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WHT mirror reflectivity

The V-band reflectivities of the WHT primary, secondary and Nasmyth (tertiary) mirrors are measured at regular intervals with a hand-held reflectometer. The results can be found here.

Primary mirror

The primary mirror is aluminised every few years. Between 1997 and 2011, it was cleaned with CO2 snow every month or so. The CO2 snow cleaning appears to maintain the reflectivity, but the scattering coefficients gradually worsen with time. Since 2011, it is snow-cleaned once every 2 - 3 months and water-washed about once a year, or whenever the reflectivity appears to be dropping. The mirror is not water-washed or CO2-cleaned during scheduled polarimetry runs (to avoid changing the polarisation characteristics of the telescope).

The UV reflectivity cannot be measured with the reflectometer, and experience at other telescopes suggests that it might not be fully restored by snow-cleaning. The UV reflectivity is occasionally monitored via observations of standard stars. The UV/V reflectivity ratio (measured more accurately than the absolute values) was within a few % of the expected values when measured in Jan and Jul 1999, and in Feb 2000, suggesting that snow cleaning is as effective in the UV as at V band.

The primary mirror suffered its most serious accident yet on 6 May 1999, when a 1.5-kg component of the top-end focus drive fell vertically onto the bare mirror. The surface now bears a few deep scratches, which were ground out prior to the Jun 2000 aluminising to avoid spreading of any cracks. The area affected is negligibly small.

Secondary mirror

The secondary mirror is less exposed to the elements than is the primary, and is aluminised less frequently.

Nasmyth flat

The Nasmyth flat is also well protected (particularly when stowed to allow light to reach the Cassegrain focus). Before ~ 1997, it was cleaned about once per year with PL25 spray-on/peel-off film. More recently, we have used alcohol to clean it.

Chris Benn (

2 June 2012

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Contact:  (WHT Manager)
Last modified: 23 February 2016