Installing the Telescope Mirror

In January 2001 we removed the NSRT telescope primary and secondary mirror and sent them to Pittsburgh, PA to be re-aluminized. This is the story of how we re-installed the mirror on its return.

What and Why Re-aluminize?

If you know little about telescopes, our telescope primer may come in useful.

Telescope mirrors are most often made of glass which is shaped to focus the light it gathers. The glass is coated with a very thin (a few atoms thick) layer of silver or aluminum that gives the mirror its reflective properties. Silver and aluminum reflect slightly different ranges of light wavelengths (colors, see our primer on astronomical spectra or our primer on photometry to learn about different wavelengths of light). But another difference between silver and aluminum is that silver is a lot more expensive. Aluminum does the job we want for our mirror so that is what it is coated in.

Over time the thin layer of aluminum on our mirror wears off as a result of periodic cleanings and the corrosive properties of oxygen in the air. When we took out the secondary mirror to ship off for re-aluminization this time we could actually see through the reflective layer it had worn so thin.

How do they Re-aluminize the mirrors?

When the mirrors get to the company that is going to give them their new coat of aluminum, first they strip the old aluminum off the glass and clean the surface very well. The last thing you want is to trap dust and dirt under your new coat of aluminum. Also, removing the old coating ensures that the new layer is smooth and uniform. The contractors are also very careful not to do anything to the glass to change the shape of the mirror even by less than a micron so this is done very carefully.

Next the mirrors are put in a vacuum chamber and all the air is pumped out. On the end of the chamber opposite the mirrors there are aluminum coated electrodes that are superheated to vaporize the aluminum, which then recondenses on the mirror surfaces in a thin layer. During this process the mirrors or the super-heated aluminum electrodes may be moved to ensure an even coating on the glass.

After the glass is evenly coated in aluminum the surface is carefully polished to give it an immaculate shine and the mirrors are carefully packed up and shipped back to the observatory.

Do you take an insurance policy out on the mirror when you move it?

Definitely! We are very careful moving the mirror and it puts a lot of pressure on you to not screw up. But none of us would want to be personally responsible for a 600 pound piece of glass so the mirror is insured for every part of its move and re-coating.

That's great but how do they do this for REALLY BIG telescope mirrors?

Our largest mirror is only 0.9 meters (36 inches) across so it is relatively easy to pack up in a box and ship with a moving company even though it weighs about 650 pounds. But imagine a really big telescope mirror like the 4 meter (about 160 inches) mirror on Kitt Peak or the 8 meter (about 320 inches) mirror at the VLT? Well those are a bit harder to contract to move but they still have to be re-coat periodically so they normally have the equipment to do it right there in the telescope building so they don't have to move the mirror very far. Basically there is a point where the mirror gets so big that it would cost more to insure it and move it than to just buy your own coating and polishing equipment. Also there are very few coating chambers built big enough to fit a mirror that is 4 or 8 meters across.

Let's see the pictures:

	Before we get the mirror back we scrubbed the inside of the telescope clean to minimize the dust falling on our new mirror coating. Most chimney sweeps don't do telescopes and grad students (John Martin) work cheaper anyway. Being the skinniest guy in the department normally gets you this job.
The mirror was shipped in a large custom built wooden box on a moving truck. It was hoisted into the dome on a crane affixed to the outside wall and then moved around the inside of the dome by a crane attached to the dome. Here we use the crane attached to the dome to hoist the mirror out of the box. Charlie Knox operates the crane and supervises.
		Slowly the mirror is lifted to about 5 1/2 feet off the floor
Once the mirror is raised the mirror cell (that holds the mirror in place on the back of the telescope) is rolled into place underneath.
		Lowering the mirror into the cell is a non-trivial operation. It is a snug fit so the mirror has to be centered in two dimensions as well as rotated in the correct orientation so that it properly rests on the floating adjustment supports. At the same time we have to be careful not to get our fingers crushed between the 1/2 inch steel and the 650 pound mirror.
The mirror cell is wheeled on its cart under the telescope and then raised and bolted in place.
		The secondary mirror will later be mounted at the top end of the telescope. After the primary is in place, the telescope is tipped over into a nearly horizontal position and the secondary is screwed into its mounting.