Issue: Problem with centering Target 5/59 (156P/Russell-LINEAR, pointing error 21,435 arc min poiting error), causing RA Slew Adjustment to be set to a very large number, which then caused subsequent targets to fail slew: Target 6/59 (DY Per) and Target 7/59 (SS Cyg, first attempt) until the problem was noticed and a user intervention made.

Description:  Slew File shows very large numbers 21,435.51,  435.622. Centering Graph shows Av Slew Error after 5 slews of 4288.59 arc min,  Slew Adj W 3062.52' .   Resolved in session by a) going to manual and confirming that telescope coordinates were sound by taking a CCD Image and checking that it links ok with coordinates very close to those of the telescope) b) using back door facility to reset slew adjustments,  allowing a second attempt at imaging SS Cyg and subsequent targets to work ok. 

Later in the session the 156P/Russell-LINEAR was reattempted as target 29/62 by submitting it as a ToO   Again the same problem happened again and the slew adjustment was immediately reset to prevent it affecting other targets.  So the problem is repeatable with the same target and therefore it must relate to something specific about the 156P/Russel (Linear) target or its coordinates.  Issue is categorised as major because without spotting the problem by remote monitoring and intervening the problem would have impacted some 55 targets.An alert needs to be issued if a high slew adjustment numbers are generated, and the slew adjustment reset to 0,0 or the number it had before the target with problematic slew/pointing error calculation.
 
In-Session Fix
After acknowledging the error (by clicking on dialog window's Ok button), the session automatically resumed without further incident.

Investigation:  

Looking at first case,  the slew to Target Coords  (RA: 23 59 41.731, Dec: +06 58 00.48 (J2000)) was normal.  Locate Frame 1 linked ok (UCA/USNO,  Stars 8), but the reported pointing error was an impossibily large number  21,435.52 arc mins (W: 21,435.52', N: 11.28')  given the range of sky searched by the linking operation.

Looking at second case,  the slew to Target Coords  (RA= 23 59 58.279 Dec= +07 05 40.91) was normal, and although first locate image failed to link (too few stars), the 2nd locate image eventually linked ok (UCA/USNO* , 6 stars). Again the reported pointing error was an impossibily large number 21,426.29 arc mins (W: 21,426.28', N: 17.41')  given the range of sky searched by the linking operation.

The pointing error in both cases corresponds to some  357 deg suggesting that the error has somehow been wrapped by nearly a full circle.  

Coordinates show that the target coordinates of the comet where very close to RA 0 / RA 24

Object Coords (2000)  Data  | RA= 23 59 58.279 Dec= +07 05 40.91 | Epoch 2000
Object Coords         Data  | RA= 00 01 01.959 Dec= +07 12 38.29 | Epoch Current

Slew Adjustments      Data  | 2.62 arc min (W= -0.23', N= 2.61')

Object                Data  | RA= 23 59 58.395 Dec= +07 05 44.07 | Epoch 2000
 Image                Data  | RA= 00 00 31.884 Dec= +07 23 05.46 | Epoch 2000

Examing in the logfile shows that by one method a pointing error of  19.2 arc mins was calculated and logged
But subsequently in CalculatePointingError an exception was captured :

22:54:47.11 | Get Image.RaDectoXY    Fail | Exception getting XY coordinates in CalculatePointingError
22:54:47.11 | Get Image.RaDectoXY    Fail | Exception: Error: command failed.    Error code = 206 (0xce).

After outputting the target and image coordinates

22:54:47.11 |  Target Coords (2000)  Data | RA= 23 59 58.279 Dec= +07 05 40.91 | Epoch 2000
22:54:47.11 |  Image Coords (2000)   Data | RA= 00 00 31.884 Dec= +07 23 05.46 | Epoch 2000

the very large pointing error was output

22:54:47.11 | Pointing Check (Image) Data | Object Offset:             [W: 21,426.28', N: 17.41']  21,426.29 arc mins
22:54:47.11 | Pointing Check (Image) Data | Pointing Error:            [W: 21,426.28', N: 17.41']  21,426.29 arc mins
22:54:47.11 | Pointing Check (Image) Data | Correction Required: [W: -21,426.28', N: -17.41'] 21,426.29 arc mins

Although jog adjustment wasn't actually performed ("Jog aborted as jog amount is > 2 deg"), the pointing error had already been incorporated into a revised slew adjustment that throws of the slewing to subsequent targets>

22:54:47.44 |   Get SlewAdj          Data | Previous Slew Adjustments: [W= -0.23', N= 2.61' ]
22:54:47.46 |   Update SlewWAdj      Ok   | Updated Slew Adjustments: [W= -1,071.55', N= 1.74', nSlews= 18 ]

In this second cases were there were 18 prior slews (rarther than just 4)  the affect of the single anomalous pointing error is more smeared out but still leads to updating the slew adjustment to a value of 17.86 degs !

Conclusion:
There is a design flaw in OB.CalculatePointingError() routine, where if Ra2 (RA Image) and Ra1 (RA Target) fall either side of RA=0 then
large negative dErrorW and large postive dErrorW values can be produced.   

dJogWcorrection = (Ra2 - Ra1) * 15 * 60
dErrorW = -dJogWcorrection * Math.Cos(Dec1 / DR)

This does occur that often, but the consequence is large as demonstrated by the issue discovered in this session

Fix:
The OB.CalculatePointingError() routine was fixed to correct 'RA error' value where RA Target and RA Image fall either side of RA 0 (either adding to 24 to the difference or subtracting the difference from 24).  

' Calculate West Correction (in rotational arc mins), correcting for cases where Ra1 and Ra2 fall eitherside of the Ra=0 line
' -------------------------
  If Ra2 - Ra1 < -12 Then
    dJogWcorrection = ((Ra2 - Ra1) + 24) * 15 * 60
  ElseIf Ra2 - Ra1 > 12 Then
    dJogWcorrection = (24 - (Ra2 - Ra1)) * 15 * 60
  Else
    dJogWcorrection = (Ra2 - Ra1) * 15 * 60
  End If

  dErrorW = -dJogWcorrection * Math.Cos(Dec1 / DR)

As an extra precaution pointing errors in excess of 20 arc mins are excluded from the calculation of Av. Slew Adjustments values.

With these corrections it is not necessary to include an alert.

Fixed  2020-11-29 (AstroMain 3.32.11)