David's Astronomy Pages
Notes - Session 720 (2019-11-08)

 
Bullet Session Aims & Highlights
 - Observing Result
 - Night Summary Plot
 - Session Event Log
 
Bullet Operational Issues
  - Critical Issues (0),  Major Issues (3),  Minor Issues (3),  Continuous Improvement (3)
 
Bullet Secondary Scope - Focusing Issue
Bullet Images from 2019-11-08  >>
 
2019-11-11
Bullet Secondary Scope - Focusing Backlash
Bullet Secondary Scope - Focusing using SharpCap Focus Measurements
 
2019-11-12
Bullet Secondary Scope - Focusing with New Backlash Compensation
 

Session Aims & Highlights (2019-11-08)

Main aims

  1. Smarter Suspension:  Test bug fixes to new code that suspends session if 'AllSkyStars' count falls below 20
  2. Targets: Acquire images of a selection of new targets plus monitoring of previous variable stars, nearby stars and comets.
    (Rain and/or cloud was expected to interrupt or possibly end the session at some point)

Equipment & Software

Highlights Image

Lowlights Image

Summary Plots & Logs

Observing Plan
Image
  
Observing Result
Image
   
Slew/Centering Performance
Image
  
Dome Slewing Performance
Image
  
Sky Conditions (Locate Frames)
Image
  
Night Sky Summary Plot
Top axis: Sky Brightness at Zenith (in ADU/s)
Lefthand axis: Local Time (hh LT). Righthand axis: Sun Altitude (degs)
Image   
  
Pre-Session Weather Forecast (API data from darksky.net)
Forecast showed some initial clear patches, but with cloud progressively developing through the night, but otherwise dry and calm
In the event there was a moderately clear interval between 18:30 and 22:30, with another clear interval at 02:30-03:45. 
Transparency wasn't great at any stage.
Operations didn't start till suspended at 21:50 and took advantage of clear intervals, suspending when sky deteriorated. 
Image   
  
