How to Clean Fungus From a Telescope Mirror

Updated February 21, 2017

Reflector telescopes are among the best in amateur astronomy for magnifying deep-space objects, such as nebulas, galaxies and distant twin stars. A combination of a wide aperture and a large primary reflective mirror at the rear of the telescope tube permit more light to be reflected toward the lens than any other type of telescope. However, because reflector telescopes have an open front end, dirt and moisture can settle on the large primary mirror, which can then allow mould and mildew to grow on the mirror surface. The mirror may be cleaned, but with extreme caution during the process.

Refer to your telescope operator's manual, and study the section pertaining to the way your telescope's primary reflective mirror is mounted to the tube body of the telescope. Become familiar with the mount assembly and how the mirror is removed and installed.

Use an adequately sized screwdriver to remove the primary mirror mount screws. Slowly and carefully remove the mirror assembly from the rear of the telescope tube body without allowing anything to touch the reflective surface of the mirror. Place the mirror into a plastic wash tub or basin, with the reflective side facing up.

Add 10 to 15 drops of scent-free, dye-free dish soap to the bottom of the wash basin, and then pour distilled water into the tub until the entire mirror is covered with about 1 inch of water.

Rock the basin left-to-right and front-to-back for 15 to 20 minutes, gently, allowing the soapy water to run across the reflective surface of the mirror. Then, observe the mirror to spot any areas on which fungus or dirt remains.

Place one cotton ball into the soapy water, and allow it to absorb water until it sinks. Grab the cotton ball between your fingers and gently, without applying any pressure, roll the cotton ball across the difficult fungus or dirt stain until it is removed. If necessary, use additional cotton balls until the stain is removed. Always roll the cotton ball lightly across the mirror surface; never drag the cotton ball, which may produce scratches on the thin metallic mirror surface.

Dump the soapy water out of the wash basin. Hold the mirror at a slight diagonal tilt with one hand, and rinse the soapy water off of the mirror assembly by slowly pouring 1 or 2 gallons of distilled water over the entire surface. Remove the mirror from the basin, and set it on a soft towel, with the reflective surface facing upward. Allow the mirror to air dry. Refrain from attempting to manually dry the mirror because it is easily scratched.

Install your cleaned mirror back into your telescope's tube body, and fasten the mount screws tightly.


Reflector telescope mirrors have a special thin metallic coating that is easily scratched. Never drag a cloth or any other object, no matter how soft, across the mirror surface; even one small dust particle can cause a deep scratch in the mirror surface. Only use cotton balls on the surface if absolutely necessary and while the mirror is under soapy water, which allows for a lightly lubricated surface, and only rolling the cotton ball across the surface rather than dragging. Only clean your telescope mirror when absolutely necessary and at seldom intervals, never frequently. Cleaning too often can also damage your mirror.

Things You'll Need

  • Telescope manufacturer's operating manual
  • Screwdriver set
  • 5- to 10-gallon plastic wash tub or basin
  • 5 to 7 gallons distilled water (not spring water)
  • 1 bottle scent-free, dye-free dish soap
  • 1 large bag cotton balls
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About the Author

Kurt Schanaman has had several editorials printed by the Star-Herald Newspaper publication in Western Nebraska. He attended Western Nebraska Community College.