Dome Standard Operation

Quick links:

1. Upper Shutter detailed procedure that follows 1992 document Safe Opening Procedure following Wet Weather	2. Dome Rotation Safe Recovery Procedure has changed Page following rotation faults . older (1997) document on operating dome and telescope in ENG mode.
3. Lower Shutter open fully for ventilation; response to faults	4. Dome Ventilation discusses standard operation and suggests some rules

1. Upper Shutter

1.1 Main Drive Operation

See Main Drive Operation in the shutter operation page. .

1.2 Aux Drive Operation

This uses the same drive motor but it by-passes the electronics which control the motor speed. It should be used in case of a fault preventing the Dome being closed using the Main Drive.

1.3 Safe Opening Procedure following wet weather

When opening the dome after a prologned period of high humidity, water that has accumulated in the dome rails can enter the dome in considerable quantities. To ensure this does not damage 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 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. Note that any lower than this could switch off the GLAS Laser (depending on override conditions).

Open the lower shutter before the upper shutter - standard procedure.

If you see of hear water dripping, stop the upper shutter and drive it back about 2 metres before trying again. This may require several iterations, but helps to clear water out of the rails before observing begins.

When closing the dome on a night like this, the same procedure should be followed.

2. Dome Rotation

2.1 Dome Panel Operation

Operation is described in this document.

2.2 Dome Rotation Faults

Please note: as of mail from Head of Ops, 3 Sep 2007, we are no longer recommended to open the Circuit Breaker panel in the dome.
Resetting Circuit breakers is achieved by toggling the Dome Power Interlock switch in the Control Room. We only go out to check which motor has tripped.

Here is the original recommended safe procedure to resetting Circuit Breakers, including a photo of the Interlock Crate.

This page describes the two main sources of recent Dome fautls, links to relevant fault reports, and describes recommended recovery procedures.

3. Lower Shutter

3.1 Daytime checks

3.1.1 The DE will exercise the lower shutter during the daytime to avoid the motors sticking (caused by high humidity over a long period). This can be carried out even when there is high humidity and wind outside because the opening is too small to allow much humidity into the dome.

3.1.2 In icy weather, the shutters should not in general be operated, nor should the dome be rotated. In this case, mechanically qualified staff will test these mechanisms during daytime and may note in the operational logbook whether they can be operated. If there is no note, the TO should consult with the DE.

3.2 Night Time Standard Operation.

3.2.1 The lower shutter should preferably be operated in Micro speed to save wear on the motors.

3.2.2 Open the lower shutter before opening the upper shutter. 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.

3.2.3 Since Micro speed is preferred, it takes a long time (some 5 minutes) to open the lower shutter completely. Therefore the TO can first open the lower shutter just until the Main Motor proximity switch is activated (a click is heard) and later open the rest of the way, at a convenient time. Note, at its closed position, the lower shutter vignettes the entrance aperture when the telescope is at about 25 degrees elevation.

3.2.4 Since the lower shutter is important for natural ventilation of the dome (see 1.1.3) it should always be fully opened, provided external humidity is below 75%. (At higher humidities, you may have to close in a hurry).

3.2.5 The Main motor may be used to close the lower shutter at full speed in an emergency, e.g. external humidity goes above 75% or wind speed above 50 kph or causing an alarm on the Met system. Otherwise the Micro motor should be used to close the lower shutter.

3.2.6 The lower shutter may be raised as a windshield if the wind is causing objects to be moved or noise in the dome. Of course if the wind is above the permitted limit of 80 km/h the dome should be fully closed. The shutters may also be used to block direct moonlight when this becomes a serious problem for observing.

3.3 Lower Shutter Faults

3.3.1 If while operating the lower shutter the Torque Trip indicator switches on, it means one of the motors has stopped operating because it has seized or encountered too much resistance. Try to reset the trip indicator using the button and then try moving in the opposite direction to before. If the Torque Trip occurs again, then the lower shutter should not be moved further. The DE should be informed and a Fault Report entered.

3.3.2 If the overtravel indicator is lit, it means the shutter is outside its positional range and the system will prevent the shutter from being moved. If you need to move the shutter in this case, the key should be inserted into the corresponding overtravel key slot and turned. The shutter will then move in the correct direction until it moves out of the limit and the overtravel indicator switches off. The shutter can then be moved as normal.

3.3.3 If the lower shutter for some reason is not horizontal, there should be no attempt to use the motors.

4. Dome Ventilation

4.1 Efficacy of Fans:

There are several reasons to believe that the WHT Dome Ventilation Fans do not and cannot achieve their intended purpose:

4.1.1 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.

4.1.2 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.

4.1.3 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.

4.1.4 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).

4.2 Sensible Use

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.

4.3 Standard Practice

Ventilation practices differ among the TOs. Some TOs use the fans all the time, some never, others use them only in certain situations. The Head of Ops (JR) prefers that all TOs operate the Dome Ventilation in a standard way. The idea is that Visiting Observers should receive the same service independent of who is on duty.

Standard practice at the moment is to switch off the Fans when the external relative humidity goes above 75%. We can apply similar reasoning to other circumstances to obtain a set of "rules" for standard operation for ventilation. Here are the 4 rules that have been agreed on:

	DO NOT Operate Fans	Operate Fans
1st Hour	During the first hour after opening the Dome (to allow the convective chimney effect to work)	During focusing of the telescope.*
Seeing	If the the natural seeing is poor, e.g. 1.5 arcsec or more, there is unlikely to be any benefit from operating the Fans.	If the observers report poor seeing in the science data, but RoboDIMM or another telescope indicates significantly better seeing.**
Temp	If, after 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.
Humid	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.