Session Event Log
Time     Event Detail
17:17:16 Session Created Live Session Created (2019-11-08 S00720, ImageSaveNum: 720001)
17:17:52   Obs.Manager Started Obs.Manager started
17:17:54   Obs.Overseer Started Obs.Overseer started
17:17:58   Services Started Observatory Services started
17:18:24   Scope Switched On Scope Power has been switched on via UPB Powerbox.
17:20:08   Services Started Observatory Services started
17:26:03 Observatory (Auto) Observatory placed in Fully-Automated Mode
17:26:09 Session Pending Session pending (2019-11-08)
17:26:11 Session Initiating Session initiating (2019-11-08)
17:26:13   Plan Requested Observing Plan requested from AstroPlan (1.13)
17:27:29   Plan Loaded Observing Plan loaded to queue (2019-11-08 V1 BEST PLAN, Plan ID: 417)
17:27:42   Camera1 Connected SBIG Camera Connected (set point -20°C)
17:27:47   Telescope Connected Telescope Connected (TheSky6)
17:28:00 Session Equilibration Session ready for dome & camera equilibration
17:28:02 Wait on Weather Waiting for acceptable weather conditions
17:33:02 Session Suspended Session suspended
17:33:04   Shutter Closed Shutter already closed (closed at 05:00, closing time 49s)
17:33:06   Queue Started Observing Queue started (29 targets selected)
17:33:08     Target Missed (1/29) Target's time slot was missed (1/29, GCVS Z And)
17:45:36     Target Cancelled Target cancelled (2/29, 68P/Klemola) due to other condition
17:53:32     Target Cancelled Target cancelled (3/29, GCVS FG Sge) due to cloud
18:02:30     Target Cancelled Target cancelled (4/29, GCVS CY Lyr) due to cloud
18:08:59 Session Resuming Session resuming
18:09:45   Shutter Opened Shutter opened (opening time 45s)
18:09:48 Session Running Session running
18:09:50   Queue Resumed Observing Queue resumed
18:09:52     Target Started (5/29) Target started (5/29, 61 Cyg)
18:13:25     Target Completed Target completed (5/29, 61 Cyg)
18:13:28     Target Started (6/29) Target started (6/29, GCVS CI Cyg)
18:15:25       Focusing Started-Foc1 Foc1 Focusing Started (TCF-S)
18:17:15       Focusing Completed Foc1 AutoFocus Completed (Profile No 1)
18:20:51     Target Completed Target completed (6/29, GCVS CI Cyg)
18:20:53     Target Started (7/29) Target started (7/29, GCVS S Cyg)
18:23:05       Focusing Started-Foc1 Foc1 Focusing Started (TCF-S)
18:24:55       Focusing Completed Foc1 AutoFocus Completed (Profile No 2)
18:35:07     Target Completed Target completed (7/29, GCVS S Cyg)
18:35:09     Target Started (8/29) Target started (8/29, IC 5067). Using SharpCap
18:46:35     Target Completed Target completed (8/29, IC 5067)
18:49:28     Target Started (9/29) Target started (9/29, GCVS HH And)
18:52:30       Focusing Started-Foc1 Foc1 Focusing Started (TCF-S)
18:54:25       Focusing Completed Foc1 AutoFocus Completed (Profile No 3)
18:55:59     Target Completed Target completed (9/29, GCVS HH And)
18:56:53     Target Started (10/29) Target started (10/29, GCVS BL Lac)
18:59:07       Focusing Started-Foc1 Foc1 Focusing Started (TCF-S)
19:01:03       Focusing Completed Foc1 AutoFocus Completed (Profile No 4)
19:05:28     Target Completed Target completed (10/29, GCVS BL Lac)
19:06:41     Target Started (11/29) Target started (11/29, GCVS TT Ari)
19:10:26       Focusing Failed Foc1 Focusing Failed - star is too dim (TCF-S)
19:17:24     Target Completed Target completed (11/29, GCVS TT Ari)
19:18:07     Target Started (12/29) Target started (12/29, GCVS RX And)
19:21:24       Focusing Started-Foc1 Foc1 Focusing Started (TCF-S)
19:23:15       Focusing Completed Foc1 AutoFocus Completed (Profile No 5)
19:23:44       Focusing Started Foc2 Focusing Started (Secondary Scope)
19:25:54       Focusing Completed Foc2 AutoFocus Completed (Profile No 6)
19:28:34     Target Completed Target completed (12/29, GCVS RX And)
19:28:36     Target Started (13/29) Target started (13/29, GCVS U And)
19:31:26       Focusing Started-Foc1 Foc1 Focusing Started (TCF-S)
19:33:14       Focusing Completed Foc1 AutoFocus Completed (Profile No 7)
19:33:57       Focusing Started Foc2 Focusing Started (Secondary Scope)
19:36:28       Focusing Failed Foc2 Focusing Failed - failed QC check (Quadratic)
19:44:22     Target Completed Target partially completed (13/29, GCVS U And)
19:44:24     Target Started (14/29) Target started (14/29, GCVS DY Per)
19:53:13     Target Completed Target partially completed (14/29, GCVS DY Per)
19:53:15     Target Started (15/29) Target started (15/29, GCVS AM Her)
19:56:11       Focusing Started-Foc1 Foc1 Focusing Started (TCF-S)
19:58:14       Focusing Completed Foc1 AutoFocus Completed (Profile No 9)
20:02:51     Target Completed Target partially completed (15/29, GCVS AM Her)
20:02:53     Target Started (16/29) Target started (16/29, GCVS S Cyg)
20:05:08       Focusing Started-Foc1 Foc1 Focusing Started (TCF-S)
