Topics
Subaru Work in Summer Part I
September 28, 2001
We are featuring the upgrading and maintaining work of Subaru
Telescope mainly at the submit of Mauna Kea this past summer.
We first dismounted the 8.3 meters primary
mirror (The 8.2 meters in diameter is used for observations.) from
the Subaru Telescope. The primary mirror is on the 9.4 meters mirror
cell.
The Subaru enclosure has two structures; the upper
part can rotate with the telescope, and the lower part is fixed
to the ground. The observing floor where we dismounted the mirror
cell is in the upper part, and the lower part is the area where
the aluminizing the primary mirror took place (the white part of
the enclosure).
The primary mirror with the mirror cell was lifted
down from the observing floor to the first floor of the enclosure.
The total weight of the primary mirror and mirror cell is about
63 tons.
The primary mirror is dismounted from the mirror cell
with the primary mirror handling machine. The primary mirror weighs
about 23 tons.
We next updated the three fixed points that connect
the primary mirror to the mirror cell. Because we operated on the
reverse of the primary mirror, we carefully worked for about three
weeks..
The 261 actuators that correct the mirror flexure
are installed in the mirror cell. On the mirror cell, we adjusted
the actuators closely. The actuator is about 1.5 meters long, and
the weight is about 70 kilograms.
Next is cleaning the primary mirror. We used water
and detergent to clean the dust and greasiness on the mirror surface.
The washing machine like a large dishwasher is controlled with a
computer.
To remove the old aluminium coating from the mirror
surface, hydrochloric acid mixture was used. The staff wore a gas
mask and gloves because the hydrochloric acid is a dangerous chemical.
The primary mirror is made of the ULE (Ultra Low Expansion)
glass. The glass without the aluminium coating is clear, and we
can see the holes, which the actuators support, through the glass.
Then, we moved the glass into the vacuum aluminizing chamber to
be aluminized.
Five months prior to the aluminizing, we prepared
filaments and checked them at the base facility. We applied aluminium
to the surface on the twisted filaments made of 19 centimeters long
tungsten. We use about 300 filaments for the aluminizing.
After installing the filaments to the chamber, we
created a vacuum by pumping air out of the chamber. When we energize
the filaments, the temperature becomes high, and the aluminium evaporates.
The aluminium goes out from the filaments directory and makes a
uniform film on the surface of the glass. If there is air in the
chamber, the aluminium cannot go directly to the surface and the
aluminium coating does not become uniform.
We took the glass from the chamber the next day and
checked the surface on September 12th; we confirmed the successful
finish. The thickness of the aluminium coating is only 0.00017 millimeters.
Then, we put the primary mirror on the mirror cell and lifted them
to the observational floor.
We remounted the primary mirror on the mirror cell
onto the telescope, and have continued to adjust the control system
and to make test observations until mid-October when the Open Use
observation restarts.
