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Why are radio telescopes typically only a dish with a receiver above it, while optical telescopes have a primary, plus secondary and sometimes even a tertiary mirror?

In other words, why do radio telescopes have only one reflector, while optical telescopes have up to three or more?

The same wave phenomena, such as focusing, should apply in both cases. So I don't understand why the geometry would be radically different. You could just replace actual mirrors with a convenient solid, such as plastic, that reflects radio waves just as wells as mirrors reflect optical waves.

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    $\begingroup$ What makes you think that you can't use an optical telescope at "prime focus"? $\endgroup$ – Rob Jeffries Jul 11 '15 at 7:45
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    $\begingroup$ Didn't they used to have a cage you could sit in while exposing photographic plates at prime focus, inside the tube of the Hale telescope? Looks like it: google.com/… $\endgroup$ – Wayfaring Stranger Jul 11 '15 at 12:06
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    $\begingroup$ Many large optical telescopes have prime focus instruments. $\endgroup$ – Rob Jeffries Jul 12 '15 at 22:44
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They're not different. Same principles do apply. You could have secondary, tertiary, quaternary, and so on, mirrors with instruments at any wavelength, either optical, or radio, or infrared, etc. You could also have instrumentation placed directly in prime focus (so no mirrors other than the primary) with any kind of instrument - radio or infrared or visible or whatever.

See this image of the 5 meter Hale telescope on Mt. Palomar - there is no secondary mirror in this case, the observer is sitting in a little cage at prime focus, using the primary mirror directly:

enter image description here

Of course, for other scenarios, the Hale telescope employs secondary and tertiary mirrors - it depends on the particulars of the telescope, the instrumentation, the experiment or research you're doing, etc.

One reason many of the large optical telescopes very often have at least a secondary mirror is that the architecture preferred in most of these cases is the Ritchey–Chrétien - chosen often for the largest professional telescopes because it eliminates coma, an aberration that is detrimental to astrometry (with coma, images of stars are not round, so it's hard to measure angular distances between them). You could use the primary mirror of such a telescope directly, sure, but being a concave hyperbolic mirror, it has strong aberrations of its own, and so requires the convex hyperbolic secondary (often a strong hyperbola, with a large eccentricity) to correct the aberrations.

The Hale telescope pictured above has a parabolic primary, so using it directly is not a problem.

Again, all of the above are not strict rules, just statistical observations.

Some radio telescopes have instrumentation at prime focus simply because it's convenient for that particular case. Other radio telescopes have secondary mirrors. Again, it all depends on what you try to achieve. E.g., the Arecibo radio telescope could be used either in prime focus, or with a secondary mirror in a Gregorian configuration - here's the image with the prime focus instrumentation and the Gregorian mirror to the left:

enter image description here

In the case of the Arecibo scope, the N-ary mirrors are sometimes used to correct the aberrationa of the spherical primary reflector, but that's not the only reason why they're used.

Here's a discussion comparing various architectures (classic Cassegrain versus Ritchey-Chrétien versus anastigmatic aplanat) for a large radio telescope, highlighting various design, performance and operational issues for each. TLDR: classic Cassegrain is traditional for radio telescopes, but the R-C architecture performs better and is not significantly more difficult to build; OTOH, with R-C you must always use the secondary.

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Radio telescopes are shaped differently primarily because we can't see microwaves or radio waves. Optical telescopes are designed so that there is a focal point where you can look and see the image. However, radio telescopes and optical telescopes actually work very similarly, and sometimes radio telescopes do have secondary reflectors.

In an optical telescope the secondary mirrors are generally meant to redirect the light and focus the image for your eye. The primary mirror is what is gathering the light, so it's what is doing the magnification. You can see a great example of this with the image of the Newtonian telescope below (thank you, Wikipedia!).

Newtonian telescope

Radio Telescopes

Radio telescopes actually work in a very similar manner. The "dish" portion of the telescope is reflecting the waves, the same as the primary mirror in the optical scope. It is then received at the LNB/LNA/receiver part. You can think of that as the focal point where the secondary mirror is positioned in the optical telescope.

In addition to that, sometimes radio telescopes actually do have a secondary reflector. I'll use an image of a radio telescope at NASA Jet Propulsion Laboratory's Goldstone Deep Space Communications Complex to show this (also from Wikipedia). The "dish" is the primary reflector, then it's being reflected again at the secondary reflector being held up by the metal arms. After the second reflection the signal goes into the receiver attached to the primary reflector.

Cassegrain radio antenna

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One of the reasons for the difference is the sheer number of different optical (and near-infrared) instruments available. Most professional optical telescopes have two or more standard instruments (e.g., an imager and a spectrograph), with the possibility of adding guest instruments from time to time; some have as many as five standard instruments at the same time. Having the instruments mounted at the base of the telescope makes it much easier to switch between them (sometimes, as with a Nasmyth mount, by simply rotating the tertiary mirror 90 or 180 degrees) than it would be if the instruments were mounted at prime focus.

See, for example, the picture at this web page for the SOAR Telescope, which has ports for five different instruments: http://www.lna.br/soar/telescope_e.html

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    $\begingroup$ It wouldn't surprise me, though, if the majority of imagers at large professional telescopes are not at prime focus; most imagers are not very wide-field (especially near-IR and mid-IR imagers, and anything using adaptive optics) and some do double-duty as spectrographs. Of the 8-10m telescopes, I think only Subaru and LBT have prime-focus imagers.... $\endgroup$ – Peter Erwin Jul 13 '15 at 12:21
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    $\begingroup$ @PeterErwin - as a rule, a lot of the very big telescopes are Ritchey-Chretien, so they have at least a secondary. Of course, you could mix and match, and switch configurations, but the massive concave hyperbolic primary of a big R-C system is pretty much begging for a convex hyperbolic secondary to correct diverse aberrations. $\endgroup$ – Florin Andrei Jul 14 '15 at 0:06
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    $\begingroup$ @FlorinAndrei You ought to put this in your answer! $\endgroup$ – Rob Jeffries Jul 14 '15 at 6:38
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    $\begingroup$ @RobJeffries - done. $\endgroup$ – Florin Andrei Jul 14 '15 at 17:33
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    $\begingroup$ @PeterErwin - the Subaru telescope does indeed have a prime focus instrument, but here's the thing: Subaru is a Ritchey-Chrétien system just like all the big ones. That means the primary mirror is concave / hyperbolic. That means the primary, all by itself, has strong aberrations. A bare camera placed in prime focus would get a bad image. What they do instead is they made a corrector - a group of lenses designed to compensate the aberrations from the hyperbolic primary. In theory, you could do that with any R-C system canon.com/technology/approach/special/subaru.html $\endgroup$ – Florin Andrei Jul 17 '15 at 8:15

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