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How to Clean Your Telescope Mirror

Rave Uno
Cleaning a mirror involves using a rag dipped in soapy water and wiping it across the mirror's surface. This is the standard procedure followed on most mirrors. But for an extremely delicate and expensive mirror, like the one used in a telescope, you need to use a different cleaning technique.
Mirrors are cleaned only when it is obvious that they are dirty. Fingerprints, smudges, marks and the collection of dust and grime on a mirror's surface are key indicators, to start bringing out the dusting cloths and mixing soap with water.
But the mirrors in your telescope are a different kettle of fish altogether. There are 2 specialized mirrors in a telescope, a primary and a second mirror. The primary one is shaped like a parabola and is coated with a metallic reflective coating.
You may think that cleaning a telescope's mirror is like cleaning any ordinary mirror but a particular style of cleaning must be used for handling such delicate internal parts. A few questions you should ask yourself prior to cleaning your telescope's mirror are:

Can I Take Apart the Telescope Correctly?

A telescope's mirrors are located inside the telescope. The primary mirror is at the bottom of the telescope's tube and the secondary mirror is in a holder within the telescope's tube. The primary mirror is larger than the secondary mirror in diameter.
If you want to clean the mirrors, you will need to be very well-versed on how to take the telescope apart, such that you can put it back together. With most telescopes, you may need to clean only the primary mirror.
To do so, you will need to unscrew the mirror's holding cell at the back of the telescope. If you remove the secondary mirror, you may need to align the mirrors (collimation), once you are done cleaning and fitting the mirrors back in the telescope.

Does the Telescope Need Cleaning?

A telescope's mirrors are very sensitive, delicate and specialized mirrors. Repeated cleaning can wear down the sensitivity of the mirror and remove the reflective coating. Enthusiastic cleaning can scratch the delicate surface of the mirror. Actually the mirrors in a telescope are not meant to be handled at all.
So if you see a spot of dust on the telescope, that does not mean you should clean it. Shining a torch or light down the telescope and checking for smudges and dust is not a test of cleanliness. In fact, even a thin film of dust will not affect the telescope's viewing abilities.
A telescope should be cleaned only if it is really dusty and grimy and if there is a noticeable effect in its imaging. A time interval of 5 years or more should be kept between the telescope cleanings.

How to Clean a Telescope Mirror

Materials Needed

  • 5 gallon washbasin or tub (depending on the size of the mirror)
  • Distilled or de-ionized water (can be obtained from a pharmacy)
  • 1 packet surgical cotton balls
  • 2-4 towels
  • Non-alkaline detergent (do not use dishwashing or laundry detergent)


Initial Steps
  • Wash out the tub, rinse it and spread a towel on its bottom.
  • Take out any jewelry from your hands and arms, including your watch. They could scratch the mirror by accident.
  • Remove the mirror from the telescope gently, do not touch the shiny surface of the mirror.
  • You can grab the mirror by its sides. Lower the mirror on the towel, the shiny portion facing up.
  • Try to get someone to help you with this cleaning job, especially for the rinsing parts.
First Rinse
  • Carry the tub over to your sink and place it in the sink under the tap. Turn on the tap and let water fall on the mirror.
  • Do not rub or touch the mirror, let the water wash away any loose dirt and dust.
  • You can grip the sides of the mirror and move it to allow the water to wash its surface.
  • Rinse the mirror well with tap water. Then rinse the mirror once with a little distilled water.
  • Gently stand the mirror on its edge on a towel and tilt it, so that the water drips off.
  • If the mirror is still dirty, perhaps with fungus or smudges, you will need to soak it.
  • Place the mirror on the towel in the tub. Fill the tub halfway with room temperature water.
  • Add a little detergent to the water. Leave the mirror to soak in the tub for 5-10 minutes.
  • Hold the submerged mirror by its sides in the tub and swirl it slowly to get dirt to come off.
  • You can even slosh the water in the tub to get the dirt off the mirror.
Wiping the Mirror
  • If there are visible marks on the mirror, you will need to use the cotton balls.
  • Try to do this under water. Using the cotton ball, wipe the mirror in one direction.
  • Do not wipe in a zig-zag way. Wipe in one uniform direction.
  • Try to rotate the ball as you wipe. This is so that the dirt on the cotton does not get wiped back onto the mirror.
  • Keep changing the cotton. You may need to use a lot of cotton to get the mirror cleaned to your satisfaction.
Final Rinse
  • Now lift the mirror out of the tub and drain the tub. Place a towel on the tub's bottom.
  • Pour some distilled water on the mirror and let the water run off the mirror. Make sure all the soap has gone.
  • Leave the mirror on the towel to air dry. Do not dry the mirror with a paper towel or cotton, let it dry on its own.
Finally your telescope's mirror (or mirrors) is squeaky clean and dry. You can now fix them back into the telescope, taking care not to touch the mirror's reflective surface.
If the telescope is very dirty and very old, like it was stored away for a very long time in some dusty corner or attic, then do not try to clean it by yourself. Seek professional help and try to restore the telescope completely.