The V-band reflectivities of the WHT primary, secondary and Nasmyth (tertiary) mirrors are measured at regular intervals with a hand-held reflectometer.
The dates of cleaning and measured reflectivities are stored here: Reflectivity_results_page
The primary mirror was aluminised each autumn (after the summer dust storms) up to 1998, and was cleaned with CO2 snow every couple of months, humidity permitting, during 1997-9. There was no aluminising during 1999. It appears that each CO2 cleaning only partially restores reflectivity, and by June 2000 it was only 81%. The mirror was then experimentally washed with detergent and warm water, which raised reflectivity to 85%, and then aluminised, raising reflectivity to 88%. ING plans to wash the mirror in situ every few months, but this hasn't yet (May 2002) been possible. The aim of reducing intervals between aluminising is to reduce the number of nights lost, and manpower costs, and lessen the risk of damage (during craning) to ING's most irreplaceable component.
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 now being 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 1999 May 6, 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.
881126 aluminised 950620 0.88 using light-box + photometer 951120 aluminised 000524 0.87 001025 0.78 001025 0.87 after water washing 001031 0.88 after aluminising 020326 0.88
901212 aluminised 960325 aluminised 000524 0.75 000525 0.84 after water washing 000918 0.83 001129 aluminised 001130 0.89 001221 0.89 010130 0.90 010403 0.89 010404 0.88 after CO2-snow cleaning 010605 0.87 010718 0.87 010905 0.87 011105 0.79 011204 0.81 020107 0.80 before CO2-snow cleaning 020107 0.85 after CO2-snow cleaning 020326 0.87 before CO2-snow cleaning 020326 0.85 after CO2-snow cleaning 020416 0.83 020531 0.85 before CO2-snow cleaning 020531 0.83 after CO2-snow cleaning 020708 0.84 before 020712 0.88 after 020816 0.82 before 020816 0.87 after
INGRID feed flat (Ag coated, in cass A&G): 0001 re-silvered (good for 2 years?) 0011? 0.79 0012? 0.92 after cleaning 001121 0.89 after cleaning, but looks bad, needs recoating
Chris Benn (crb@ing.iac.es)
2002 Jun 3