David's Astronomy Pages Notes - Session 839  (2020-12-17)

Notes (S838)

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Notes (S840)

	Session Aims & Highlights
	- Observing Result  - Night Summary Plot  - Session Event Log
	Operational Issues
	- Critical Issues (1),  Major Issues (0),  Minor Issues (1),  Continuous Improvement (1)
	Images from 2020-12-17 >>
	Investigation:  Dome Failed To Close (Mechanical Failure)

Session Aims & Highlights (2020-12-17)

Main aims

1. Targets. Acquire images of a selection of variable stars, nearby stars, comets and deep sky targets as allowed by conditions.

Equipment & Software

• 12" LX200 and ST-10XME
• 80mm APO and ZWO ASI 178MC
• CCDApp2, TheSky6, CCDSoft5, PHD2,  DeviceHub 4.5.0.1, UPB 1.5 
• Pegasus UPB & Motor Focus, TCF-S
• SharpCap 3.2, PHD2  2.6.9
   
• AstroMain 3.32.15,  AstroGuard 1.4.6,   AstroPlan 1.18  AstroShCap 1.3.6, AstroVOE 1.5.2, AstroSN 1.1,  AstroProtect 1.4.1
  (AstroWeather 1.10.13, AstroAllSky 3.6)

Highlights

• Shutter System.  A defect in the Dome shutter system was found and a solution was implemented to correct the defect and maintain continued protection of the observatory.

Lowlights

•   Cloud.  Cloud developed faster than expected and only a single target was observed before the session had to be ended.
•   Shutter System. Mechanical failure of Dome's shutter system. Shutter overran upon opening and chain broke, leaving the observatory exposed to elements and requiring dome to be manually closed.

Summary Plots & Logs

Observing Plan

Observing Result

Dome & Scope Slewing Performance

Slew/Centering Performance

Guiding Performance

Sky Conditions (Locate Frames)

Chart unavailable for this session

Night Sky Summary Plot Top axis: Sky Brightness at Zenith (in ADU/s) Lefthand axis: Local Time (hh LT). Righthand axis: Sun Altitude (degs)

Pre-Session Weather Forecast (API data from darksky.net) Pre-Session Weather forecast showed patchy cloud conditions in first part of night (27-73% cloud cover) with minium tempertaure of 6 degC, becoming very cloudy from 20:00 onwards (74-94% cloud cover) with some rain expected between 21:00 to 03:00 (70% probability), wind up to 24-25 mph and temperature rising to 9 degC In the event skies there were initially clear, but became overcast at 19:00 with a rain shower at around 21:00.

