Kwasan and Hida Observatories, Graduate School of Schience, Kyoto University japanese Home page
Maintenance of mirrors

In order to keep high refrectivity, flatness of the surface, we perform maintenance for the mirrors during the rainy season. This is because the low refrectivity, and the bad surface conditions will cause significant scattering during the observation.

The maintenance can be roughly surmarized as 1) to remove the old metal-coating of the mirror, and 2) to newly plate the mirror with Alminuum. We use a device called "Vacuum Evaporater" (VE) which we have in Hida observatory, to plate Al to the glass.

We have performed the maintenance of the mirrors of the 70cm Celostat telescopein Kwasan, and the 60cm refractor in Hida this year (from June to July). The mirrors in Kwasan are sent to Hida, and then are proceeded with the following procedures.

  1. Removal of the old Al-coating.
  2. To wash the surface of the glass.
  3. To plate new Al.

Removal of the old Al coating

The old Al coating is removed by solution of sodium-hydroxide.

To wash the surface

After the removal of the old plate, we wash the glass (which became transparent) with the solution of ethanol and ether to remove not only dust but also oil. After this, we put the glass until it gets dry.

After the glass become dry, we put it inside the VE, and form a vacuum. Then we discharge to create plasma of oxigen inside the VE. The surface of the glass is then irradiated by the plasma, and it becomes highly clean, and activated. The activation of the surface enhances the efficiency of the plating Al to the glass surface. This procedure is called "Ion-bombered".

Plating Al.

This is the main procedure. After forming higher vacuum (about 1/70,000,000 of the air pressure), we heat the filament of tungsten up to 1600 degrees of Celsius with the electric current to vaporise the Al, and plate the surface of the glass. Then the operator stops the electric current to cease heating the tungsten and vaporising the Al to adjust the thickness of the Al plate, by looking at the lamp backside of the mirror which gradually becomes darker by the Al-coating being plated on the glass surface. (Please click here to watch inside the VE during the plating.)

This procedure of plating the Al lasts only about 10 seconds. If the thickess of the Al coating is too thin or too thick, we have to do everything again, so that this procedure requires the operator with a high skill based on many years of experience.

Actually there are much more processes which are not described here. For example, the "Anneal" procedure in which we fuse the Al into the tungsten, or to heat the tungsten-filaments in advance to the main heating to vaporise the Al are the procedures which were resulted by the trials and errors of our pioneers.