Aluminization of the 1.2m mirrors

Created 08/24/04 by EF
Updated 09/01/04 by EF

In August 2004, it was the turn of the 1.2m mirrors to be aluminized. We had planned the usual stripping of the old aluminum and re-aluminization at the UA "Sunnyside" facility in Tucson and had scheduled it for the week of Aug 16. We removed the mirrors the previous week, packed them and trucked them to the basecamp. Only to find out from Gary Rosenbaum that the schedule at Sunnyside was suddenly pushed back, so they would not be able to get to our mirrors until about Sep 20, more than 1 month too late for us. After a quick scramble, thanks to Richard Green, we found that NOAO had a break in their schedule, and would be able to aluminize for us in their 2.4m chamber on Kitt Peak the week of Aug 16, for a slightly higher fee. After discussing the possibility with Tony Abraham at NOAO, we decided to pursue this option, but to aluminize only the primary, as the secondary was in excellent condition.

The stripping and aluminization proceeded during the week of the original schedule. In the morning of Aug 16, with Rodger Harris at the wheel, we drove the mirror to NOAO in Tucson for an inspection. The goal was to determine whether any fittings would be required, and to have them fabricated in Tucson if necessary. We met with Khairy Abdel-Gawad, who would oversee the aluminization, as well as Gary and Bill Kindred at the NOAO loading docks. Fortunately, Gary found our lifting ring (aka band) at Sunnyside and delivered it to NOAO himself. Bill Kindred obtained a set of reflectivity measurements (with and without the specular component) with the Minolta reflectometer. After the inspection, we proceeded to Kitt Peak and delivered the mirror that afternoon. Fittings would be machined and the chamber would be prepped on Aug 17, and the stripping and aluminization would take place on Aug 18.

On Aug 18, Bob Hutchins and I drove to KPNO and met there with Bill Kindred, Khairy and Wally, who would be doing the hard work.

Before stripping, Bill performed several "tape tests" (see below). These showed that the aluminum was holding up well, except in the obviously degraded islands. The stripping began with a wash with filtered water and Orvis soap. The mirror was daubed dry, and then Bill took a new set of reflectivity measurements at the same positions on the mirror as before. Then, Bill tested out his power-washing machine using tap water. He applied as much water pressure as feasible. There was no damage to the aluminum that could be detected by visual inspection. After that, Wally proceeded with the 3-step standard KPNO stripping. During that process, one of the attachment points for the spreader bars on the side of the mirror fell off. These are epoxied and prone to becoming detached easily. Once the bare glass was dry, Bill repeated his reflectivity measurements.

The digital photos below illustrate the procedure. The mirror went into the vacuum chamber around midday. Two witness microscope glass slides were placed in the chamber next to the mirror. Wally started pumped down the chamber until about 3pm, when it reached about 8E-6 Torr. The normal goal is 5E-6 Torr, but the pumping was taking much longer than usual. At that point, it seemed to be going nowhere lower and Wally made the decision to "shoot." We watched through a porthole as the electrodes first glowed and suddenly, the glass was coated. Wally noted that the flux gauge was not reading out, so the coating thickness readout was unreliable. He suspected (and, once he opened the chamber, confirmed) that he had bumped the detector in the chamber when putting in the mirror, so it was pointed the wrong way. He decided to pump down again, and to fire the inner array of electrodes, as we preferred a thicker than normal coating. At about 3:30pm, we were back down to about 8E-6 Torr, and Wally fired again. That also went well, except for the lack of a reliable measurement of the coating thickness. From their past experience, we expect the coating is about 100 nm thick. At that point, the mirror was allowed to hang until the following day.

The morning of Aug 19, we drove to KPNO with the new MMT truck, again with Rodger at the wheel. By the time we arrived, Khairy and Wally had removed the mirror from the chamber and placed it with the lifting ring into its box. We saw that the coating was visually good, except for the islands of bad glass, which had increased somewhat in surface area. The "orange-peel" effect was visible on the new coating. It is clearly a property of the glass surface. I decided to accept the mirror and drive it to the basecamp. We left the mirror in its box and in the truck until Monday Aug 23, when support drove it to the Ridge.

On Aug 23, we unloaded the mirror from its box, removing the lifting ring, and placed it on the cell, but resting on the tripod fixture. Once again, Bill repeated his reflectivity measurements. He found relatively high scattering and lower than expected specular reflection, just below 90%. However, we still need to analyze the measurements. We started by gluing the attachment point for one of the spreader bars that had become detached. The epoxy required about 17 hours of curing, so little else could be done that day. On Aug 24, after Bob had mounted the spreader bars, I jiggled all of them, and one of the attachment points fell out once again (a different one this time). Thus went another day of epoxy curing. On Aug 25, we started mounting the rest of the hardware. We had the mirrors and the topbox mounted back on the telescope by Aug 27.

The following is a visual record of the aluminization. Click on the images for enlarged views.