CO2 cleaning of the telescope mirrors
This document is to be used during the CO2 / Snowcleaning of the telescope mirrors
When:
Snowcleaning is due to be done every first week of the month. Important before you start Snowcleaning is to have a look at the humidity. If this reads more than 40% don't Snowclean that day. This is because you won't get the wanted snowflakes for the cleaning and you could make condensation stains on the mirror.
Material:
For the Snowcleaning you need:
- Snowcleaning hose with the pipe and key (these can be found in the Mechanical workshop in the INT)
- CO2 tank (these will be at every telescope observing floor)
- Cryostat filing gloves and mask (present at every telescope)
- Safety harness (when cleaning the WHT mirror)
Safety:
- For safety reasons Snowcleaning is a two people job.
- The cryostat filling gloves and the mask are for protection against the cold CO2.
- 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".
You need to connect the orange band on both sides of the telescope cube. With other end you have to span the orange band. Then you can secure the harness on to this orange band.
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.
Procedure:
- Move the mirror to access park (for the WHT do this after the reflectivity measurements)
- Make reflectivity measurements of the mirror
- Open up the mirror covers
- Connect the hose to the CO2 tank using the spanner, which is with the hose set.
- Connect the pipe to the hose
- Connect the earthing wire to the edge of the coating on the mirror with a piece of tape and the other end onto an earthing point (only tough the mirror at this place of connecting)
- Open the CO2 tank
- Climb into the mirror cell with the CO2 hose and pipe
- Open the valve a little. Do not point the pipe 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.
- After you have done the whole mirror wait for 10 minutes and do it again
- Clean up the material
- Make the reflectivity measurements in a new file