How to Aluminize Telescope Mirrors

Written by andrea helaine
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How to Aluminize Telescope Mirrors
If the aluminium coating chips, the image seen through the telescope will be less bright and more fuzzy. (Telescope à l'aube image by Antares from

Applying aluminium to the surface is the last step in mirror building before installing the mirror inside the telescope. The aluminium is applied using evaporation. The mirror is placed in a vacuum chamber and aluminium is vaporised using electrical currents. The mirror needs to be cleaned to avoid having the aluminium coating chip or become uneven.

Skill level:

Things you need

  • Vacuum chamber with operator's manual
  • Tungsten filament
  • 99.99 per cent or Grade A-1 aluminium
  • Paint thinner
  • Lint-free cloth
  • Ethanol
  • Pan
  • Hydrochloric acid
  • Distilled water
  • Silver (optional)

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  1. 1

    Use a lint-free cloth to clean the surface of the mirror with paint thinner.

  2. 2

    Wipe the mirror with semiconductor grade ethanol using a lint-free cloth.

  3. 3

    Dip the mirror for 10 minutes in a pan with one part hydrochloric acid in 10 parts distilled water.

  4. 4

    Install the tungsten filament in the vacuum chamber and add the aluminium strip on top of the filament.

  5. 5

    Install the mirror into the mirror holder inside the vacuum chamber.

  6. 6

    Seal the vacuum chamber and turn the vacuum pumps on. Wait until the pressure is 0.00001mm of mercury.

  7. 7

    Turn the mirror rotators and evaporator on.

  8. 8

    Wait for the cycle to complete.

Tips and warnings

  • You can also used silver to coat mirrors. Silver has better reflectivity and lasts longer but is more expensive.
  • Refer to your vacuum chamber operator's manual to know the amount of aluminium to vaporise, and the cycling time for vaporising aluminium.
  • Use 99.99 per cent or A-1 grade aluminium or you risk having the coating degrade quickly.

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