<snipped pages of JJs excellent information>
>JJ
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I would like to add some information to this discussion from a slightly different perspective. I am employed on a research project to observe ultra-high energy cosmic rays. The project uses many large (50" - 63" diameter) aluminized front surface spherical mirrors. We wash these mirrors on an annual basis.
Things to do:
What: Use dH2O (de-ionized water or distilled water). Why: Good quality dH2O is very clean. It does not contain dissolved materials which will precipitate out on your mirror when the water evaporates away. It is also enhances the cleaning ability of detergents.
What: Support the mirror in a steeply inclined position.
Why: The dH2O will drain rapidly towards the bottom of the mirror. This speeds drying and aids in rapidly removing contaminated wash water from the mirror.
What: Do not let any part of the support touch the front surface of the mirror. This includes anywhere near the edges of the front surface.
Why: Any supporting structure will tend to trap contaminants. It is nearly impossible to sufficiently clean the cracks and crevices in supports or the small gap between the support and the mirror. Surface tension will pull the contaminants from these places onto the surface of your mirror as it drys.
What: Rinse the entire mirror vigorously using dH2O first.
Why: This removes any loose grit which may scratch the mirror surface in subsequent steps.
What: Use a wet cotton ball as suggested in JJs procedure to wash the mirror surface. Use very light pressure. Use lots of water and just a bit of detergent. We use clean sponges for our large mirrors.
Why: Obviously you should use the least destructive instrument to wash the mirror surface hence the wet cotton ball. Lots of water ensures that the surface remains well lubricated, also to prevent abrasion. Use just enough detergent to do the job. More detergent just means more to rinse off later. In my experience a 5 ml detergent is plenty for 1 liter of dH2O. I use a lab detergent called Liquinox but JJs suggested dish soap should work fine.
What: Wash up and rinse down. That is, when washing start at the bottom of the mirror and work your way up. When rinsing the mirror start at the top and work your way down.
Why: This is an old washing maxum. Washing up brings fresh cleaning solution to unwashed parts of the mirror. The contaminated cleaning solution runs off the bottom of the mirror. Rinsing down brings fresh dH2O to the mirror surface. The contaminated water drains off the bottom of the mirror. If you were to rinse up instead the dirty water draining from the higher parts of the mirror surface would continually contaminate the rinse water below.
What: Rinse the entire mirror front and back and the support.
Why: This reduces the possibility of having surface tension drag contaminated water from other parts of the mirror or support onto the front surface of the mirror.
What: Finally rinse the front surface of the mirror over and over. Start each pass at the top of the mirror sweeping back and forth and end up at the lowest part of the mirror.
Why: Rinsing is by far the most important step. It is almost impossible to over rinse a mirror and very easy to under rinse one. Alway start at the top and work you way down to keep the dH2O on your mirror face as clean as possible.
What: Use hot dH2O for washing and rinsing! Not too hot.
Why: Hot cleaning solution works better. Most chemical reactions including the action of detergents proceed faster with increasing temperature. Higher temperature also enchances the rinsing ability of the dH2O. The mirror will dry rapidly. I use water the feels hot but is not hot enough to be painful. Too hot might crack your mirror or damage the surface. I have never seen this actually happen but I suppose it is possible.
What: Do not use alcohol to speed drying of the mirror.
Why: I have found that using reagent grade ethanol (not isopropyl) alcohol to speed the drying of mirrors leaves a small residual film which reduces the reflectivity of my mirrors in the near UV. This may not be the case for isopropyl alcohol, however, I find that hot dH2O and careful procedure does the job just fine.
What: If drops form near the top of the mirror then wash it again. You will often get a drop or two near the bottom of the mirror.
Why: A clean mirror will dry rapidly starting at the top. A continuous interface between the dry and wet surface will propogate from the top of the mirror down to the bottom as the mirror drys. If an island of water forms this indicates that there is a local anomally in the surface tension (aka: dirt).
What: If you get a few drops forming near the bottom of the mirror as it drys then use hot dH2O to rinse them away before they dry. Only rinse near the bottom of the mirror. It is usually a good idea to re-rinse away this last bit of water near the bottom of the mirror anyway.
Why: As the mirror drys the surface tension will kindly drag along the minute amounts of contaminants which are always left. They will concentrate as the last bit of water collects near the bottom of the mirror. A few drops may form on the mirror surface. Re-rinsing this last bit at the bottom of the mirror clears away the contaminated rinse water.
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I should note that our mirrors are NOT protected by either SiO or MgF. We use Aluminum Oxide.
-- Stan Thomas <thomas@cosmic.physics.utah.edu>