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

 
Bullet Session Aims & Highlights
 - Observing Result
 - Night Summary Plot
 - Session Event Log
 
Bullet Operational Issues
  - Critical Issues (1),  Major Issues (0),  Minor Issues (1),  Continuous Improvement (1)
Bullet Images from 2020-12-17 >>
Bullet 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

Highlights

Summary Plots & Logs

Observing Plan
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Observing Result
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Dome & Scope Slewing Performance
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Slew/Centering Performance
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Guiding Performance
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Sky Conditions (Locate Frames)
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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)
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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. 
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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)

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Critical Issues

Major Issues

Minor Issues

Continuous Improvement

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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.
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Photo showing Shutter's Normal Open Position
with arrows indicating how shutter was driven beyond its normal position
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Photo showing Roof Line and position of Upper Limit Switch and Upper Pulley
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Photo showing original position of Limit Block
(block lies flush with shutter and only grazes the upper limit switch)
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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)
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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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