Cleaning Lenses and Mirrors

You have got that telescope that you have always wanted and have already taken it out a few times. The sights you have seen are still fresh in your mind as you are (for the 200th time) looking over your telescope and now notice dust on your lens/mirror. The first thing you think is the dust will ruin the views that you have been seeing and it needs to get cleaned NOW! Not so, a little dust will not ruin your views as near much as you might think. I have heard that a lot of professional scopes get very dirty before they are cleaned.

The heart of your scope is your lens/mirror and if it gets defective your views will suffer. A quality lens has a coating that allows more light to go through and a mirror has the reflective coating on the first surface (not on the back surface as in the bathroom mirror). Because of this, these coatings are very sensitive to scratches and being rubbed off. Before going into how to clean here is a list of don'ts:

1. Do not let water dry on your lens/mirror (I had this happen to me and water spots are impossible to remove without removing the coating)

2. Never rub hard or even kind of hard. If you thought that stubborn spot was bad....
3. Don't touch the lens/mirror with your fingers, and if you do clean it now.
4. Do not touch the part of the tissue you clean with, this will only transfer to oils off your hand to the lens/mirror.

Materials Needed

Before you start cleaning your lens/mirror make sure you have read and understand the procedure for the condition of your lens/mirror and that you have all the materials needed at hand. This is the heart of your telescope don't take unnecessary chances or be unprepared in the middle of a procedure.

Dust only

To remove just dust, use a clean lens brush and gently brush off the dust. You can use compressed air if necessary but never use aerosol air.


To remove fingerprints treat the lens/mirror as you would for dust only first. This is to remove any dust or grit that may scratch the lens/mirror as you remove the prints. Use a facial tissue and rubbing alcohol. Take a tissue and dampen with alcohol and gently swab the area with the print. Using another tissue dry the alcohol off. Repeat swabbing with new tissue and drying with new tissue if necessary.

Dust and Light Film

If you lens/mirror has become coated with a light film; first use the Dust only procedure, then use the same procedure you would for fingerprints except on the complete surface.

Heavy Grease Film

If some how you have a very dirty lens/mirror use the following procedure. Remember do not let water set for any time at all as water marks are hard to remove without removing the coating. Read entire procedure before starting and have swabs ready before hand. Place lens/mirror (mirror face up) on a towel in a sink, run a cold water stream over the lens/mirror and rinse off any loose particles. Using a large wad of paper towels dipped in solution of dishwashing detergent (1/2 tsp of detergent to a pint of water), gently swab the surface. Keep the water running so that you may rinse the detergent solution off immediately. IMPORTANT DO NOT LET THE SURFACE BECOME DRY OR LET ANY WATER BEADS DRY. I let this happen to a 4-inch mirror I use to have; those watermarks don't come off without taking the coating with it. Now take a wad of paper towels dipped in alcohol and swab the surface. Warning do not turn over the swab as it will take the dissolved oils from your skin a deposit them on the surface of your lens/mirror. If you do this you will have to repeat the procedure. IMMEDIATELY take a large wad of facial tissues and gently wipe the complete surfaces of the lens/mirror. Repeat with more wads till completely dry.

Remember your mirror / lens has to become quite dirty before it seriously affects the performance of your telescope. Weigh the need with the risks you will be taking.

Thanks to Coulter Optical for the instructions they have given me in taking care of my telescope.