4
$\begingroup$

Would it be somewhere in a desert a long way from water and mist, or high on a mountain above the lower atmosphere? Where is the best location in theory or in practice?

$\endgroup$
7
  • $\begingroup$ Why specifically "refractor", given that nearly all large telescopes are reflector telescopes. Moreover what do you mean by "best" - best for what? Large refractors are nowadays only used in specialist applications, such as solar telescopes. $\endgroup$
    – James K
    Oct 24, 2023 at 8:00
  • 1
    $\begingroup$ @James K I have used the word optical instead of refractory now. $\endgroup$
    – user52681
    Oct 24, 2023 at 8:58
  • 1
    $\begingroup$ Probably at the summit of Mt. Everest, but that's not super practical. $\endgroup$
    – T.E.D.
    Oct 25, 2023 at 14:52
  • 1
    $\begingroup$ @T.E.D. Almost certainly not. I imagine the seeing would not be good from the top of Mt Everest and nor are there likely to be ~80% clear nights per year as there are at the best sites. $\endgroup$
    – ProfRob
    Oct 25, 2023 at 16:18
  • 1
    $\begingroup$ Could you not first look at a few sites of real observatories and - ruling out politics or economics - try to analyse why they might have been chosen? Look, for instance, at T.E.D.'s Comment, above. $\endgroup$ Oct 25, 2023 at 21:03

2 Answers 2

25
$\begingroup$
  • High altitude, so there is less atmosphere to look through.
  • Dry climate, so there is less water vapour, fewer clouds, less rain.
  • Low latitude, so more of the sky is visible for at least part of the year.
  • Remote from non-natural light sources.

And there are lots of non-science considerations: political stability, good transport links, good network connectivity, skilled local labour, no indigenous land rights issues. All these factors have to be balanced against each other.

There isn't a single "best" place, but Mauna Kea in Hawaii, La Palma in the Canary Islands and Paranal in the Atacama desert in Chile tick a lot of the above boxes.

$\endgroup$
7
  • $\begingroup$ Surely the steppes can be quite an attractive option if they meet all the requirements? $\endgroup$
    – dtn
    Oct 24, 2023 at 17:45
  • 4
    $\begingroup$ Well, if the meet all the requirements. $\endgroup$
    – James K
    Oct 24, 2023 at 20:01
  • 2
    $\begingroup$ Also, geologically stable. $\endgroup$
    – gerrit
    Oct 25, 2023 at 12:03
  • 4
    $\begingroup$ There is arguably an additional factor promoting good seeing, which is being on a mountaintop near the ocean, because the inland airflow will tend to be smoother (more laminar) going up and over the mountains than it will be further inland. This is part of why observatories on mountainous islands (Mauna Kea, La Palma) or mountain ranges next to the ocean (Chile) are particularly good. $\endgroup$ Oct 25, 2023 at 12:22
  • 4
    $\begingroup$ @gerrit I'm not sure geological stability is actually that important compared to other considerations. Mauna Kea and La Palma are active volcanoes, and the Atacama Desert is right next to a major fault line. $\endgroup$
    – 8bittree
    Oct 25, 2023 at 18:14
18
$\begingroup$

The best sites for telescopes are where they currently are and where the even bigger next generation of telescopes are being built. These sites are Mauna Kea in Hawaii, Roque de los Muchachos on La Palma in the Canary Islands, the Chilean Andes (sites near La Serena—Cerro Tololo, Las Campanas—and in the Atacama desert south of Antofagasta—Cerro Paranal), and in the Arizona mountains near Tucson—Kitt Peak and Flagstaff—Lowell Observatory.

For optical astronomy, the main factors that determine the "quality" of the site are the "seeing" (this is the extent to which the atmosphere around and above the observing site blurs the images of stars), the fraction of clear observing nights and the darkness of the night sky when the Moon is below the horizon. All of the sites listed above do pretty well on all these scores.

It's unlikely that there are significantly better sites elsewhere in the world. There has been extensive site testing at various other promising locations, but the ones I've listed generally come out on top.

Of course if you move to longer (infrared) wavelengths then the answer would be different. High altitude is then really important, to reduce the mass of water vapour overhead, and also the atmosphere and telescope are significant thermal sources of background—and so in theory putting a large infrared telescope on a mountain near the South Pole would be a good idea, but very difficult in practice.

$\endgroup$
10
  • $\begingroup$ Is it possible that the telescope will be placed at some suitable height above the south pole (in the format of an orbital observatory) and maintain a fixation above it? $\endgroup$
    – dtn
    Oct 24, 2023 at 17:54
  • 2
    $\begingroup$ @dtn There would be no advantage to that and it isn't possible. $\endgroup$
    – ProfRob
    Oct 24, 2023 at 18:32
  • 2
    $\begingroup$ @dtn That's not an optical telescope. Considerations for radiotelescopes are different. $\endgroup$
    – gerrit
    Oct 25, 2023 at 12:07
  • 2
    $\begingroup$ @dtn I have used a (small) infrared telescope sited at the south pole (no longer there), called SPIREX. $\endgroup$
    – ProfRob
    Oct 25, 2023 at 14:24
  • 2
    $\begingroup$ @dtn remotely unfortunately. Before the Spitzer spacecraft it was world-beating at 3-5 microns. Observing trip to La Palma next month though :) $\endgroup$
    – ProfRob
    Oct 26, 2023 at 7:33

You must log in to answer this question.