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A couple years ago I moved near Lowell Observatory and our city, Flagstaff, has a number of Dark Sky ordinances, which are great. I've seen so many more stars t

A couple years ago I moved near Lowell Observatory and our city, Flagstaff, has a number of Dark Sky ordinances, which are great. I've seen so many more stars than ever in my life. We just bought our kids a Newtonian reflector telescope. Most things have trade-offs. Hopefully, the reduced cost of access to space will allow launching of more space-based telescopes, which don't have problems with atmosphere. Any astronomers here that care to explain what only ground-based observatories can do?

_verandaguy 7y ago
Not an astronomer, but ease of maintenance comes to mind. If a mirror is defective on a ground-based telescope, it's easier to build a replacement and install it. Hubble had a defective mirror which required an essentially dedicated Shuttle mission in the mid 90s.
petschge 7y ago
Mounting new detectors, fixing broken things, swapping out (optical) filters and shear mirror size are all a big problem for satellites. No atmosphere is nice for IR, but current optical telescopes are 10 meters and up, much larger than even a Falcon 9 heavy can fit.
- tropo 7y ago
  10 meters should be doable. You'd need a custom fairing. Probably you'd want a fairing that holds the mirror on edge, so that it travels edge first as if looking to the side during launch. This would save on air resistance during launch.
  - _verandaguy 7y ago
    Spoken like someone who's spent a bit too much time in KSP :) There are more issues than just sheer mirror size in this case. - Resistance to vibrations is a _major_ issue if the mirror is monolithic (as with HST) as opposed to segmented (as with JWST). - The mirror also has to either be very resistant to extreme temperature changes (and the contraction that comes with it), or must be cooled before launch. For extremely large mirrors, the cooling assembly would add _tons_ of extra weight, since this would most likely be active/liquefied gas cooling, and the weight of the cooling medium alone would be significant in this case. - Using a shape that's close to being a cylinder is great for cooling too, because cylinders have a very good surface area-to-volume ratio, which matters when you have to account for heat exchange. If memory serves, only ellipsoids are better (with spheroids being a better than ellipsoids, and spheres being ideal). - The support structure for the mirror (the satellite fuselage proper) would probably have to fit in there too, at least for the first launch. That's not small, even though it can be made collapsible. - All of the optical elements have to stay outstandingly precisely positioned within insanely tight tolerances. Any shifting could result in a lack of clarity, chromatic aberrations, or other issues, even if the mirror(s) aren't directly warped or damaged. Even accounting for that, a fairing that's irregular around the long axis of the rocket is often undesirable. While it reduces (compared to the alternative), it can cause issues with rolling after launch, and introduces additional complexity.
NikolaeVarius 7y ago
You can make really really really big mirrors,. Space telescopes mirrors are limited by Fairing size, so barr folding mirrors ala James Webb, there is a fairly hard limit. https://en.wikipedia.org/wiki/Extremely_Large_Telescope https://en.wikipedia.org/wiki/Extremely_Large_Telescope
- marvin 7y ago
  Do you know anything about development work that's been made regarding manufacturing mirrors in space? I'm not thinking assembling them from pieces, I'm thinking more about melting some raw material and using some physical process to create in space something larger.
  - clmul 7y ago
    The manufacturing tolerances for large mirrors are very small, and it seems unlikely that we would be able to do this - note that barely any manufacturing has been done in space. Assembling from pieces on the other hand, is more doable.
  - ceejayoz 7y ago
    I wonder if something akin to glassblowing would work in zero-G.
  - HorstG 7y ago
    Gravity is an issue for mirror production. But it is far less of an issue than tension and deformation during the months- to years-long cooling of the substrate. The temperature and environment control to do that is challenging on earth, fairly impossible in space. Except with maybe a really really huge spacestation as a thermal sink, which we won't get for the next few hundred years I'd guess...
    - m4rtink 7y ago
      Aren't the terrestrial mirrors so thick due to the need to survive all the tilting under the strong terrestrial gravity ? I would assume a micro gravity only mirror could be much thinner & thus easier to cool down. Or possible alternative techniques could be used to get the needed reflective surface geometry if it does not need to take gravity and atmosphere into account.
Rebelgecko 7y ago
Not an astronomer but things you can do on the ground: 1. Bigass telescopes. Some radio telescopes are 100 or even 1000 feet across. You can't put something like that easily in a rocket fairing. JWST has a 20 foot mirror and they probably spent hundreds of millions of dollars figuring out how to fit that thing in an Arianne 5. 2. Easy maintenance and upgrades. There are 80 year old telescopes still doing useful science since you can make incremental upgrades over the years. (shoutout to Hubble: at 30 I think it's probably the longest lived space telescope, even if it's getting a bit long in the tooth) 3. Interferometry. This would potentially be really cool to do in space, since you could theoretically make a telescope whose diameter is tens of thousands of miles. However, interferometry requires you to be able to position yourself really accurately (amount of accuracy you need depends on wavelength and a few other factors, but potentially at the micro-meter level). That's really hard to do in space. 4. You can build a bunch of ground based observatories for the cost of a single space one. 5. Data downlink. Some telescopes generate a lot of data. It's not easy to get a terabit per second of data down to the ground.

