Always check that you have plenty of pressure and reserve in the nitrogern gas cylinders before you start the washing process in order to make the drying fast and optimum.

  • 1.5M TEL: washing done in-situ. Remove chimney. Put telescope at ZD60deg to the south. Raise the platform as much as possible. Remove northern petal covering M1, install the pneumatic seal around M1 so that it protrudes about 20mm above the mirror edge, and inflate it. Seal VERY carefully the space between the chimney base and the inner hole of the mirror with duct tape. If you don't remove the instrument from the telescope, make very sure all the seal are effective! Prepare a warm and highly concentrated solution of orvus soap and water (lot of foam). Rinse with tap water and hose. Use the vacuum cleaner to suck water. Contact-wash with the natural sponge and the soapy solution, and try to maintain the glass wet and covered with foam for 5 minutes (the idea is to unstick the dust and also degrease). Rinse with tap water. As usual, observe how the water is flowing on the surface, any abnormal surface tension showing up will indicate a residual grease that has to be removed. Never let the mirror dry. Finally rinse with bidistilled water (about 6-8 liters needed in small 1 liter bottle) and dry with at least one high pressure nitrogen gas gun (check you have plenty of pressure in the nitrogen cylinder before you start). Dry carefully with KImwipes all the water drops remaining around the mirror. With lot of care to avoid trapped water jumping, remove the inflatable seal and unstick the adhesive tape.

  • 0.9M: remove the cell from the tube, incline it by 5-10deg. Mirror stays in cell. Seal the inner hole by forming around it a 2"-high cylindrical wall with duct tape and seal the outer diameter with the special plastic round skirt and duct tape. Contact-wash with sponge and soapy solution. Suck the water from the lower side of the mirror with vacuum cleaner. Dry with 2 nitrogen gas guns. The entire process takes about 3-4 hours (the washing itself takes 30min at most) and can be done without engineering time as the telescope collimation is not affected once the cell is reinstalled.

  • 4m CFADC top surface (sol-gel AR coated): bring the telescope at D=30deg North (same position as M1 cleaning) and work inside the chimney with proper working light. First blow off dust with dry nitrogen. Only about 100ml of dichlorodimethylsilane (DDMS) is used in a goose-neck plastic bottle. A large plastic bag is taped on the lower half of the cell perimeter to force water to flow directly into it. Two Orings seal the top element with the cell and the cell itself so that no liquid can get inside the corrector assembly (see CH2903-E003) but you still need to take some care. Use some Kim-wipes towels placed at the bottom of the lens to suck liquid as it flows down the glass, and change the towels once they are too wet. Dry off with nitrogen gas. The entire process takes at most 1h.

    IMPORTANT: solgel coatings are hygroscopic (they absorb water) and deteriorate over time. Waterproofing is achieved by rinsing the coating with a solution of DDMS with ethanol to a concentration of 5 parts per million. Alcohol is not good because it will wash off the DDMS and leave the solgel unprotected against humidity! Be careful, DDMS is very nasty stuff. Prepare the dilution in 2 steps: first 0.1ml of DDMS into 100ml of ethanol and then 0.5ml of this solution into 100ml of ethanol.

  • M3 in F/14 TT BOX (gold coated): remove cell from box, remove mirror from cell (it is pushed against a reference corner with 2 plungers), and wash in a soapy and warm bath, rinse and dry with nitrogen gas. Some minor tilt adjustment is needed when its cell returns into the box (do it with Osiris pupil imagery).


    April 11, 2001, Maxime Boccas