The mirror covers and safety brakes on the telescope are pneumatically operated. Pressure for the system is supplied by a Husky air compressor located in the "compressor shed" on the east side of the dome, south of the roll-up door. Pressure is maintained at roughly 80-90 psi and should never require observer itervention. Air pressure can however be checked via the gauge on the western side of the south wall inside the dome, where the air is distributed via manifold to the mirror cover and brake systems.
The control solenoid for operating the brakes is located in a locked box mounted under the louvres near the south west corner of the dome, at the same point as the pressure gauge for the system. The two position valve on the west side of the box should be set to the COMPRESSOR(up) position. A pressure gauge on the side of the box shows the condition of the line, and should read approximately 85 psi. A second pressure gauge with built in filter is mounted on the east wall inside the dome.
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 psi in the cylinder. The output regulator should be set for ~80 psi. Put the two-way valve handle on the solenoid box to the CYLINDER (down, or sideways) position.
There is only enough nitrogen in the cylinder to operate the mirror covers for a few nights.
Brakes can be actuated from the Control System GUI at Telescope>Misc>Switches
tab. Typically, the brakes can simply be left disengaged.
If the brakes are cycled, the calipers will squeak unless
vigorously tapped upon.
A second Craftsman air compressor is also located in the "compressor
shed". This feeds air to the primary mirror support system, which
maintains a pressure of 23" H20 ((<1psi) to the
airbags. The 'clacking' sound typically heard within the dome is
the actuation of air to the mirror support system.
9.2 Pressurizing LN2 Storage Dewars
Unless there is a problem, you should never have to worry about
pressurinzing a nitrogen tank. The primary storage dewar, known
as the "O-50", is filled under pressure, and so maintains a constant
pressure of roughly 11-13psi. This tank should never require
pressurization. The other tank must be brought to atmospheric
pressure to be filled, and so immediately after filling, will have
insufficient pressure to expel the liquid nitrogen. It takes roughly
24-36 hours for the tank to come up to operational pressure.
There is a black line coiled on the west wall in the dome that is
attached through a brass-knobbed valve to the gaseous N2 bottles.
To
pressurize the storage dewar, first open the tapped N2 bottle in the SW
corner of the dome, then crack open the valve, placing your finger
over the end of the
nitrogen hose. Adjust the pressure so that you are able to stop
the
flow with light finger pressure, roughly 1/4-1/2 turn of the brass valve. 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. Pressurize
until the gauge on the storage dewar reads 3-4psi. Remember
to close the storage dewar valves and turn off the HOSE valve along the
west wall once the instrument dewar has been filled in the
usual manner. Lastly, close off the N2 bottle as well.
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.
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 psi.
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.
The telescope must be pointed to the zenith when cycling the mirror covers. Failure to first point to zenith could result in one or more petals remaining closed (or open).
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 attempting to cycle the mirror covers faster than they 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, open the covers through the TCS Control System 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: 2018Sep20 (Galayda/MDM)