Dome Standard Operation

 

See 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.

 

Upper Shutter

Main Drive Operation

See 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.

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    Dome Rotation

    See Detailed procedure for Dome Rotation Operation.

     

    Lower Shutter


  • 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 upper shutter.
     
  • Operate the lower shutter in Micro speed to save wear on the motors.

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  • Before operating the lower shutter turn the dome so you can observe the gap between the shutters while moving. 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.

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  • 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.

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    Dome Ventilation

    There are several reasons to believe that the WHT dome ventilation fans do not and cannot achieve their intended purpose:


  • 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).
     

    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.
     

    Standard Practice

    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
    		 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.