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© Ciel Extrême, 1998
Cleaning closed-tube telescope mirrors without removing the mirror from the cell.
to be rolled down

 © Philip J. Tramdack, 1999.
  unpublished article

1./ Cover the secondary mirror with a plastic bag unless you want to clean the secondary at the same time.

2./ Remove the cover from the bottom of the telescope tube (cell end) if there is one.

3./ Work outside only on a sunny day when the temperature is at least 10 degrees centigrade.

4./ Assemble the following materials:

  • A garden hose attached to a cold water faucet. (Not essential)
  • A 1 liter squirt bottle filled with distilled water and a few drops of common liquid dish detergent.
  • A 1 liter squirt bottle filled with steam distilled water.
  • An empty squirt bottle and a fresh bottle of 95% isopropyl alcohol OR 95% grain (suitable for drinking) alcohol. Do not use commercial denatured alcohol: it may not dry perfectly clear.
  • 5./ Wash the mirror down from the front of the tube with the garden hose at low pressure. This removes a lot of the dust.

    6./ Squirt the entire bottle of detergent-water on the mirror, aiming at visible dirty spots. The point of the detergent is to break the surface tension of the water only so only a few drops of detergent should be used. This will have sufficient cleaning power to dissolve the dirt on the mirror. You may position the telecope upright and allow the mirror to soak for a few minutes if there are dried substances stuck to the mirror (insects). On very warm days the water will evaporate off the surface of the mirror. On those days one must work continuously without stopping even between steps.

    7./ Immediately squirt the entire bottle of steam distilled water on the mirror to remove all traces of the detergent water. Make sure to squirt around the mirror cell clips.

    8./ Immediately squirt the alcohol onto the wet mirror to drive off the water. Usually about 300 cc are needed to guarantee that the water has been displaced by the alcohol. Leftover alcohol can be stored in a smaller airtight container with no air space. Storing otherwise will cause the alcohol to lose its anhydrous properties.

    9./ Do not disturb the mirror until the alcohol is dried.

    If the soap solution is not too rich; if you use steam distilled water only; and if the isopropyl or grain alcohol is pure, the mirror will dry in about 10 minutes perfectly clean. I have washed my 250mm mirror five times per year since 1992 in this manner and have never needed to pull the mirror from the cell and have never touched the mirror. Although there are two or three stubborn spots caused by dried insects, the mirror always comes up very clean. I do not agree that dust does not interfere with the performance of the mirror. My experience is that a coating of dust visible in the red flashlight is sufficient to degrade the performance of the 250mm mirror one or two tenths second of arc-- enough to make a difference in double star or planetary observing.

    The key is to use pure materials and to work fast if the day is warm enough to cause rapid evaporation. Test the purity of your alcohol by putting one drop on a perfectly clean microscope slide. The alcohol should dry without leaving a ring.


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