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This is a limitation known to several radio telescopes. For example, the Arecibo telescope couldn't aim much until they added the arm in 1997. They could aim

This is a limitation known to several radio telescopes. For example, the Arecibo telescope couldn't aim much until they added the arm in 1997. They could aim the telescope not by moving the dish, but by moving the focal point/receiver. This only works with spherical dishes, but it is an option.

sennight 5y ago
Arecibo was distance limited by the Earth's rotation speed, since radar returns had to make it back before the target shifted out of the telescopes steerability window. As the Moon is tidally locked - that long stare limitation remains.
- ncmncm 5y ago
  Advantage is, the rotation rate will be 28x slower.
  - sennight 5y ago
    If you want to take a radar measurement for something once every 28 days, its great - except for the whole needing to be on the Moon part. Even if you were to build a network of radio telescopes on the dark side of the Moon, you'd still have an unavoidable revisit period - and building on the Earth facing side to overcome that defeats the purpose of putting radio telescopes on the Moon. The possibilities for optical telescopes are pretty interesting though.
    - ncmncm 5y ago
      You don't need to be there yourself. And, you can build a dozen of them, half-way around the far side, and get a lot more opportunities to point at anything. Build two close together, and range things hours away.
      - sennight 5y ago
        > You don't need to be there yourself. lol, dunno if you are aware or not - but the Arecibo telescope catastrophically collapsed on itself. Serviceability is the concern. The maintenance logs are long for scientific instruments, there is no such thing as a 'set it and forget it' telescope.
        ncmncm 5y ago
        Hubble has been visited I think twice in 3 decades.
        sennight 5y ago
        That is an interesting choice for a counter example, because when I think of Hubble I think of failed gyroscopes. Hubble has required 5 servicing missions and always seems to be at risk of moving from a state of crippled-but-still-useful to flat-out-busted-forever. Look at the program's weekly observation logs, keep an eye on the file size - the interesting ones are smaller than average. Gyros and safemode. If you suddenly feel the urge to blurt out "Bbut funding and politics!", then consider my point proven - a Moon based telescope would be far more vulnerable to such externalities. https://www.stsci.edu/ftp/observing/weekly_timeline/ https://www.stsci.edu/ftp/observing/weekly_timeline/
        ncmncm 5y ago
        A telescope on the moon will not need any gyroscopes. The first Hubble visit was to fix a manufacturing/testing flaw impossible in a radio telescope. Arecibo's failure would also be impossible, as there is no oxidation and no weather on the moon. The only likely failure mode would be from metal fatigue as a consequence of temperature swings, but they are 100% predictable. Maybe you are not aware of the very large difference between an optical telescope and a radio telescope? Optical telescopes always have many precision moving parts. Radio telescopes often have none at all. Anyway, in ten years, given promised SpaceX Starship progress, a visit will be much cheaper than a single Shuttle visit to ISS. Probably such progress will end up turning on thousands of heat-shield tiles not each needing a diaper change after each flight, as the current design seems to require.
- deleted 5y ago
  [deleted]
- rtkwe 5y ago
  That's just if you're doing active radar ranging to objects instead of passive radio observations which is most of what radio telescopes do anyways.
  - sennight 5y ago
    > > > ...can’t do a long exposure of anything... > > They could aim the telescope not by moving the dish, but by moving the focal point/receiver. > As the Moon is tidally locked - that long stare limitation remains. See the point now? Passive radio observation doesn't change the fact that the Moon orbits the Earth - and has an even longer delay between revisits to the same patch of sky.
    - rtkwe 5y ago
      Yeah it's not going to be able to point for days at a single target but that will be helped by the relative silence of where it is. > Arecibo was distance limited by the Earth's rotation speed, since radar returns had to make it back before the target shifted out of the telescopes steerability window You were talking about active radar ranging a lot though which is not at all what these are meant for. It's like complaining a car can't tow a trailer, it's technically true but it's orthogonal to it's intended purpose.
      - sennight 5y ago
        > ...it's technically true but it's orthogonal to it's intended purpose. Well that is the more verbose and less helpful way of saying the same thing: can't do long stare for the same reason this recognizable thing couldn't - rotation. Sticking with your car analogy: Upon hearing somebody else answer the question of "Can this car tow a trailer?", you swing in on a chandelier and declare "Actually it can, if the trailer is imaginary!"

