Archive for the ‘How to Clean a Telescope Mirror’ category

How to Clean a Telescope Mirror That Will Exceed Your Expectations and Will “Do No Harm”

March 18, 2013


     The following is the method I’ve used many times over many years to “successfully” clean telescope mirrors.  

     I reviewed a variety of sources to get my ideas. 


Cotton balls  (100% cotton)  – Be sure to store the cotton balls in a sealed zip-lock bag

Blower bulb from a photo supply shop

100% Camel hair brush – Do not touch the bristles with your fingers or hands, and be sure to keep the brush or brushes in a sealed bag. 

Three gallons of distilled water

One teaspoon of liquid “Ivory” dishwashing detergent

Two clean folded bath towels  (not to have been washed using fabric softener) 

     First, wash your hands and remove all rings.   (You may  use surgical gloves if you so desire)

     Now its time to remove the mirror cell (with mirror) from the telescope.  Using a permanent marker, mark two lines on the side of the mirror and also on the cell.  This will allow the same mirror/cell orientation after the cleaning process.  Carefully remove the mirror from the cell.  Tilt the mirror on its side and blow any loose particles from the surface with the blower bulb.  Now, take the camel hair brush, and using care, very lightly “whisk” the surface, removing any loose debris or particles that might have remained.  

     Typically the mirror cleaning process is carried out in the kitchen sink.  It’s best to wash the sink thoroughly using dishwashing detergent and rinsing with water from the faucet.  After this, wipe the sink with alcohol to remove any oil or grease residue.  Place one of the folded towels in the sink and carefully lay the mirror on the towel.  Turn on the water and flood the mirror surface for a couple of minutes.  Next, plug the sink and fill with distilled water to a level that is about one inch above the mirror.  Pour one teaspoon of liquid dishwashing detergent into the sink with the mirror, and let soak for about five minutes.  

     Secure one cotton ball by the corner to insure that finger oil is not deposited onto the mirror.  Next, saturate the cotton ball with water and pull it gently across the mirror…using only the weight of the saturated cotton ball.  After each crossing, dispose of the cotton ball, and proceed with another cotton ball (make sure the mirror is completely submerged during the entire cleaning  process.)  If the mirror is really dirty, I’ve found it necessary to use the same procedure, but pulling the cotton balls crossways.   

      The rinse process:  Turn on the faucet and flood the mirror, and then gently put one hand under the mirror and lift out of the cleaning water.  Rinse the mirror for at least two minutes, then turn off the faucet, and immediately start pouring one gallon of distilled water over the mirror for the final rinse.  

Caution!  Be careful not to hit the faucet with the mirror….


      During my most recent cleaning, my final process was to pour an entire bottle of 91% isopropyl alcohol over the mirror as the final rinse, and it seemed to have worked fabulously! 

      Carefully position the mirror onto the second folded towel at an angle to allow the remaining distilled water, and or 91% isopropyl to drain from the surface.  You can use the blower bulb to chase drops of water over the edge.  Do not use any other means to dry the mirror.  

      You’re almost through.  Now, carefully install the mirror back into the cell using the alignment marks.   You are now ready to tighten the mirror clips.  I use a sheet of notebook paper as a gauge.  I leave just enough space between the clips and the mirror to be able to slide a corner of  the notebook paper between the two.  This will leave a very tiny amount of space.  The clips should not be tightened down against the mirror, as their only purpose is to keep the mirror secure, but not tight.  This will insure that the mirror is not stressed, especially during temperature changes. They should, however, be tight enough to keep the mirror from moving or “flopping around” in the cell.  The clips should have cork pads glued onto the area that touches the mirror. 

      Disclaimer:  This method has allowed me to clean telescope mirrors with excellent results.  It should, however, be noted that your results may vary.  And mirrors coatings are very delicate, so be careful and use good common sense and care.   

Roger Ivester