In-Situ Wash: Mirror Cleaning at Gemini South


A fish eye view of the primary mirror during the last In-Situ wash

In-Situ Wash

Periodically, the primary mirrors of both Gemini telescopes need to be cleaned. The day crew at the Gemini South telescope perform this task regularly (Gemini North’s primary mirror does not receive as much dust, and does not require cleaning as frequently).

In Chile, the in-situ washing happens twice a year and involves as many as 17 members of the engineering and optics groups working up to a 10 hour day. The next scheduled wash will not happen until after the primary mirror has been stripped and re-coated, later this October (the previous re-coating occurred in October of 2010).


This panoramic photo shows the primary mirror after to receiving a new silver coating in 2010. This procedure is scheduled to occur again in October of this year.

During the most recent in-situ wash, on april 22nd, staff from the Magellan Telescope and the Large Synoptic Survey Telescope were invited to observe Gemini’s procedures, thus providing cross-observatory sharing of knowledge, which is critical for every observatory’s operations.

The in-situ wash is just one of the procedures that assure that Gemini meets tight specifications on infrared emissivity and overall light reflectivity.  The mirror is also cleaned with carbon dioxide on a weekly basis.  Both of these procedures aim to extend the durability of the primary mirror’s silver coating, and is an important factor in reducing the need for re-coating to about every 5 years.

Learn more in this video of the in-situ washing process:

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