How to Repair and Re-coat a Telescope Mirror

How to Repair and Re-coat a Telescope Mirror

Telescope mirrors collect and then reflect light to the viewer's eye. Damaged or old telescope mirrors distort and reflect less light, thereby reducing the effectiveness of the mirror to provide a clear, sharp image. Atmospheric corrosives, like those caused by air pollution or salt water, are the primary cause of damaged mirrors. Over time, all telescope mirrors need to be repaired and re-coated to restore them to their original, or better, condition.


  1. Wash the mirror thoroughly with a mixture of 3 drops sulphonated organic detergent to 1 litre (or quart) of distilled water. Rinse thoroughly with distilled water until no beads form as the water runs off of the mirror.

  2. Swab the mirror with a cotton ball placed on the end of a glass rod, dipped in concentrated nitric acid, to oxidise any remaining residue left on the mirror. Continue to swab the mirror with nitric acid until all regions of the mirror produce a slightly grating feeling.

  3. Rinse the mirror thoroughly with distilled water when finished. Place the clean mirror into a bath of distilled water, covering the mirror completely so it is not exposed to the air.

  4. Mix the silvering solution using the following formula. For a mirror of area x square inches, mix x/5 grams of silver nitrate with 17cc of distilled water per gram of silver nitrate (i.e., 1 gram of silver nitrate requires 17cc of distilled water, 2 grams of silver nitrate requires 34cc of distilled water, and so forth).

  5. Pour the required amount of silver nitrate solution into the glass container that will hold the telescope mirror, then add concentrated ammonium hydroxide while stirring constantly. A brown silver oxide will begin to form and cloud the mixture, but continuing to add ammonium hydroxide will redissolve the precipitate. Continue to add ammonium hydroxide until the brown silver oxide is just dissolved.

  6. Prepare a mixture of 60 grams of potassium hydroxide with 1000cc of distilled water, and slowly add this mixture to the silver nitrate solution. When a dark-brown precipitate forms, stop. Add, drop by drop, just enough ammonium hydroxide to almost clear the solution (If the solution clears, add a few drops of silver nitrate solution to re-form a slight brown cloud in the mixture).

  7. Mix 100 grams of table sugar, 40cc of nitric acid and 1000cc of distilled water, and bring to a boil in a glass container able to withstand high temperatures. Once the solution has reached a boil, cool to room temperature and add to the silvering solution while stirring vigorously for a few seconds. Lower the telescope mirror, reflective side up, into the bath and keep the solution moving across the face of the mirror by tilting the bath from side-to-side and by gently rocking the bath in a rotary motion (always keep the mirror face submerged).

  8. Remove the small black specks that will begin to form on the mirror face by gently swabbing with a cotton ball attached to a glass rod. Remove the mirror as the silvering solution begins to clear and the black specks become granular. Immediately rinse the mirror with distilled water. If the reflective silver film that has formed on the mirror face is too thin, repeat this process from the beginning (rewashing with detergent is not necessary).

Tips and warnings
  • Try this procedure with a small mirror before re-coating your telescope mirror.
  • Preparing the solution described in section 7 may be done in advance so that it is ready when the silvering solution has been prepared; however, this solution will not last longer than a few hours.
  • Wear protective gloves and eye wear when mixing and using the chemicals needed for re-coating.
  • Do not use soap and tap water instead of a sulphonated organic compound, as they will form insoluble compounds with calcium and magnesium that will deposit upon the mirror.
  • Do not touch the new mirror surface with your fingers.
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