The title isn't entirely accurate, it's more of a spring refurbishment. UKIRT is currently closed because the primary mirror is about to be realuminised.
Telescope mirrors need to reflect light as efficiently and accurately as possible. In most cases the accuracy comes from the initial production of the mirror but the reflectivity can change over time and that's due to a number of factors. Telescopes like UKIRT use a thin coating of aluminium (or aluminum if you're from the USA) which is highly reflective in the infrared but the local weather and environment erodes this surface and it needs to be renewed every few years. Unfortunately UKIRT doesn't have the facilities to do this itself so over the last couple of decades we've sent the primary mirror to the CFHT for coating as they have a coating tank suitable for the tonnes (tons!) of glass we send their way.
Not that long ago we were considering coating the primary with silver. It would have been more expensive than using aluminium but bear in mind we're only talking about a thickness of a few hundred microns and in general the process is the same. In the end we backed out of this because although silver is better in the infrared, it really only benefits wavelengths longer than around 2.5 microns and our current survey only goes to about 2.3 microns, so the benefits would have been limited. It would also have meant using a new facility for the process and at the time silver coating were still experimental with some ambiguous results, so we stuck with the old but tried and tested method.
Tomorrow I'm back at the summit again, but during the day. The telescope will have been taken to pieces and it'll be interesting to see what remains. Our current instrument, WFCAM, will have been removed from above the primary to allow us to remove the mirror and it'll allow a look at the guts of one of the most productive and successful telescopes in modern astronomy.
I took a shot of WFCAM from inside the dome a couple of weeks ago and you can see it above. I'll try and take a couple of shots tomorrow so you can see the difference. I'm back up to recommission the telescope in early June so hopefully it'll look as though nothing ever happend - except the primary will look beautifully pristine!