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Cleaning Optics - Part 2
Mirrors

As explained in Part 1, it takes a LOT of contaminants before your views will be compromised.  More importantly, mirrors are much more delicate than glass surfaces and cleaning always puts a least a few "micro-scratches" on the surface (one exception - dielectric coated mirrors used in the best diagonals are very durable and not likely to scratch).  Under normal use an open-tube design like a Newtonian reflector should not need cleaning more than once every year or two.  You can prolong the need to clean by always keeping the dust covers on your scope when not in use.  And if the mirrors don't look very dirty - don't clean them!  In the case of closed-tube reflectors like Schmidt-Cassegrain and Maksutov telescopes, you most likely will never need to clean the mirrors.  The only mirror that might need more frequently cleaning is the mirror in the diagonal, that is, if your scope uses a diagonal.

When it is time to clean, keep one thing in mind - BE GENTLE.  The surface of a mirror should never be touched directly.  Also, use absolutely no pressure when actually cleaning.

Gather together a batch of pure cotton balls (natural cotton, not synthetic substitutes), a box of lint-free tissues (refer to Part 1 for recommendations), a large blower bulb and two towels.  Prepare a sink large enough to easily hold the mirror with plenty of room for you to work.  Line the bottom of the sink with one of the large towels and then fill the sink to at least 4 inches with warm water.  Add 2 or 3 drops of mild liquid dishwashing soap (I use Ivory) and mix well.  Place the other towel (dry) on top of a pillow laying on a bed or large sofa - we will be laying the mirror at an angle against the pillow to allow it dry completely.

Next, you will need to remove the mirror from the telescope or diagonal.  Removal varies from scope to scope, but it will most certainly require tools.  If you are not mechanically inclined, it may be best to have a telescope shop do the cleaning or get help from an experienced friend.  The goal is to remove the mirror entirely from its mounting as you will be submerging it in the sink.  Once the mirror is free from its mount, use the blower bulb to remove any loose dust.  Next, lay the mirror, shiny side up, in the water.  Allow it to soak for about 5 minutes to allow more dust to work loose.  Then, lift the mirror from the water and look for remaining contaminants.  To remove them, submerge the mirror again and gently drag a cotton ball across the surface.  Remember - apply absolutely no pressure!  Let the weight of the cotton do all the work and only drag the ball in a straight line.  Do not reuse the cotton - one trip across the mirror and in the trash it goes.  Lift the mirror from the water and check again for remaining contaminants.  Do not be too concerned if you cannot get the mirror perfectly clean - it is more important that you do not scratch the surface by overdoing it.

Once you are satisfied, drain the sink, hold the mirror at an angle and run warm water over the entire surface to rinse it.  Drain the water off the mirror.  If there are any remaining drops of water, touch the drop with the corner of a tissue to blot it away.  Set the mirror at an angle against the towel-covered pillow and allow it to dry completely.  It might be best to dry the mirror in a closed room to prevent pets, kids and significant others from touching the mirror.

One other trick I've learned works well if there are small bits of dust stuck to the surface of a mirror.  Take a lint-free tissue and wet one corner with isopropyl alcohol.  Touch this corner to the dust to wet it fully, depositing a small drop of alcohol on the surface of the mirror.  Immediately touch a dry corner of a tissue to the drop to blot it, and the dust, away from the mirror.

After the mirror is completely dry, reassemble your scope or diagonal.  A scope will then require complete collimation, so be prepared in advance.


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