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MMT primary mirror coating removed for re-aluminization

The 6.5-meter MMT primary mirror was stripped of its aluminum coating on July 23, 2010, in preparation for re-aluminization. A video of the mirror stripping can be seen at:

Removal of the aluminum coating is done with a series of scrubbing and rinsing treatments, using ordinary mops, paper towels, various cleansers, solutions, and water.  Great care must be taken to remove all of the existing aluminum coating as well as any contaminants from the mirror surface.

Unlike many other mirrors, the MMT is re-aluminized in situ with the mirror remaining on the telescope within its mirror cell during the entire aluminization process.  The mirror is enclosed in a large (>20-foot  diameter) vacuum bell jar from which all air and contaminants are removed.  Aluminum filaments are then vaporized, depositing a thin, even coating of reflective aluminum across the mirror face.

Although the aluminization event itself occurs in less than a second, weeks of preparation are required prior to the event.  Much of the hardware and electronics within the mirror cell must be removed prior to vacuum sealing.  All surfaces that will be inside the vacuum chamber must be thoroughly cleaned.  The hardware is then reassembled after aluminization and prior to any telescope operations.  The previous re-aluminization of the MMT primary mirror occurred in 2005.

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Elevation Tracking Report

The MMT elevation tracking has been evaluated for the 3rd trimester of 2008 and was found to be at a median of 0.065″ +/- 0.04″. There is a lower limit to the tracking smoothness at about 0.02″, or +/- 2 encoder counts, with a non-linear dependence on the tracking rate. Wind rejection remains an issue, with tracking degradation a (nearly linear) function of the wind speed, regardless of the telescope position. However, positions at high elevation angles away from the prevailing wind are more favourable, as the following 3D plot with RMS error population as a surface over elevation and relative wind angle shows.

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