Session Event Log

Time     Event Detail 18:18:42 Session Monitoring AutoStart monitoring for Live Session opportunity starting between 18:18 & 06:33 18:18:47   AutoStart Delayed Session delayed due to cloud 18:18:57 Session AutoStarting Session autostarting (18:18) 18:19:27 Session Created Live Session Created (2020-12-17 S00839, ImageSaveNum: 839001) 18:19:29   Scope Switched On Telescope Power has been switched on via UPB Powerbox. 18:21:24   Services Started Observatory Services started 18:21:33 Observatory (Auto) Observatory placed in Fully-Automated Mode 18:21:37 Session Pending Session pending (2020-12-17) 18:21:39 Session Initiating Session initiating (2020-12-17) 18:21:41   Plan Requested Observing Plan requested from AstroPlan (1.18) 18:22:17   Plan Loaded Observing Plan loaded to queue (2020-12-17 V1 BEST PLAN, Plan ID: 574) 18:22:29   Camera1 Connected SBIG Camera Connected (set point -15°C) 18:22:35   Telescope Connected Telescope Connected (TheSky6) 18:22:45 Session Equilibration Session ready for dome & camera equilibration 18:24:57   Dome Opened Dome opened (opening time 40s) 18:24:59 Session Running Session running 18:25:01   Queue Started Observing Queue started (40 targets selected) 18:26:16       Focusing Started-Foc1 Foc1 Focusing Started (TCF-S) 18:29:36       Focusing Completed Foc1 AutoFocus Completed (Profile No 1) 18:31:27       Focusing Completed Foc1 AutoFocus Completed (Profile No 1) 18:31:53       Focusing Started-Foc2 Foc2 Focusing Started (Secondary Scope, using ShCap) 18:34:06       Focusing Completed Foc2 AutoFocus Completed (Profile No 2, wide) 18:35:49       Focusing Completed Foc2 AutoFocus Completed (Profile No 2) 18:36:04       Focusing Checked Foc2 Focusing Check Succeeded 18:36:37     Target Started (1/40) Target started (1/40, UGC 10909 w/AT2020abkz) 18:40:03       Focusing Skipped Foc1 focusing skipped - star is lost (TCF-S) 18:50:29   SoftSuspend Called Soft Suspend is called due to Deteriorating Conditions (too few stars) 18:52:49   SoftSuspend Cancelled Soft Suspend is cancelled due to Improved Conditions 18:57:49     Target Completed Target completed (1/40, UGC 10909 w/AT2020abkz) 19:02:25   SoftSuspend Called Soft Suspend is called due to Deteriorating Conditions (too few stars) 19:02:27     Target Started (2/40) Target started (2/40, UGC 678 w/AT2020abjq) 19:06:00     Target Failed Target failed due to cloud (2/40, UGC 678 w/AT2020abjq) 19:06:03   Queue Paused Queue paused for Session Suspension 19:06:06 Session Suspended Session suspended 19:16:06   Camera1 SetPoint -15°C SBIG Camera Set Point Temperature Updated (new set point -15°C) 19:17:21     Target Cancelled Target cancelled (2/40, UGC 678 w/AT2020abjq) due to cloud 19:26:52   Dome Closed Dome closed (closing time 1,247s) 19:27:07 User Intervention User request to close and finish session 19:27:10   Queue Aborted Job Queue aborted 19:27:12 Session Closing Session closing 19:27:17   Dome Closed Dome already closed (closed at 19:26, closing time 1,247s) 19:28:04   Dome Parked Dome parked (parking time 39s), Az: 90.1 deg 19:28:24   Telescope Parked Telescope parked (parking time 14s) 19:28:54   Telescope Switched Off Telescope Power has been switched off via UPB Switch. 19:29:50   Services Stopped Night Services stopped 19:29:52 Session Housekeeping Housekeeping Started (Cleanup FITS, Create Fits Summary, Transfer Files) 19:29:55 Session Finished Session Finished

Session Alerts

Time     Alert Detail 19:07:51 Dome Alert Dome hasn't reached 'Closed' state after 100s during SoftSuspend

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Operational Issues (2020-12-17 S839)

• Critical Issues (1)
• Major Issues (0)
• Minor Issues (1)
• Continuous Improvement (2)

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Critical Issues

• Shutter unable to close when clouds developed due to mechanical failure of shutter pulley system.   After opening the shutter at 18:24:57, the Shutter was later noted to be 'stuck' in the 'Closing' state for excessive amount of time after clouds developed and the shutter had ordered the closure of the shutter at 19:06. A Red Session Alert was raised  at 19:07:51 'Dome hasn't reached 'Closed' state after 100s during SoftSuspend'.   After ruling out an issue with DeviceHub (similar error message through Sept/Oct when Dome Comms appeared stalled), the observatory was visited and it was found that the shutter drive was still busy turning, but the plastic drive chain had 'delinked' and was no longer attached to the drive cog. The shutter was completely open. In fact the shutter was in a position was 'over-open' such the chain fixture and the U shaped metal limit block on the shutter had been driven beyond there normal range and had overridden the rear roof line. The chain had come apart adjacent to the point of fixture of the chain to the shutter roof.   The shutter was manually closed at 19:26 and a user interventation made to close the session (including parking dome and telescope).  The issue is deemed critical as automated operation was unable to close the open shutter and observatory  equipment would probably have been damaged by a rain shower that occurred at 23:00.   Further investigation was made the following day and a repair made to the chain by replacing section of chain with some spare chain (one link was missing and couldn't be found).  Although the shutter had never done this before it was evident with hindsight that this was a problem waiting to happen as the limit block lies  too close to the shutter surface and lies too far from the liimit switches, especially the upper one.  A wood insert was used to raise the height of the limit block in order to prevent the issue from ever occuring again. 
  Fixed 2020-12-18 (Observatory)
  
  Later review of Pulsar logs showed that Shutter State changed to Opening (2) at 18:24:18, changed to Open (0) at 18:24:55, changed to Closing (3) at 19:06:07,  and then remaining as 'Closing' (3) it changed to 'Unknown State (5) at 19:22:49,  changing to Closed (1) at 19:26:26.