20:07:00       Focusing Completed Foc1 AutoFocus Completed (Profile No 10)
20:17:20     Target Completed Target completed (16/29, GCVS S Cyg)
20:17:22     Target Started (17/29) Target started (17/29, GCVS SS Cyg)
20:24:51     Target Completed Target completed (17/29, GCVS SS Cyg)
20:27:22     Target Started (18/29) Target started (18/29, GCVS SS Cyg)
20:28:03       Focusing Started-Foc1 Foc1 Focusing Started (TCF-S)
20:30:04       Focusing Completed Foc1 AutoFocus Completed (Profile No 11)
20:33:46     Target Completed Target completed (18/29, GCVS SS Cyg)
20:41:44     Target Started (19/29) Target started (19/29, C/2017 T2 (PanStarrs))
20:45:20       Focusing Started-Foc1 Foc1 Focusing Started (TCF-S)
20:47:09       Focusing Completed Foc1 AutoFocus Completed (Profile No 12)
20:55:34     Target Completed Target completed (19/29, C/2017 T2 (PanStarrs))
20:55:36     Target Started (20/29) Target started (20/29, C/2018 N2 (ASASSN))
20:57:52       Focusing Started-Foc1 Foc1 Focusing Started (TCF-S)
21:00:09       Focusing Completed Foc1 AutoFocus Completed (Profile No 13)
21:00:41       Focusing Started Foc2 Focusing Started (Secondary Scope)
21:03:14       Focusing Failed Foc2 Focusing Failed - failed QC check (Range)
21:11:50     Target Completed Target completed (20/29, C/2018 N2 (ASASSN))
21:11:52     Target Started (21/29) Target started (21/29, GSC 1224:29)
21:14:25 Critical RainThreat Alert Critical Rain Threat Alert (Obs.Manager will close the Shutter)
21:14:27   HardSuspend Called Hard Suspend is called due to Critical Conditions (threat of rain)
21:14:29     Target Aborted (21/29) Target aborted (21/29, GSC 1224:29) due to threat of rain
21:14:31   Queue Paused Queue pausing for HardSuspend
21:14:33 Session Suspended Session suspended
21:15:20   Shutter Closed Shutter closed (closing time 49s)
21:18:56     Target Cancelled Target cancelled (21/29, GSC 1224:29) due to cloud
21:24:31   Camera1 SetPoint SBIG Camera Set Point Temperature Updated (new set point -25°C)
21:37:36     Target Cancelled Target cancelled (22/29, GCVS CM Tau) due to cloud
21:47:12     Target Cancelled Target cancelled (23/29, GCVS SU Tau) due to cloud
21:57:31     Target Cancelled Target cancelled (24/29, GCVS RR Tau) due to cloud
22:07:04     Target Cancelled Target cancelled (25/29, GCVS ER UMa) due to cloud
22:15:25     Target Cancelled Target cancelled (26/29, GCVS FU Ori) due to other condition
22:22:54     Target Cancelled Target cancelled (27/29, WDS GRB 34) due to other condition
22:30:35     Target Cancelled Target cancelled (28/29, GCVS GK Per) due to cloud
22:44:18 Session Resuming Session resuming
22:45:04   Shutter Opened Shutter opened (opening time 46s)
22:45:07 Session Running Session running
22:45:09   Queue Resumed Observing Queue resumed
22:45:11     Target Started (29/29) Target started (29/29, GCVS FO Ori)
22:51:30     Target Completed Target completed (29/29, GCVS FO Ori)
22:51:33   Queue Completed Job Queue completed
22:51:34 Session Continuing Session continuing
22:51:46   Plan Requested Observing Plan requested from AstroPlan (1.13)
22:52:02   Plan Loaded Observing Plan loaded to queue (2019-11-08 V2 BEST PLAN, Plan ID: 418)
22:52:04 Session Running Session running
22:52:06   Queue Started Observing Queue started (34 targets selected)
22:54:46     Target Started (1/34) Target started (1/34, GCVS BL Lac)
22:57:53       Focusing Started-Foc1 Foc1 Focusing Started (TCF-S)
22:59:40       Focusing Completed Foc1 AutoFocus Completed (Profile No 15)
23:04:01     Target Completed Target completed (1/34, GCVS BL Lac)
23:04:34     Target Started (2/34) Target started (2/34, GCVS HH And)
23:07:13       Focusing Started-Foc1 Foc1 Focusing Started (TCF-S)
23:09:06       Focusing Completed Foc1 AutoFocus Completed (Profile No 16)
23:10:44     Target Completed Target completed (2/34, GCVS HH And)
23:11:59     Target Started (3/34) Target started (3/34, GCVS TT Ari)
23:15:12       Focusing Started-Foc1 Foc1 Focusing Started (TCF-S)
23:17:23       Focusing Completed Foc1 AutoFocus Completed (Profile No 17)
23:24:01     Target Completed Target completed (3/34, GCVS TT Ari)
23:24:03     Target Started (4/34) Target started (4/34, GCVS Z And)
23:27:16       Focusing Started-Foc1 Foc1 Focusing Started (TCF-S)
23:29:10       Focusing Completed Foc1 AutoFocus Completed (Profile No 18)
23:32:59     Target Completed Target completed (4/34, GCVS Z And)
23:33:01     Target Started (5/34) Target started (5/34, GCVS RX And)
23:40:31   Telescope Disconnected Telescope Disconected (TheSky6)
23:40:35   Telescope Connected Telescope Connected (TheSky6)
23:44:01     Target Completed Target partially completed (5/34, GCVS RX And)
23:44:03     Target Started (6/34) Target started (6/34, GSC 1224:29)
