This
document is to be used during the CO2 / Snowcleaning of the WHT telescope
mirrors.
CO2 cleaning of the telescope
mirrors
When:
Currently we use snow cleaning on WHT M1 every 6-8 weeks, depending on weather and R% measurements.
If the Internal Humidity reads more than 40% do not Snowclean that day, to avoid condensation
stains on the mirror.
Casual observation also suggests that when RH% is below 20, snowflakes do not stick as much to the mirror surface, which may reduce the effectiveness of the cleaning.
Power down the mirror temperature sensor amplifier before snowcleaning. The
switch is located behind the Sauron PC in the control room.
The mirror now has an earth cable connecting the edge of the reflective surface to ground,
which prevents damage to the temperature sensor by static.
Material:
For the Snowcleaning you
need:
- Snowcleaning hose with
the pipe (wand) and large spanner (these can be found in the OPS team office
behind the wooden slide door)
- CO2 tank (these will
be at every telescope observing floor)
- Cryostat filing gloves
(present at every telescope). Wearing the face protection helmet while CO2
cleaning is considered unnecessary and a hazard for the mirror surface.
- Safety harness (when
cleaning the WHT mirror) and (NEW, Nov 2010)
Fall Arrestor.
Safety:
- For safety reasons it
is preferred to Snowclean with two people.
- The cryostat filling
gloves protects hands from the freezing temperature of the metal part of the
CO2 wand.
- For Snowcleaning the
WHT there is a safety harness which will keep you in the mirror cell. These
can be found in the cupboard reading "Snow cleaning safety equipment".
Before parking at AP3, the fixed end of
the Fall Arrestor
must be secured to the hook above the broken CASS port and the extesible lead trained out
between the spider vanes. Finally place the clamp at the end of the extensible lead over the
ladder hooks on the opposite side of the cube. When opening the mirror petals, one of these will
slightly brush the spool housing of the fall arrestor, but should not trap it when fully open.
Important to know:
Snow cleaning is done to
remove the dust on the mirrors. The slow flow of snowflakes pickup the dust and
falls down from the mirror.
If you use a high pressure
you might damage the mirror when hard snowflakes bounce against the soft
aluminium and cause pits in the mirror coating.
The uSMS reflectometer head
is sensitive to temperature. It should be left in the dome for some time,
preferably overnight, to adjust to ambient temperature. The unit can be put on
the electrical charger during this time.
Procedure:
- Secure the Fall Arrestor as decribed above.
- Make reflectivity measurements at each of the ports on the mirror petals.
- Park the telescope at AP3 (WHT) or Access Park (INT) and open the mirror
covers
- Alternatively, Reflectivity can be measured at park position, but only in the bottom half, in the case of the WHT mirror.
- Connect the hose to the CO2 tank by hand and then finally tighten (without excessive force) using the spanner,
which is kept with the hose set.
- Connect the pipe to the hose
- Climb into the mirror cell with the CO2 hose and pipe
- Open the CO2 tank while the pipe is opened. (Make sure the pipe is NOT
pointing at the mirror!)
- Adjust the flow of snowflakes until you have a nice slow flow of
snowflakes leaving the pipe
- Now point the pipe at the mirror and spray the whole mirror with the
snowflakes. Make sure you don't stay too long on one place. Keep the pipe
moving. The snowflakes should fall on the mirror and glide down with the
dust.
- Attempt to clean uniformly by sweeping the plume of snoflakes in a regular way accross the surface, like when painting.
- When finished, close the valve on the CO2 tank but do NOT close the valve on the wand.
- After you have done the whole mirror wait for 10 minutes and do it again
(in the WHT this time can be used to do the Nasmyth flat)
- When finished cleaning at WHT, check the control box for the Nasmyth
turret mechanism, on the inside of the cube, where you are standing. The
switch should be set to ON, otherwise the Nasmyth flat mechanism will not
move.
- The WHT mirror cover petals on which you have been standing should be
pushed upward until you feel resistance from the mechanism. This will ensure
they will close fully later.
- Tidy away the equipment
- Once at least 10 minutes have passed since cleaning finished, you can
make the final reflectivity measurements.
- Move the telescope to Zenith park
- Alternatively take Reflectivity measurements through the ports at senith park.
