In November we will be pulling the 200-inch mirror out of the Hale Telescope and re-coat the surface with a new layer of aluminum. We do this every year and a half to two years. To say that it is an involved procedure is a huge understatement.
So when you have a complex procedure that involves most of the staff working on the most precious thing you have (the 200" mirror) and you only do this every couple of years or so it is a good idea to practice. We've been working a practice session this week.
The way we practice is to take a large aluminum disc, that is the same size as the real mirror, and use it as a stand-in. This dummy mirror will then get enclosed in a giant bell jar. The air will be pumped out and a small amount of aluminum is vaporized and deposited on the surface. The procedure is similar to what we did for our 60-inch mirror, only more so. If you click on the image above to see the larger version you might notice some 14-inch glass photographic plates and some glass slides that have been placed on the disc.
After all is said and done we will be able to examine the glass samples to study the coating that we applied.
To get all of this to happen you've got to lift the 17.5-ton top of the bell jar up off of the dome floor and down over the mirror, or in this case the dummy mirror. Here are some views of yesterday's lift.
From this vantage point you can see into the aluminizing chamber (aka the bell jar). Here's a better look inside:
Inside are over 200 tungsten wire filaments that have had a small amount of aluminum melted into them.
Once everything is together, the pumps begin to work and a day or so later it is time to "fire" and apply the new coating.
Pumps are working as I type and the firing of the coils should happen tomorrow.