23:44:09     Target Failed Target failed due to slew error (6/34, GSC 1224:29)
23:48:46     Target Started (7/34) Target started (7/34, GCVS GK Per)
23:48:54     Target Failed Target failed due to slew error (7/34, GCVS GK Per)
23:56:37     Target Started (8/34) Target started (8/34, GCVS DY Per)
23:56:44     Target Failed Target failed due to slew error (8/34, GCVS DY Per)
00:05:55     Target Started (9/34) Target started (9/34, 61 Cyg)
00:06:03     Target Failed Target failed due to slew error (9/34, 61 Cyg)
00:12:59     Target Started (10/34) Target started (10/34, GCVS SS Cyg)
00:13:06     Target Failed Target failed due to slew error (10/34, GCVS SS Cyg)
00:23:41   SoftSuspend Called Soft Suspend is called due to Deteriorating Conditions (too few stars)
00:27:20     Target Started (11/34) Target started (11/34, C/2018 N2 (ASASSN))
00:27:28     Target Failed Target failed due to slew error (11/34, C/2018 N2 (ASASSN))
00:27:30   Queue Paused Queue paused for SoftSuspend
00:27:33 Session Suspended Session suspended
00:28:21   Shutter Closed Shutter closed (closing time 49s)
00:38:12 Session Resuming Session resuming
00:38:57   Shutter Opened Shutter opened (opening time 45s)
00:38:59 Session Running Session running
00:39:01   Queue Resumed Observing Queue resumed
00:41:05     Target Started (12/34) Target started (12/34, GCVS FO Ori)
00:41:13     Target Failed Target failed due to slew error (12/34, GCVS FO Ori)
00:47:54   SoftSuspend Called Soft Suspend is called due to Deteriorating Conditions (too few stars)
00:49:13     Target Started (13/34) Target started (13/34, GCVS FU Ori)
00:49:20     Target Failed Target failed due to slew error (13/34, GCVS FU Ori)
00:49:22   Queue Paused Queue paused for SoftSuspend
00:49:24 Session Suspended Session suspended
00:50:13   Shutter Closed Shutter closed (closing time 49s)
01:15:13     Target Cancelled Target cancelled (14/34, GCVS CM Tau) due to other condition
01:16:36 Session Resuming Session resuming
01:17:21   Shutter Opened Shutter opened (opening time 45s)
01:17:24 Session Running Session running
01:17:26   Queue Resumed Observing Queue resumed
01:17:28     Target Started (15/34) Target started (15/34, GCVS RR Tau)
01:17:30     Target Failed Target failed due to slew error (15/34, GCVS RR Tau)
01:26:47     Target Started (16/34) Target started (16/34, GCVS CN Ori)
01:26:54     Target Failed Target failed due to slew error (16/34, GCVS CN Ori)
01:35:08     Target Started (17/34) Target started (17/34, GCVS SY Cnc)
01:35:16     Target Failed Target failed due to slew error (17/34, GCVS SY Cnc)
01:44:17     Target Started (18/34) Target started (18/34, GCVS RZ LMi)
01:44:27     Target Failed Target failed due to slew error (18/34, GCVS RZ LMi)
01:54:54   SoftSuspend Called Soft Suspend is called due to Deteriorating Conditions (too few stars)
01:55:35     Target Started (19/34) Target started (19/34, GCVS U Gem)
01:55:42     Target Failed Target failed due to slew error (19/34, GCVS U Gem)
01:55:44   Queue Paused Queue paused for SoftSuspend
01:55:46 Session Suspended Session suspended
01:56:34   Shutter Closed Shutter closed (closing time 49s)
02:11:58     Target Cancelled Target cancelled (20/34, WDS GRB 34) due to cloud
02:24:10     Target Cancelled Target cancelled (21/34, GCVS U And) due to cloud
02:38:39     Target Cancelled Target cancelled (22/34, C/2017 T2 (PanStarrs)) due to cloud
02:48:00     Target Cancelled Target cancelled (23/34, GCVS SU Tau) due to cloud
03:07:26     Target Cancelled Target cancelled (24/34, Scholz's Star) due to cloud
03:18:02     Target Cancelled Target cancelled (25/34, LAMOST-HVS1) due to cloud
03:49:22     Target Cancelled Target cancelled (26/34, C/2019 Q4 (Borisov)) due to cloud
04:11:15     Target Cancelled Target cancelled (27/34, NGC 3365) due to cloud
04:26:16     Target Cancelled Target cancelled (28/34, GCVS CN Leo) due to cloud
04:45:28     Target Cancelled Target cancelled (29/34, UGC 4596) due to cloud
05:07:20     Target Cancelled Target cancelled (30/34, NGC 3454) due to cloud
05:29:12     Target Cancelled Target cancelled (31/34, NGC 3455) due to cloud
05:51:04     Target Cancelled Target cancelled (32/34, SHK 1) due to cloud
06:03:40     Target Cancelled Target cancelled (33/34, 38P/Stephan-Oterma) due to cloud
06:23:52     Target Cancelled Target cancelled (34/34, GCVS DX Cnc) due to cloud
06:23:57   Queue Completed Job Queue completed
06:24:04 Session Closing Session closing
06:24:09   Shutter Closed Shutter already closed (closed at 01:56, closing time 49s)
06:25:09   Dome Parked Dome parked (parking time 60s)
06:25:20   Telescope Parked Telescope parked (parking time 11s)
06:26:04 Session Housekeeping Housekeeping Started (Cleanup FITS, Create Fits Summary, Transfer Files)
06:26:18 Session Finished Session Finished
   

