>On the other hand: some of the telescopes have been there many years and are probably somewhat obsolete by today's standards, producing little new science. I

>On the other hand: some of the telescopes have been there many years and are probably somewhat obsolete by today's standards, producing little new science. I can assure you most of those telescopes are still being used for new science. There is a serious lack of "big" telescopes, globally. We simply don't have enough instruments to follow up everything we want to look at, and getting time on these "old" instruments is still competitive. A big part of this is location: Mauna Kea has fantastic skies and is good for things like IR astronomy as a result of its elevation. Telescope optics are fairly simple. They're a bunch of mirrors which focus light in one place - a 2m mirror is a 2m mirror. The main maintenance task is cleaning or re-coating, which happens once in a while. While larger telescopes really need adaptive optics to fulfill their potential, you don't need it unless you need to resolve tiny objects in crowded fields.* One good use for these smaller telescopes is long observations, or repeat observations. Time domain astronomy is a Big Thing at the moment, and for a lot of objects you don't need a 30 metre mirror (the main benefit of which is seeing extremely faint stuff). Another use is follow-up work, which is rather mundane, but necessary. For example you get a bunch of targets back from a sky survey and you want to observe them all for a period of time. Doing that on something like the TMT or ELT would be a fantastic waste of resources. Usually you would need to prove, using a smaller telescope, that it would be worth pointing a O(10m) mirror at it. What often changes is the instrumentation: lower noise or higher resolution CCDs, fancier spectrographs, etc. These can all be retrofitted to fit onto older telescopes. * more specifically, if you want to do diffraction-limited imaging and your telescope can theoretically resolve better than atmospheric seeing.