_verandaguy 7y ago

Not an astronomer, but ease of maintenance comes to mind. If a mirror is defective on a ground-based telescope, it's easier to build a replacement and install it. Hubble had a defective mirror which required an essentially dedicated Shuttle mission in the mid 90s.

petschge 7y ago

Mounting new detectors, fixing broken things, swapping out (optical) filters and shear mirror size are all a big problem for satellites. No atmosphere is nice for IR, but current optical telescopes are 10 meters and up, much larger than even a Falcon 9 heavy can fit.

tropo 7y ago
10 meters should be doable. You'd need a custom fairing. Probably you'd want a fairing that holds the mirror on edge, so that it travels edge first as if looking to the side during launch. This would save on air resistance during launch.
- _verandaguy 7y ago
  Spoken like someone who's spent a bit too much time in KSP :) There are more issues than just sheer mirror size in this case. - Resistance to vibrations is a _major_ issue if the mirror is monolithic (as with HST) as opposed to segmented (as with JWST). - The mirror also has to either be very resistant to extreme temperature changes (and the contraction that comes with it), or must be cooled before launch. For extremely large mirrors, the cooling assembly would add _tons_ of extra weight, since this would most likely be active/liquefied gas cooling, and the weight of the cooling medium alone would be significant in this case. - Using a shape that's close to being a cylinder is great for cooling too, because cylinders have a very good surface area-to-volume ratio, which matters when you have to account for heat exchange. If memory serves, only ellipsoids are better (with spheroids being a better than ellipsoids, and spheres being ideal). - The support structure for the mirror (the satellite fuselage proper) would probably have to fit in there too, at least for the first launch. That's not small, even though it can be made collapsible. - All of the optical elements have to stay outstandingly precisely positioned within insanely tight tolerances. Any shifting could result in a lack of clarity, chromatic aberrations, or other issues, even if the mirror(s) aren't directly warped or damaged. Even accounting for that, a fairing that's irregular around the long axis of the rocket is often undesirable. While it reduces (compared to the alternative), it can cause issues with rolling after launch, and introduces additional complexity.

NikolaeVarius 7y ago

You can make really really really big mirrors,. Space telescopes mirrors are limited by Fairing size, so barr folding mirrors ala James Webb, there is a fairly hard limit. https://en.wikipedia.org/wiki/Extremely_Large_Telescope https://en.wikipedia.org/wiki/Extremely_Large_Telescope

marvin 7y ago
Do you know anything about development work that's been made regarding manufacturing mirrors in space? I'm not thinking assembling them from pieces, I'm thinking more about melting some raw material and using some physical process to create in space something larger.
- clmul 7y ago
  The manufacturing tolerances for large mirrors are very small, and it seems unlikely that we would be able to do this - note that barely any manufacturing has been done in space. Assembling from pieces on the other hand, is more doable.
- ceejayoz 7y ago
  I wonder if something akin to glassblowing would work in zero-G.
- HorstG 7y ago
  Gravity is an issue for mirror production. But it is far less of an issue than tension and deformation during the months- to years-long cooling of the substrate. The temperature and environment control to do that is challenging on earth, fairly impossible in space. Except with maybe a really really huge spacestation as a thermal sink, which we won't get for the next few hundred years I'd guess...
  - m4rtink 7y ago
    Aren't the terrestrial mirrors so thick due to the need to survive all the tilting under the strong terrestrial gravity ? I would assume a micro gravity only mirror could be much thinner & thus easier to cool down. Or possible alternative techniques could be used to get the needed reflective surface geometry if it does not need to take gravity and atmosphere into account.

Rebelgecko 7y ago

Not an astronomer but things you can do on the ground: 1. Bigass telescopes. Some radio telescopes are 100 or even 1000 feet across. You can't put something like that easily in a rocket fairing. JWST has a 20 foot mirror and they probably spent hundreds of millions of dollars figuring out how to fit that thing in an Arianne 5. 2. Easy maintenance and upgrades. There are 80 year old telescopes still doing useful science since you can make incremental upgrades over the years. (shoutout to Hubble: at 30 I think it's probably the longest lived space telescope, even if it's getting a bit long in the tooth) 3. Interferometry. This would potentially be really cool to do in space, since you could theoretically make a telescope whose diameter is tens of thousands of miles. However, interferometry requires you to be able to position yourself really accurately (amount of accuracy you need depends on wavelength and a few other factors, but potentially at the micro-meter level). That's really hard to do in space. 4. You can build a bunch of ground based observatories for the cost of a single space one. 5. Data downlink. Some telescopes generate a lot of data. It's not easy to get a terabit per second of data down to the ground.