9 The Pneumatic System
The mirror covers and safety brakes on the telescope are pneumatically operated. Pressure for the system is supplied by an air compressor located in the utility room adjacent to the kitchen area. It can be heard cycling on and off and occasionally venting to expel moisture from the tank. It automatically maintains 70 p.s.i pressure and does not require observer intervention.
The control solenoids for operating the mirror covers and brakes are located in a locked box mounted under the louvres near the south west corner of the dome. The two position valve on the side of the box should be set to the COMPRESSOR position. A pressure gauge on the side of the box shows the condition of the line, and should read approximately 55 p.s.i. A pressure gauge mounted on the south side of the utility room gives the pressure in the line near the tank.
In the unlikely event of a compressor failure, pressurized nitrogen gas from a cylinder located in the south-west corner of the dome can be used to operate the mirror covers and brakes. There are two gauges mounted on the cylinder, showing the cylinder pressure and the working pressure of the output line. If the cylinder pressure reads zero check that the main cylinder valve is open. You need at least 500 p.s.i. in the cylinder. Adjust the output regulator for 55 p.s.i. Put the two-way valve handle on the solenoid box to the CYLINDER position.
There is only enough nitrogen in the cylinder to operate the brakes and mirror covers for several nights.
9.2 Pressurizing LN2 Storage Dewars
The liquid nitrogen storage dewars are normally filled every Friday. When they have just been filled the pressure in the storage tanks will be insufficient to expel the liquid nitrogen. To pressurize the storage dewar crack open the valve marked HOSE located above the nitrogen cylinders. Place your finger over the end of the nitrogen hose and adjust the pressure. You should be able to stop the flow with light finger pressure! Stuff the end of the hose onto the lower horizontal nipple of the storage dewar and open both valves; liquid nitrogen will be expelled down the fill tube. Remember to close the storage dewar valves and turn off the HOSE valve near the gas cylinder once the instrument dewar has been filled in the usual manner.
9.3 Changing Nitrogen Gas Cylinders
PLEASE LET THE STAFF CHANGE THE BOTTLES!
In a dire after-hours emergency, proceed as follows:
Put the COMPRESSOR/CYLINDER valve to COMPRESSOR (default).
Close the main valve located on the top of the cylinder.
Open the HOSE bleed valve to release pressure in the line; close it after a few seconds.
Loosen the brass nut attaching the regulator to the cylinder. It has a common right-hand thread. You will hear residual gas pressure escaping. This is normal. Remove the regulator.
Screw the large safety cap onto the neck of the empty bottle.
Place ``EMPTY'' sign on the bottle.
Do NOT move the full cylinder; the hoses will reach either stand.
Unscrew the large safety cap off the neck of the full cylinder. WARNING. Do not move the gas cylinders.
Attach regulator to the full cylinder and tighten firmly with adjustable wrench.
Crack open cylinder valve keeping face away from gauges. The cylinder pressure should read around 2500 p.s.i.
Put the COMPRESSOR/CYLINDER valve handle to CYLINDER only if the compressor is out of service.
Inform the staff that you have changed the cylinder; put note on the board in the observing room.
9.4 Mirror Cover Failure to Open or Close
You should only open and close the mirror covers when the telescope is pointing at the zenith.
The mirror covers consist of four petals that have to open and close in a particular order. They open in the order south and north first, then west and east and they close in the reverse order. If only one cover fails to open or close, manually slew the telescope using the hand paddle in the dome so that the desired motion of the cover is aided by gravity. For example, if the east cover fails to open, slew the telescope east. If the east cover fails to close, slew the telescope west..
The most common reason for the mirror covers failing to open at all is that the west cover has overlapped the edge of the south one when they were last closed, preventing the south cover from opening first. If all of the mirror covers fail to open contact the mdm staff.
Sometimes the pneumatic valves which operate the mirror covers get into a confused state; only some of the covers open. This condition can be caused by switching the open/close toggle on the TELESCOPE CONTROLLER faster than the covers can open and close. (Don't do this: the covers might end up closing in the wrong order, see above paragraph). To return the covers to their normal state, put the toggle switch to open and move the two-position valve on the control box located at the south-west corner of the dome from COMPRESSOR to CYLINDER. Wait 10 seconds and then return it back to COMPRESSOR. The remaining covers should now open within one minute.
Updated: 2006 December 01 (rab/MDM)