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.






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