This document is to be used during the CO2 / Snowcleaning of the WHT telescope mirrors.

CO2 cleaning of the telescope mirrors

Snowcleaning is due to be done every month. Important before you start Snowcleaning is to have a look at the humidity. If this reads more than 40% do not 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.

For the Snowcleaning you need:

• Snowcleaning hose with the pipe and key (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 and mask (present at every telescope)
• Safety harness (when cleaning the WHT mirror)

• For safety reasons it is preferred to Snowclean with two people.
• 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".

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.

1. (For WHT cleaning) Move the telescope to Zenith park and lock off
2. (For WHT cleaning) Move the Nasmyth flat to the GHRILL position
3. (For WHT cleaning) Climb in the cube via GHRILL and measure the mirror's reflectivity
4. Move the mirror to access park and open the mirror covers (for the WHT do this after the reflectivity measurements)
5. (For INT cleaning) Make reflectivity measurements of the mirror
6. Connect the hose to the CO2 tank using the spanner, which is with the hose set.
7. Connect the pipe to the hose
8. Stick the earthing (yellow/green) wire to the edge of the coating on the mirror with a piece of tape (only tough the mirror at this place of connecting). Connect the other end on an earthing point.
9. Climb into the mirror cell with the CO2 hose and pipe
10. Open the CO2 tank while the pipe is closed
11. Open the valve on the pipe a little. Do not point the pipe at the mirror
12. Adjust the flow of snowflakes until you have a nice slow flow of snowflakes leaving the pipe
13. 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.
14. 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)
15. Clean up the material
16. (For WHT cleaning) Move the telescope to Zenith park
17. Make the reflectivity measurements.