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Official Statement on the Re-coating of the MMT Primary Mirror


By admin - Posted on 02 November 2010

The MMT was taken offline for an extended period during this year’s summer shutdown to re-coat the primary mirror.

The previous coating was applied during 2005 and with careful use, frequent CO2 cleaning and contact washing we have been able to keep that coating in very good condition. Measured reflectivity of the primary indicated it was time to give the mirror a new coating.

Although the process appeared to have gone smoothly, a visual inspection following the removal of the bell jar revealed patches of the surface that are “discolored”. We are still investigating exactly what happened, and though we have a working theory we wish to verify exactly the issue with the process before we make an official statement to this end.

There is one main affected area, which we estimate, is ~15% of the mirror surface and two small discreet patches separate from the larger area that are effectively non-reflective. The combined effect of these “blemishes” is to give a reduced throughput (or effective collecting area) of the telescope as they have a greatly reduced reflectivity with respect to an ideal Aluminum coating. As a guide, the measured reflectivity is worse in the blue and increases red-wards, e.g. at 400nm the worst measured reflectivity is 18% and increases to 50% at 700nm (for reference ideal Al is ~91% reflectivity).

We estimate that the imperfection of the coating delivers an equivalent collecting area of a telescope approximately 6.2m in diameter. For numerous reasons including budgetary and staffing restrictions, at this time we intend to re-coat again during the summer shutdown, 2011.

For more information and images please see Grant’s presentation from the SO Observer’s Lunch on 10/5/10. If you have any further questions, or comments please direct them to Morag Hastie.