Washing the Gemini South
Primary Mirror
A few times annually, the Gemini South primary mirror is subjected to an in situ wash
in which the mirror, left in place on the telescope, is cleansed with specialized detergents to remove dust particles. The latest
wash occurred in April; we show a picture of
the process here (Figure 8). An in situ wash requires that we very carefully seal the bottom
end of the telescope so that the instruments,
science fold mirror, etc. do not get wet.
If you’re wondering whether we do this in
Hawai‘i also, the answer is no, and it’s because of geology. The wind-blown dust
on Cerro Pachón is granitic, while that on
Maunakea is light basalt. Granitic dust on
the Gemini South primary does not fully
come off with CO2 cleanings, whereas basaltic dust on the Gemini North primary
does. Therefore, given only carbon dioxide
cleaning, reflectivity of the Gemini South
primary would decrease more quickly than
Gemini North’s.
Figure 8.
Washing the Gemini
South Primary Mirror
in situ.
15
GeminiFocus
July 2015