Dome Standard Operation
Detailed procedure that follows
original 1992 document.
If there is
ice or snow on the shutters they should in general not be operated.
In this case, mechanically qualified staff will test these mechanisms during
daytime and note in the operational logbook whether they can be operated. If
there is no note, the OSA should consult with the operations leader.
Main Drive Operation
Detailed procedure for Main Drive Operation.
Aux Drive Operation
This uses another, very slow drive
and by-passes the electronics which control the Main Drives. This is
for emergency purposes and should only be selected if the main drive has failed preventing the
dome being closed using the main drive. It takes close to 30 minutes to close or
open the upper shutter completely.
Before using the Aux Drive the OSA should consult with the operations leader
even this is late at night. The key for the key switch is kept in the operations
room. See 'WHT keys 1' key press, position 15.
To select Aux Drive insert the key in the Mode Selection switch and turn so that
the arrow on the barrel of the switch points to AUX DRIVE.
Important: The Machinery Selection switch must not be interfered with.
Its normal position is I + II MACH.A+B.
The shutter can then be operated by pushing the buttons marked AUX DRIVE DIR 1 or AUX DRIVE DIR 2 depending upon which direction you want to travel. The button
must be pressed all the time you want the shutter to move.
Safe Opening Procedure following wet weather
When opening the dome after a prolonged period of high humidity, water that has accumulated in the dome rails can enter the dome in considerable quantities.
To ensure this water does not reach the mirror or other important equipment, the following procedure should be followed:
Ensure mirror covers are closed before opening or closing dome.
Align the dome azimuth with the telescope azimuth e.g. using the DOME command on TCS. This ensures that any water that does enter falls well to the side of the mirror cell.
Lower the telescope elevation to 25 degrees.
If you see of hear water dripping, stop the upper shutter, wait 5 seconds and drive it back about 2 metres before continuing. This may require several iterations.
When closing watch out for water dripping.
Detailed procedure for Dome Rotation Operation.
The lower shutter should be lowered only if required for night observing. If
operated at the begin of the night open the lower shutter before opening the
Operate the lower shutter in Micro speed to save wear on the motors.
Before operating the lower shutter turn the dome so you can observe the gap between the shutters while
if the upper shutter is already open and you have to lower the lower shutter
during the night, use a torch to watch the lower shutter while opening. Does it not open
or close parallel or if the drives do not sound right you must stop and not try again. Submit a fault report.
When closing, bring the upper shutter completely down before finally closing the lower shutter in Micro speed.
This is to provide the seal at the top of the upper shutter.
There are several reasons to
believe that the WHT dome ventilation fans
do not and cannot achieve their intended
The fans work in the opposite direction to the "chimney effect", which is what allows warm air to rise by convection and exit through the top of the dome shutter.
This means their capacity to change the dome air is reduced.
The fans blow in the direction opposite to what they were designed and installed for, by reversing the electrical polarity on the fan motors.
This means they will not operate to their specified capacity, which in any case in not known.
The fans are very far below the telescope and shutter, while the consensus in dome design is to position ventilation openings around the height of the telescope axes.
This in turn suggests that fully opening the lower shutter is quite important.
However, the lower shutter must only be lowered if required for night
observing (see above).
Even with fans on, internal temperature generally remains about 1.5 degrees warmer than external.
This is mostly because external temperature tends to fall throughout the night, and the dome can't keep up.
There is no positive evidence for better dome temperature difference or better seeing when dome fans are ON in either the seeing.wht log or in JOSE observations (1995-98).
Since the chances of reducing
the internal temperature diminish as the night goes on, common sense suggests
that at some time during the night the fans can be switched off. Secondly, it is clear that 6 fans working all
night will consume a considerable
amount of electrical power. Therefore
if the fans are not helping, we ought to prefer to switch them off.
Standard practice at the moment is to switch off the
fans when the external
relative humidity goes above 75%. Furthermore the following rules have been agreed on:
DO NOT Operate Fans
the first hour after opening the Dome (to allow the convective chimney effect
During focusing of the telescope.*
the natural seeing is poor,
e.g. 1.5 arcsec or more, there is unlikely to be any benefit from operating
observers report poor seeing in the science data, but RoboDIMM or another
telescope indicates significantly better seeing.**
halfway through the night, Internal Temp trend is no longer falling.
If Internal Air Temp is 3 deg or more
above External Temp, except during the first hour after opening.
If External Humidity > 75 % (alarm)
* The fans may help to
suppress convection and therefore seeing fluctuations, although there is
no real evidence for this.
** Again there is no evidence the Fans will help, but it is a reasonable
response to the situation.