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The question and answer about telescopes sent to different planets got me wondering. There are so many solar system bodies that have been photographed by deep space spacecraft, planets, dwarf planet(s?) moons and rings thereof, comets, asteroids...

I always imagine that these instruments look roughly like the 6 inch Maksutov or Cassegrain telesopes you see in telescope/binocular stores, or the backs of astronomy magazines. There are "mirror lenses" which are rougly similar instruments that you can buy at camera stores as well. So this size instrument could be called both a telescope, and a camera with a really long focal length lens.

For deep space or planetary probes, spacecraft that are no longer in the immediate vicinity of Earth, Sun-Earth $L_1$ or $L_2$ or the Moon or any of it's Lagrange points (or just say more than 2 million km from Earth), what is the largest aperture telescope (camera) that has been used for science?

Is the answer different if you exclude everything roughly 1 AU from the sun (STEREO, SOHO, DSCOVR, KEPLER, etc.)

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    $\begingroup$ Instead of asking "why don't we", you should ask "why would we". Why would you send large aperture telescope to other planets or into deep space? What problem are you trying to solve? Does a big telescope solve that problem? Do you need a telescope on Mars? Why? $\endgroup$
    – Noah Spurrier
    Commented Jul 6, 2016 at 18:28
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    $\begingroup$ @NoahSpurrier the question is about aperture sizes, not about rationale. "Why" is not asked here at all. Possibly you were aiming at a different question here and accidentally added your comment to the wrong question? $\endgroup$
    – uhoh
    Commented Jul 6, 2016 at 23:30

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Voyager's Infrared Interferometer Spectrometer (IRIS) has an aperture of 0.5 m (19.685").

This is not an imaging instrument though (resolution=1 pixel). The large aperture was needed to provide enough sensitivity.

IRIS mirror

If we take the question literally, Kepler would qualify with its heliocentric orbit at a distance of 1 AU. It has a 1.4 m primary mirror.

Kepler primary

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  • $\begingroup$ Is IRIS used for producing images, or just light collection for a spectrometer? I'm stuck because a collection system can be called a telescope even if it doesn't image. For example the question about single dish radiotelescope imaging before focal plane arrays. Looks like in diameter it's nearly identical to HiRISE, but the secondary occlusion is bigger, so the area of the aperture might be smaller than HiRISE's. $\endgroup$
    – uhoh
    Commented Jul 5, 2016 at 18:06
  • $\begingroup$ Beautiful! Kepler's mirror is huge! $\endgroup$
    – uhoh
    Commented Jul 5, 2016 at 18:13
  • $\begingroup$ Now I'm confused why haven't I accepted this (Kepler) as the answer. I only asked "besides Kepler" in the last sentence as a secondary question. Kepler is in fact a deep-space telescope, right, and therefore the largest? $\endgroup$
    – uhoh
    Commented Sep 1, 2018 at 18:44
  • $\begingroup$ I believe Kepler is the correct answer, accepting. Heliocentric orbits associated with Earth-Moon Lagrange points are still Heliocentric orbits, they just happen to be in a resonance with Earth. If the Earth disappeared, Kepler would keep going around the Sun in nearly the same orbit. $\endgroup$
    – uhoh
    Commented Sep 19, 2018 at 7:47
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After looking through various mission articles on Wikipedia, the Mars Reconnaissance Orbiter's HiRISE camera has an aperture of 19.7 inches (50 cm), which Wikipedia claims is "the largest so far of any deep space mission". This camera allows it to take extremely detailed pictures of the surface.

HiRISE Camera

If you're looking for other large-diameter telescopes which we have sent beyond the Earth-Moon system, they're most likely going to be mapping cameras. As was stated in the answer to your previous question, we don't have a lot of interest in sending space telescopes out very far from Earth. But when trying to make detailed surface maps of planetary bodies, we use telescopic cameras.

Another large camera is on NASA's New Horizons mission. The Long-Range Reconnaissance Imager (LoRRI) is 8.20 inches (20.8 cm) across.

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  • $\begingroup$ WOW!!! Now I understand why those incredibly beautiful and varied images of Mars are so detailed. I just added some images from Wikipedia to the question - maybe that image of HiRISE in your link would look good here as well. Now @Hobbes has a picture too. $\endgroup$
    – uhoh
    Commented Jul 5, 2016 at 17:58
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    $\begingroup$ @uhoh Good idea, here it is. $\endgroup$
    – Phiteros
    Commented Jul 5, 2016 at 19:14
  • $\begingroup$ I've quoted this here $\endgroup$
    – uhoh
    Commented Aug 4, 2020 at 2:46
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Just for reference, here are the largest apertures (visible or almost visible) sent to every planet that I can find. Note that some of them are kind of difficult to quantify. (Sorry for the mixed units...)

  • Mercury- 15 cm- Mariner 10 (Messenger's largest was a 12cm x 12cm square)
  • Venus- 7 inch from Galileo, but if you don't count that, likely only a few inches, from which spacecraft I can't identify, but likely Venus Express. Will be similar to Mercury. In fact, both Mercury cameras are also Venus candidates.
  • Moon- 8.7 inches
  • Mars- HiRISE, 19.7 inches, the largest sent beyond Earth
  • Jupiter- LORRI from New Horizons
  • Saturn- Probably Cassini ISS camera, can't find it's aperture, but the focal length is 2m.
  • Uranus/ Neptune- Voyager 2 camera, of course. At least 17 cm, the exact value I can't find.
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