In the comments of another question on this Stack Exchange site, I talk with another user about the possibility of building an optical space telescope with a 6.5 km aperture.

Obviously, this is much larger than any rocket we have constructed, so it would need to be launched from Earth in a number of components and assembled in orbit in a similarly massive number of spacewalks, and the sheer scale of it would likely require active heat management systems to prevent heat expansion of the components from sunlight warping the mirrors or stretching the distance between the mirrors.

Are there any other obstacles that would prevent this project from being possible?

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    $\begingroup$ A 6.5 km aperture, assuming circular, would have an area of ~33 sq. km. The Square Kilometre Array en.wikipedia.org/wiki/Square_Kilometre_Array will ultimately have a total collecting area of 1 sq. km. The challenges/obstacles of building a 33 sq. km collector would not be 33 times that of a 1 sq. km, but orders of magnitude more challenging. $\endgroup$ – Mick May 2 '19 at 14:22
  • $\begingroup$ SKA is a radio telescope. The requirements are far more forgiving at radio wavelengths. $\endgroup$ – Rob Jeffries May 2 '19 at 18:23
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    $\begingroup$ Do you require that your 6.5 km aperture telescope have a collecting area over that entire diameter? One could launch two separate, small telescopes which fly in uniform at a separation of 6.5 km that could observe in tandem using interferometry. The collecting power would be significantly less than a single large telescope of diameter 6.5 km, but it would meet the requirements as you posed in your question. $\endgroup$ – zephyr May 2 '19 at 19:39
  • $\begingroup$ potential obstacles include: a 6.6 km asteroid directly in front of it ;-) $\endgroup$ – uhoh May 10 '19 at 4:10

The basic issue with a large optical telescope is to keep all of the mirror in an extremely stable position relative to the rest of the mirror. Depending on the wavelength of your light you probably want to limit deviations from your perfect shape to less than about 100nm, so, you need to account for and deal with somehow any force which might act on one part of your mirror differently from another with enough force to bend the whole mirror by more than this tiny amount. At a glance, therefore you will need to deal with:

  • tidal forces from Earth, Moon and Sun (I think you can ignore tides from Venus and Jupiter, but I'm not certain)
  • Orbital perturbations arising because Earth is not spherical
  • temperature variations from moving in and out of Earth's shadow at different times and from sunlight and Earthlight (and maybe moonlight) hitting different parts of the structure differently
  • interactions of any conducting components of the mirror with Earth's magnetic field
  • flexing and oscillations of the structure arising from thrusts used to point the mirror

and probably a lot more that I haven't thought of. These problems would become somewhat simpler if you put the telescope much further from the Earth, or even better much further from the Sun (say a very distant orbit around Neptune) but that would make assembling it by spacewalking impossible.

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Large-aperture space-based telescopes are all based on a segmented primary design. Not only is it impractical to launch any rocket with mega-diameter cross-section, but thermal and gravitational stress relaxations would cause the mirror to go ugly after deployment.
These segmented primaries are surrounded by heat shields and other goodies specifically to avoid thermal effects. We already have the technology and computer power to align and phase a zillion (that's the technical term :-) ) segments into the desired base curve for this telescope.
OK, so aside from cost, why not do this? For one thing, it's far easier to improve the Quantum Efficiency (QE) and the dark noise(signal-independent electronic noise) in the sensors than to struggle to increase the total light collected. That takes care of concerns about seeing dim objects.
Next, it's also much easier to deploy very long baseline interferometric (VLBI) systems, essentially a widely spaced array of small telescopes, to achieve the diffraction-limited resolving power of an equivalent giant single telescope. "Easier" here is still very difficult, since you need to know and maintain the telescopes' spacing to a fraction of a wavelength.

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  • $\begingroup$ QE and dark noise do not presently limit the performance of optical telescopes. We are struggling to increase the amount of light collected. Hence the E-ELT and TMT projects. $\endgroup$ – Rob Jeffries May 2 '19 at 18:24
  • $\begingroup$ @RobJeffries yes and no :-) (background: I worked in Adaptive Optics Assoc for a zillion years). I agree that, particularly for ground-based systems, there's much more noise in source than the one or 2 electrons per readout even for long exposure times. But let's envision a L-5 location and a 1000-hour exposure system. $\endgroup$ – Carl Witthoft May 2 '19 at 18:31

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