I haven't been able to get a good answer to this one -- but I have a feeling I'm going to feel really silly when I find out. The submillimeter telescopes all seem to be idle over the day. But, submillimeter telescopes operate below IR wavelengths. What keeps you from operating over the daytime?
Not a daft question at all, neither is the opposite question (why do submm telescopes work during the day) :D. To answer the part you already know, the Sun isn’t very bright at all in the wavelengths we look in, so it’s always “night” for the sub-millimetre in terms of light, hence we aren’t restricted
There are a few reasons why we don’t observe all day, that’s because 1) with the doors open, the primary mirror would heat up. This would cause it to warp a bit, which we could compensate for in some cases, but when it came to night time we’d have to wait a long time for the mirror to thermally relax.
2) (Have I got this right?) Even if the weather looks good, apparently it’s really hard to get a good pointing? Even if the atmosphere is perfectly transparent to submm light, which is awesome if it is we love that, the atmosphere can be a tad granular during the day, causing the submm to twinkle. This means when we try and calibrate the telescope with a pointing by finding, say, Mars, it moves all over the place, and there’s just no point in observing since all your observations would be smeared and messy. So we can get all the light, just it doesn’t sit still.
3) Money, sadly :( nowadays we can switch for extended observing in the morning for a few hours, we switch over control to the team in the Joint Astronomy Centre in Hilo when the weather is good, but that finishes about 10/10:30/11.
All the telescopes have a day-crew of engineers who come up and test/update/fix everything that’s needs it during daylight hours, too and they are the most important people on the telescope team. I haven’t met any of the team sadly, but they always get everything done because they’re the real pros - and as a side-note someone even curls up the ethernet cables too (which I felt bad about when I noticed so I tried to do the same when I leave I’m so sorry if everything’s a mess aaa). The day crew makes everything possible :)