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Observing Result |
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Dome & Scope Slewing Performance | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Slew/Centering Performance | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Guiding Performance | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Sky Conditions (Locate Frames) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Chart unavailable for this session |
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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 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Session Alerts | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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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.
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This Web Page: | Notes - Session 839 (2020-12-17) |
Last Updated : | 2024-09-30 |
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