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Operational Issues (2019-11-08, S720)

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Critical Issues

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Secondary Scope - Focusing Issue

Although a motor focus was installed on the guidescope/secondary scope in Nov 2018 (Installation Notes 2018-11-07), using it to focus the scope and in particular using it to fully automate the focusing of the scope has proven to rather challenging.  Trying to focus using images/and HFD values from PHD2 proved difficult and swapping to and fro with SharpCap was difficult.  Attempts to use CCDSoft to collect focus star image from ASI ZWO camera on the secondary scope whilst it worked in test environment but in practice it resulted in instablity in my telescope/camera control software which I was unable to quickly overcome. 

However in late 2019 I'm having another bash at finding a solution to the issue of focusing the guidescope/secondary scope.

Example of Automated Focus Profile taken on Main Scope
Orange dots are FWHM measurements made with CCDSoft
Blue dots are HFD measurements calculated from the CCDSoft image
Yellow curve is best-fit quadratic equation that leads to best focus position
Profile has a good shape there is high confidence in the
auto-calculated position of best focus position
Image

Attempts to automate the focusing process using PHD2 ran into some difficulties
-  PHD2 isn't very good at finding / locking onto a star when it is out of focus, due to starmass changes and
   difficulty distinguishing hot patches of pixels from stars
-  HFD values would often stay flat for relatively large changes in focuser position when running a focus profile even though profiles centered on
a starting position of reasonably focus

Example of Automated Focus Profile taken on Secondary Scope, 2019-11-08
Using Backlash Compensation of 400. Blue dots are HFD measurements from PHD2
Profiles are flat without any peak focus even though profiles centered on
a position of reasonably focus  (~ HFD 5.5 - 6)
Image
    
  Image
    
Example of Automated Focus Profile taken on Secondary Scope, 2019-11-09
Using Backlash Compensation of 800
  Image

Suspecting that there might be an issue with the amount of combined backlash on the motor focus/rack & pinion focuser the backlash compensation was increased from 400 steps to 800 steps to see if this might resolve the issue. Although it didn't resolve it did prompt me into doing some more testing in the following days

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2019-11-11


Secondary Scope - Focusing Backlash

Having suspected that there might be an issue with insufficient compensation of backlash a daytime test was carried out in which having driven the focus motors in one direction to the point in which the focuser in moving, the focus motors were reversed and a careful watch made for the point at which the focuser then began to move in the opposite direction.   This was most easily done by spotting the point at which the fine focus knob began to turn and seeing how steps the motor focuser had changed by.  Although this doesn't necessarily include the backlash associated with the rack and pinion focuser on the scope it does evidently show the bulk of the backlash in my system.  

