Mirror Cleaning

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Steward Observatory on Mount Lemmon

This procedure for cleaning large mirrors was demonstrated to us by Joseph Hochscheidt when he cleaned the Manner Telescope primary and secondary in June 2018. Use it with caution since this is based on notes taken at the time. Obviously the first rule is do no harm, and that means be reasonable, gentle, and follow the advice of experts when it is available.

For this cleaning process the mirror was left in the telescope. It was a warm sunny day, and the telescope enclosure was opened slightly to allow for ventilation and to provide illumination needed to judge the effectiveness of the process.

1. Blow off all dust and loose particles with dry nitrogen gas from a compressed gas cylinder.

This requires bringing a cylinder to the telescope, attaching a regulator, and using a flexible hose that provides enough gas flow for the purpose. Do not use compressed air! It will contain moisture that will condense on the optics, and it may also contain fine particulates that could damage the coatings. An alternative that is often used with large optics is to use gas withdrawal carbon dioxide. This is available in small cylinders, and is released at full pressure through a hose to direct the flow onto the mirror and its shrouds. Carbon dioxide cleaning will produce CO2 snow which can effectively scrub a mirror of dust. However it will be insufficient to remove deposits left by evaporating dew or by insects. If a supply of dry nitrogen or carbon dioxide is not available the surfaces may be dusted with a new very clean and soft brush. A photographic lens camel's hair brush, or a soft paint brush with fine bristles will be safe for this task. Compressed gas is preferred, however.

Assure that all loose material is blown out or wiped from the housing that baffles the mirror.

2. With the telescope pointed approximately horizontally, use large quantities of Kimwipes or paper towels rolled to absorb any fluids that drain from the mirror. These may be replaced several times during the cleaning process. The purpose is to prevent fluids from draining into critical components behind the primary, or auxiliary instruments or components that may be on the lower side of the truss. Each case will be unique and should be evaluated before beginning. It may be beneficial to point the telescope slightly below the horizontal if the light baffle around the primary is solid and would hold liquid that could drain to an area which is not critical.

It is not advisable to remove the primary. This adds considerable risk to the cleaning process, and would surely require telescope realignment. Also, primary removal would normally be preceded by mechanical work to assure the telescope remains stable, and would add days to a job that can be done in hours.

3. Wash the interior and all exposed surfaces with water.

4. Prepare a solution of distilled water and detergent. The distilled water can be purchased at a grocery or drug store in gallon jugs. It should be mixed in a clean 5 gallon bucket reserved only for mirror washing. For mirrors of the order of 0.5 meter in diameter, 2 gallons of water for the washing solution should be adequate. Add a small quantity of Liquinox detergent to the water. Test the solution to see that it wets a surface easily. Excess detergent leaves a residue that requires more rinsing and the compromise needed is to have enough in solution to clean, and not so much it is difficult to rinse. Only a few milliliters of liquid detergent should be needed in 2 gallons (about 8 liters) of distilled water. If Liquinox is not available, then Dove dishwashing liquid also works well.

5. Prepare suitable washing wipes. At Steward the preferred ones are treated seasponges that have been soaked in vinegar to remove minerals and then very thoroughly washed and rinsed. The final wash and rinse is done with TX609 Technicloth wipes bundled to make a poof ball perhaps 10 cm in diameter and held by the back where the edges of the cloth are gathered. You will need many of these and it helps to have an assistant making them as you proceed. At this stage both the person doing the mirror cleaning and the assistant should have on new clean nitrile gloves so that skin oils are not collected and left on the mirror.

6. Apply the water and detergent mix to the mirror working in thirds. It may take two or more treatments to achieve a clean surface, and a preliminary rinse should be done on the work area before moving to the next third.

7. Rinse twice with distilled water and pat dry with a TX609 wipe poof. The rinse should be using a separate rinse bucket of water into which to dip the wipe. The final rinse is with a wet but not dripping wipe. The goal is not to leave water droplets on the surface, and not to pick up debris into the wipe that will scratch the coating. By using a large clean bucket and fresh wipes on each pass, you are assured that contamination of the water and the wipe is minimized.

File:Ulmt:471px-primary cleaning.jpg

The primary mirror of the 0.6-meter Manner Telescope on Mt. Lemmon during cleaning. Packing around the lower edge absorbs cleaning fluids. The upper right third of this mirror has been cleaned and washed.

Materials required:

  • TX609 Technicloth
  • Texwipes
  • Bounty paper towels (for catching dripping water)
  • Dry nitrogen gas or gas withdrawal carbon dioxide with appropriate connectors and hoses
  • Soft brush
  • Isopropol alcohol 90% for removing stubborn organic contaminants
  • Distilled water (several gallons)
  • Clean 5 gallon buckets
  • Liquinox detergent
  • Dove dishwashing liquid (alternative to Liquinox)