Re-Aluminization of the 200-inch Primary Mirror of the Hale Telescope at the Palomar Observatory

Time-lapse movies:

Washing the 200-inch mirror filmed October, 2006 (17.7 mb)

October, 2006 full aluminizing movie with narration (Warning: Large File! ~352 mb)
Here's a smaller version posted at YouTube.

June, 2005 aluminizing filmed June, 2005 (~150 mb)

Telescope mirrors get a lot of use.  The 200-inch Hale Telescope at the Palomar Observatory is used on average about 300 nights a year. 

Even with weekly cleanings, mirrors periodically need to be given a new coat of reflective aluminium.  It is a process that takes several days. 

How do you re-coat a 14.5 ton mirror?  What follows is how it happens at the Palomar Observatory.  Click on the images to see them at full size.

 First the astronomical instrument located in the Cassegrain focus at the bottom of the telescope is removed.  The instrument cage and all wiring are also removed from the telescope (left).  This exposes the mirror cell (right) that holds the 200-inch mirror to the telescope.

A large cart is then pushed by the Observatory staff to a position directly under the telescope (above).  A disc on the cart is lifted up to catch the mirror cell which is then unbolted from the telescope. The mirror cell is lowered and then cart with the mirror cell and its mirror is then pushed away from the telescope (right) and over to a position where it can be worked on (below, left).

The mirror is given an inspection and its first rough cleaning (below).

After a wash with soap and water the mirror is ready to have its old coating of aluminum stripped away.  An acid wash is used to strip off the old layer of aluminum (right).  This reveals the structure of the mirror's underside.  The mirror's ribbed backing creates hollow spaces that reduce the mirror's weight and provide locations for the mirror to be supported from underneath.  The supports prevent gravity from distorting the shape of the mirror no matter which way it is pointed.


 After the coating has been stripped it is given an additional cleaning and is carefully inspected.  When all is ready the delicate operation of lifting the top of the aluminization chamber begins.




A crane operator at the top of the dome carefully lifts the 17.5 ton chamber over the top of the 200-inch mirror.  The chamber is lowered over the mirror and secured to the cart.  Over a period of about a day, all of air is then pumped out of the chamber forming a vacuum.  Once the process of pumping the air out has completed the mirror can be re-coated.   Located inside the chamber are hundreds of coils of tungsten wire.  Each of the coils has a small amount of aluminum draped on it.  In the proper sequence the coils are electrified and heated in excess of 1,000o F (below). 

The heated coils vaporize the aluminum which then coats everything in the chamber with a thin film of the shiny metal.  A successful coating deposits just a few grams of aluminum across the 200-inch disk making a layer only 3 millionths of an inch thick.

Before and after shots of the 200-inch mirror.