Major Issues

• None

Minor Issues

• Outdoor Temperature/Humidity Sensor data has flat-lined . Readings not available to Weather Data.   Outdoor temperature/humidity suddenly went offline on 2020-12-15 at c.11:30 .   Session had to be run with Dew Heater set to a manual setting of 60% power and lowered to 20% power after the session. Attempts to get the sensor working/re-connected the following day failed (no connection after inserting new batteries or after performing resets of the Sensor or the base Unit.  The particular weather station is no longer produced and it was not possible to order the same T/H sensor part, however an equivalent one (Oregon Scientific Temperature & Humidity Sensor with display OEM (THGR221) × 1) was ordered from Weather Spares for £32 (incl P&P) which arrived 2020-12-22. It was installed as a new outdoor Channel 1 T/H sensor, and is now successfully passing T/H readings to the weather station and on to the Observatory Computers.  The new sensor was installed by removing the old sensor and taping the new sensor in the compartment using a piece of wide duct tape.
  Fixed 2020-12-22 (Weather Station)

Continuous Improvement

• Reinstate Light Bafffle for AllSky Camera to block out security/work light associated with nearby building site.  Light baffle had been used in 2019/2020 was removed in the summer 2020 when the supporting fence had to be replaced. The light baffled hasn't been restalled until now.
  Done 2020-12-18 (AllSky Camera)
   
• Bring mechanical failure of shutter pulley system (S839, 2020-12-17) and fix made to attention of Pulsar Observatories.   EMail sent to Pulsar Observatories on 2020-12-20 with reply received 2020-12-21.
  Done 2020-12-20 (Pulsar Observatories)

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Investigation:  Dome Failed To Close (Mechanical Failure)

Issue: Shutter unable to close when clouds developed due to mechanical failure of shutter pulley chain system.   Dome left open to potential damage from rain.

Description.
Shutter opened at 18:24. All logs/reports indicate the shutter had successfully opened (no indication at this time of any problem). Autofocusing conducted and 1st target successfully acquired. After clouds developed at 19:06 the Shutter was automatically commanded to close. An alert was issued saying dome hadn’t closed after 100s (normally takes 45-50s to close). 10 minutes later remote monitoring showed that the dome shutter was still in the 'Closing' state and hadn’t reached the ‘Closed’ state. After spending a couple of minutes to rule out a problem with the DeviceHub connection, the observatory was visited to check what had happened.

It was found that the shutter drive was still busy turning, but the plastic drive chain had 'delinked' and was no longer attached to the drive cog. The shutter was completely open. In fact the shutter was in a position that was 'over-open' such the U shaped metal limit block on the shutter and the chain fixture had been driven beyond their normal range and had overridden the rear roof line and causing the shutter to be loosely stuck. The chain had come apart adjacent to the point of fixture of the chain to the shutter roof. The shutter was manually closed at 19:26 and a user intervention made to close the session (including parking dome and telescope).

The issue is deemed critical as automated operation was unable to close the open shutter and observatory equipment would probably have been damaged by a rain shower that occurred later in the evening at 23:00.

Investigation

Further investigation was made the following day. Although the shutter has never done this before (multiple open/close operations over last 2.1/2 years) it was evident with hindsight that this was in fact a problem waiting to happen. It is noted that the height of the limit block compared to the gap between the shutter and the roofline means that the limit switch grazes the rounded end of the limit block rather than hitting the end of the limit block square on.

It would seem upon opening the dome, the limit switch has been triggered and the message passed on to dome controller and dome driver, but in this particular case the rounded end of the limit block had immediately slipped over the limit switch and over-rode the roofline, causing the shutter to become effectively ‘stuck’. It is supposed that when the command was made to close the dome the chain delinked due to a combination of the very angle at which the chain rubbed on the roofline and the excess force required to pull the shutter back from its ‘stuck’ position.   Alternatively the chain had delinked when it hit the roofline when the shutter overran it normal range upon opening.

Repair and Fix to prevent reoccurence.
A repair was made to the chain by replacing a section of the chain with some spare chain left over from the original installation
(Note: one link was missing and couldn't be found). 

A wood insert was used to raise the height of the limit block in order to prevent the issue from ever occuring again. 

Photo Showing position of break in shutter chain.

Photo showing Shutter's Normal Open Position with arrows indicating how shutter was driven beyond its normal position

Photo showing Roof Line and position of Upper Limit Switch and Upper Pulley

Photo showing original position of Limit Block (block lies flush with shutter and only grazes the upper limit switch)

Photo showing new position of Limit Block (block raised using an 8mm thick piece of plywood so that the limit block can hit the upper limit switch square on)

Photo showing new position of Limit Block when Shutter is Open (limit switch now hits the side of the Limit Block rather than its rounded top)

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