sennight 5y ago

Arecibo was distance limited by the Earth's rotation speed, since radar returns had to make it back before the target shifted out of the telescopes steerability window. As the Moon is tidally locked - that long stare limitation remains.

ncmncm 5y ago
Advantage is, the rotation rate will be 28x slower.
- sennight 5y ago
  If you want to take a radar measurement for something once every 28 days, its great - except for the whole needing to be on the Moon part. Even if you were to build a network of radio telescopes on the dark side of the Moon, you'd still have an unavoidable revisit period - and building on the Earth facing side to overcome that defeats the purpose of putting radio telescopes on the Moon. The possibilities for optical telescopes are pretty interesting though.
  - ncmncm 5y ago
    You don't need to be there yourself. And, you can build a dozen of them, half-way around the far side, and get a lot more opportunities to point at anything. Build two close together, and range things hours away.
    - sennight 5y ago
      > You don't need to be there yourself. lol, dunno if you are aware or not - but the Arecibo telescope catastrophically collapsed on itself. Serviceability is the concern. The maintenance logs are long for scientific instruments, there is no such thing as a 'set it and forget it' telescope.
      - ncmncm 5y ago
        Hubble has been visited I think twice in 3 decades.
        sennight 5y ago
        That is an interesting choice for a counter example, because when I think of Hubble I think of failed gyroscopes. Hubble has required 5 servicing missions and always seems to be at risk of moving from a state of crippled-but-still-useful to flat-out-busted-forever. Look at the program's weekly observation logs, keep an eye on the file size - the interesting ones are smaller than average. Gyros and safemode. If you suddenly feel the urge to blurt out "Bbut funding and politics!", then consider my point proven - a Moon based telescope would be far more vulnerable to such externalities. https://www.stsci.edu/ftp/observing/weekly_timeline/ https://www.stsci.edu/ftp/observing/weekly_timeline/
        ncmncm 5y ago
        A telescope on the moon will not need any gyroscopes. The first Hubble visit was to fix a manufacturing/testing flaw impossible in a radio telescope. Arecibo's failure would also be impossible, as there is no oxidation and no weather on the moon. The only likely failure mode would be from metal fatigue as a consequence of temperature swings, but they are 100% predictable. Maybe you are not aware of the very large difference between an optical telescope and a radio telescope? Optical telescopes always have many precision moving parts. Radio telescopes often have none at all. Anyway, in ten years, given promised SpaceX Starship progress, a visit will be much cheaper than a single Shuttle visit to ISS. Probably such progress will end up turning on thousands of heat-shield tiles not each needing a diaper change after each flight, as the current design seems to require.
deleted 5y ago
[deleted]
rtkwe 5y ago
That's just if you're doing active radar ranging to objects instead of passive radio observations which is most of what radio telescopes do anyways.
- sennight 5y ago
  > > > ...can’t do a long exposure of anything... > > They could aim the telescope not by moving the dish, but by moving the focal point/receiver. > As the Moon is tidally locked - that long stare limitation remains. See the point now? Passive radio observation doesn't change the fact that the Moon orbits the Earth - and has an even longer delay between revisits to the same patch of sky.
  - rtkwe 5y ago
    Yeah it's not going to be able to point for days at a single target but that will be helped by the relative silence of where it is. > Arecibo was distance limited by the Earth's rotation speed, since radar returns had to make it back before the target shifted out of the telescopes steerability window You were talking about active radar ranging a lot though which is not at all what these are meant for. It's like complaining a car can't tow a trailer, it's technically true but it's orthogonal to it's intended purpose.
    - sennight 5y ago
      > ...it's technically true but it's orthogonal to it's intended purpose. Well that is the more verbose and less helpful way of saying the same thing: can't do long stare for the same reason this recognizable thing couldn't - rotation. Sticking with your car analogy: Upon hearing somebody else answer the question of "Can this car tow a trailer?", you swing in on a chandelier and declare "Actually it can, if the trailer is imaginary!"