' rise in Alt , ie pretty much the same value)

For a centre time position that is 10.1 deg west of Meridian (ie  Az 190.1°), prehaps the required rise in telescope position is 18 * Cos (10.1°) ?
This would be still be a rise of 17.7'  which is not significantly less than 18' and pretty much the size of the adjustment made.

Conclusion is that the TPoint assessment of Polar Alignment Error was inaccurate and that its recommendation to lower the Polar Axis by 19.3' was misleading.

Reviewing the Earlier TPoint Model again
With the 16 terms used in the final TPoint Model the Polar Alignment Information indicates
  MA +40s  (rotate polar axis west, counterclock, by 1.2 mins),  ME -1172 (lower polar axis by 19.5 mins)

When just 6 equatorial terms are used the TPoint's Polar Alignment Information indicates
  MA +183s  (rotate polar axis west by 5.7 mins),  ME -1576 (lower polar axis by 26.3 mins)

When Fork Flexure and Tube Flexure terms are added the TPoint's Polar Alignment Information indicates
  MA +68s  (rotate polar axis west by 2.1 mins),  ME -3635 (lower polar axis by 60.6 mins)
 
When 4 further terms are added (12 in total) the Polar Alignment Information indicates
  MA + 44s (rotate polar axis west by 1.4 mins), ME -1111 (lower polar axis by 18.5 mins)

When 2 further terms are added (14 in total) the Polar Alignment Information indicates
  MA + 41s (rotate polar axis west by 1.3 mins), ME -1127 (lower polar axis by 18.8 mins)

None of these alternate models suggest lowering the polar axis by any less than 18.5 mins.  So with the 17.7 mins adjustment made it is hard to see how the Polar Axis elevation has been over adjusted.

The 16 term model has an ME sigma value of 50. So taking +/-2 sigma range (-1172 +/- 100)  would indicate a lowering of the polar axis by between 17.6 and 20.9 mins (ie 19.3 mins +/- 8.5%).   So with the 17.7 mins adjustment that has been made it is again difficult to see how the Polar Axis elevation has been overadjusted.

Forward Plan
Given the indications from direction of star drift in new guide runs that the Polar Axis is now too low,  it is tentatively proposed to raise the Polar Axis by say 3 arc min, before performing a new full-sky mapping run for a new TPoint Model.  However prehaps it is better to create a new TPoint Model based on the current alignment to see how TPoint views the recent change.

Previous attempts to use Star Drift Method for assessing Polar Alignment have proven to be reliable for assessing and correcting Azimuthal Error. However they have proved unreliable / difficult to apply for assessing Elevation error since the absence of star drift at a West Sky point, has been found to not correspond to an absence of star drift at an East sky point (and vice versa).

A series of measurements of Dec Backlash measurements were made using PHD2 Guiding Assistent with a LX200 guide rate of x0.90 sidereal.  This follows Drive Training the previous day (see Ra/Dec Drive Training, 2021-11-22).

Tests were running with scope's anti-backlash setting for the Dec/Alt axis set at either 90% or 100%, and therefore represent residual backlash measurements.

Residual Backlash Results

No.	Antibacklash in Scope %	Az (deg)	Alt (deg)	Dec Drift Rate (arcsec/min	Polar Alignment Error (arc min)	Dec Backlash (ms)	Dec Backlash (arc sec)
1 (S)	100%	183	32	0.36	1.4	0	0
2 (S)	90%	184	32	-0.07	0.3	0	0
3  (S')	90%	182	70	0.79	3.9	2753 *	31.6
4 (E)	90%	88	19	0.88	3.6	1078	13.6
5 (E)	100%	88	19	1.44	5.9	641	8.2
6 (W)	100%	270	15	-3.69	14.5	- (cloud)	-
7 (W)	100%	270	35	-2.01	8.7	674	8.1

Analysis
Previous measurements of Dec backlash (2019-12-10) gave an average backlash equivalent to 3590 ms at x0.90 guide speed (3457 to 3705 ms).  Recent Measurements (2021-11-21) gave an average backlash of 3587 ms at  x0.90 guide speed (range 3294 to 4001 ms) .
(These were taken with a 0% Anti-Backlash setting in the Scope ).

After training the Dec Drive on 2021-11-22 (which effectively measures and records the Dec Backlash in the Scope), the Anti-Backlash setting in the scope was set to 90% - 100% and the residual backlash remeasured using PHD2's Guiding Assistent.  In general these showed Dec Backlash ranging from 0 to 1078 ms dependant on telescope position,  but reaching a high value of 2753 ms at certain positions.  

In the South position (Az 183 - 184, Alt 32) whilst PHD2 records residual backlash as being 0 the charts themselves show possible signs of excess motion . At this position the antibacklash setting of 90-100% is too high, whilst at East (Az 88/Alt 19) and West position (Az 270/Alt35) the antibacklash setting of 90-100% is ok (or even too low) leaving a residual backlash of 641-1078 ms.

In each case the Backlash chart show a guide anomaly part way down the slope where 500ms guide pulses can be fairly non-effective at moving the scope for periods last 1500-2500ms.   This seems to be the same feature that shows as non-effective guide pulses during guiding runs (typically these have been associated with sky positions west of Meridian where Dec Guide Mode South was formerly being used.

Polar Alignment

Polar Alignment Errors indicate by PHD2 vary with position as anticipated. Since Polar Alignment Error is mainly in Altitude rather than Azmiuth, the Error seen by a single guide assistent run is low for positions near due South (1.6') and near due north (4.6'), positions and greatest for positions in Western Sky (14.5', 15.7', 18.7') and Eastern Sky  (10.0', 14.5').

PHD2 measurement are broadly consistent with the Polar Alignment Error of 19.5' determined from most recent TPoint Mapping run that comprised  Az/Alt errors of  -19.5' ME and +0.7' MA  (see TPoint Mapping Run & Model, 2021-10-19).