When I left off last week I had described the process of how we took Palomar's 60-inch mirror out of its telescope, transported it from its dome to the 200", cleaned it & stripped off the aluminum coating. Finally, when everything was clean and dry it was loaded into the aluminizing tank as shown above.
The tank is then sealed and vacuum pumps are engaged to pump the air out.
When the tank approaches a pressure of 10x-6 torr a process known as glow discharge is applied. Essentially a plasma (ionized gas) is created inside the tank.
In Procedures in Applied Optics John Strong (1988), the guy who invented the process of vacuum deposition of aluminum onto telescope mirrors, describes the glow discharge as an "old but still useful method" that essentially further cleans the mirror surface of any organic residues "which may also positively influence the film structure" (i.e. the aluminum coating).
Here is the view looking into the aluminizing tank during one of the three glow discharges. The mirror is on the left, the plasma is produced at the bright spot in the upper right.
The tank is pumped further and when finally ready it is time to apply the aluminum coating. The inside of the tank contains tungsten wire coils that have had aluminum melted onto them. I didn't manage to get a shot (I got stuck on the phone) but one of the staff throws the "Frankenstein" switches seen in front the chamber to send electricity into the coils. This then vaporizes the aluminum. This newly formed aluminum gas spreads outwards and applies a thin coating of aluminum to everything inside the chamber, including the 60-inch mirror.
When the process is concluded you (hopefully) end up with a mirror looking like this:The next step is to evaluate the coating and, if things are good, to return it to the telescope.As you can see the process produces a remarkable improvement on the reflectivity of the mirror.Hats off to the Palomar crew for another fine job!