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How to Clean the Rear Mirror of a Cassegrain Telescope

Cassegrain telescopes are in the reflector class. The light of the object enters the front opening of the telescope and is reflected by the large primary mirror at the rear of the telescope. The light is then collected by the secondary mirror at the eyepiece and sent upwards into the lens where the object is magnified and viewed. The primary mirror doesn't need cleaning very often, but over time enough dust may be collected on this mirror where it must be removed and cleaned.

Things You'll Need

Large plastic bowl (larger in diameter than the mirror)
Screwdriver set
Cotton balls
Distilled water
Can of compressed air

Instructions

- 1
  Remove the rear primary mirror from the Cassegrain telescope. Although some telescope manufacturers use "thumb nuts" which can be turned with the fingers to release the mirror mount from the rear of the telescope, some require the use of a screwdriver to remove screws to pull the mirror away from the telescope's body tube. Remove the entire rear mirror assembly using the appropriate method.
- 2
  Hold the mirror assembly by the edges, taking ample precaution not to touch the mirror surface with your fingers, as skin oils cannot be removed easily from these delicate mirrors without possible damage. Place the mirror into the bottom of the plastic bowl with the reflective surface pointing upwards.
- 3
  Pour distilled water over the mirror surface, using enough until the entire surface of the mirror is submerged in the water.
- 4
  Throw two or three cotton balls into the bowl of water and allow them to become completely saturated with water. Grab one of them with your fingers and gently wipe it across the mirror surface with very light pressure. Move the cotton ball across the mirror from left to right, and right to left.
- 5
  Remove the mirror assembly from the bowl of water and dump the water out of the bowl. Hold the mirror at an angle in the bottom of the empty bowl and pour distilled water over its surface one final time as a rinse.
- 6
  Set the mirror in a reasonably dust-free location with the reflective surface pointing upwards and allow to air dry.
- 7
  Blow off any dust particles from the mirror surface with a can of compressed air immediately before replacing the mirror assembly onto the rear of the telescope. These dust particles will blow off quite easily. Install the assembly back onto the telescope.

Astronomy