Surprisingly this backlash amounts to some 1500 steps, equivalent to about 2.7mm of take-up (based on my focus tube travel of 0.0018mm per step). This is also equivalent to a 1/4 turn of the coarse focus knob (based on 0.06 deg per step), or 2.5 turns of the 1:10 fine focus knob.

With this measurement I entered a backlash compensation value of 1800 step into my motor focus settings, pending a more detailed measurement in the next observatory session (either a live or test session).

The precise origin of this large backlash is uncertain.  A large backlash was noticed during original manual focusing prior to the fitting of the motor focus, and was not expected from the Pegasus Universal Motor Focus Unit (PEG-MF3-MOTOR-UN) which was meant to have a "gearbox with very low backlash".  Prehaps something became loose during the installation ?

Unfortunately I don't have a record or any recollection of why I had chosen the original value of 400 for backlash compensation. WIthout this I can't  understand whether the backlash has always been as high as 1500 steps or has gradually or suddenly increased..

[ This daytime backlash estimation was done as part of setting up for transit of Mercury however lack of prior preparation on my part and then cloud prevented Transit observation. Got as far as focusing on the sun and the cloud came over and staying covered for the remainder of the afternoon. ]

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Secondary Scope - Focusing using SharpCap Focus Measurements

Having tried focusing solution using PHD2 to provide HFD measurement on a single star,  a new method has been developed using SharpCap to provide a multi-star FWHM measurements.  Whilst SharpCap has manually initiated tools for focusing using a number for methods including single star and mutiple star FWHM measurements including stepping across a range of focus positions using an ASCOM connected focuser,  this was does not provide me with means for fully-automated the task from my Telescope&Camera Control Software (AstroMain), however I can accomplish this by controlling SharpCap using a SharpCap service interface written in python (AstroShCap) that listens and responds to JSON like requests from my control program.

This was accomplished by adding a new routine to my server interface called "RunFWHM monitor" that sets the SharpCap MultiStar FWHM Measurement tool running and reads the FWHM current score when requested, along with 3 new methods 'StartFwhmMonitor',  'MakeFwhmMeasurement', 'GetFwhmScore'.  An existing method 'StopCapture' is used to end the FWHM monitoring.

My Telescope & Camera Control Software (AstroMain) has been updated to call the SharpCap server interface and uses these new methods to help acquire a focus profile and derive a position of best focus.    The older method using PHD2 is still available as an option.

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2019-11-12


Secondary Scope - Focusing with New Backlash Compensation

After determining a good estimate of the true backlash in the system (much larger than previously used) and with my new SharpCap based focusing routine in place there was opportunity to try during night-time observatory testing on 2019-11-12.

- Software : AstroMain 3.17.5, AstroShCap 1.2.8

With backlash compensation value of 1800:

- Focusing in the "In" direction produced an initial best focus position of 8081 steps

- Focusing in the "Out" direction then produced a best focus position of 9560 steps

- Subsequently focusing in the "In" direction produced a best focus position of 7999 steps.

This indicate a total backlash of between 1479 and 1561 steps which is similar to the 1500+/- steps estimated during daytime testing.

Automated Focus Profiles taken on Secondary Scope,  2019-11-12
Orange dots are MultiStar FWHM measurements from SharpCap
Profiles are reasonably well formed. Quality of fit suffers due to inclusion of points at
one or both ends of the profile (off the bottom of the chart)
where SharpCap has returned FWHM Scores of 20.0
Image
    
  Image
    
  Image


Previous calculations  (TS 80mm f/6 APO - Critical Focus Zone, 2018-11-07) has shown that the the critical focus zone for my  80mm TS Photoline APO refractor is just 31 nm (0.031mm).  With my Pegasus Motor Focus Unit  (0.0018mm per step) this is equivalent to 17 steps,  i.e. a zone that is just +/-9 focuser steps wide.   With focusing previously undercompensating for backlash by around 900 steps (1500-400) it is no surprise that focusing was getting screwed up.

(Critical Focus Zone calculation is based on the "New Critical Focus Zone" (NCRZ) equation of GoldAstro, and assumes 1.5” (arc sec) FWHM total seeing and a focus tolerance of 10% .  This CRZ changes to +/- 6 steps for 1" seeing,  +/-11 steps for 2" seeing,  +/-14 steps for 2.5" seeing ,  +/- 17 steps for 3" seeing & +/- 23 steps for 4" seeing. )

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