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When I was writing this answer about the LAGEOS satellites, I notices that four of the over four hundred corner reflectors are made of optical germanium rather than fused silica.

Fused silica (FS) is transparent to some wavelengths in the infrared, but it's limited. For a wider transparency window, especially at longer wavelengths, optical-grad silicon or germanium is used (e.g. FLIR cameras). Also, there are likely to be antireflection coatings which further limits the use of the FS elements away from their optimized wavelength range.

This was probably done "just in case" someone would like to try IR, and it's good thinking, but I'm wondering if these have every been used in orbit, even for testing purposes.

This question has a new companion: Why do the LAGEOS' satellites have four germanium corner cube reflectors out of over 400?

below: Sample output from JPL Horizons.

Revised: Sep 11, 2017                 LAGEOS-2                         -122195

LAGEOS-2 (1992-070B, "Laser Geodynamics Satellite", NASA/ASI)
60-cm diameter sphere, 426 corner cube reflectors (VIS), 4 germanium (IR)
Deployed: 1993-Oct-22 from Space Shuttle Columbia (STS 52)

below: From here.

enter image description here

uhoh
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  • Using these cube reflectors for a distance measurement with laser pulses requires a very sensitive detector. A photomultiplier tube is able to detect single photons for wavelengths from visible light to near infrared. Is there another detector available for detection of single photons of a wavelength requiring germanium reflectors? – Uwe Sep 12 '17 at 20:57
  • @Uwe If you re-read my question you'll see that I mentioned that there may also be an issue of the bandpass of the antireflection coating on the FS cubes. Broadband AR coatings have steep cut-offs at their edges, so until we have specific data, we don't know what wavelength range is covered by the germanium cubes. Also, since there are only four cubes, out of over four hundred, this may be more for testing the laser feasibility than getting ultimate timing. Thus solid state detectors could be used. Further, the satellite has a long life so it anticipates future technological developments. – uhoh Sep 13 '17 at 00:30
  • But photomultipliers have low noise and very high amplification. Are solid state detectors for that wavelengths comparable in noise and timing properties? – Uwe Sep 13 '17 at 09:44
  • @Uwe there are not enough germanium corner cubes to produce a strong enough signal, or even an unambiguous signal, because the rotation of the sphere can bring one lonely reflector closer or farther by tens or centimeters, whereas the FS cubes are so numerous that it the distance spread mostly averages out. They were definitely not placed there to produce accurate measurements. Instead they are there just in case someone wanted to do some very basic testing to see if an IR signal could be reflected at all. So I'm curious if anyone every tried such a test. 426 visible vs 4 IR reflectors. – uhoh Sep 13 '17 at 09:53
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    I've never noticed this about the LAGEOS satellite: most interesting question indeed. – Selene Routley Sep 13 '17 at 10:46
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    May be the germanium reflectors should be used only for short distances from a space ship in orbit. Some kilometers instead of some hundreds and the reflected IR signal should be measurable. The radar equation is applicable here too, 1/100 of the distance and the signal is 100 million times stronger. – Uwe Sep 14 '17 at 09:33
  • @Uwe that's a really good point! I never thought about that, but you are right. From a satellite in a similar orbit, or one that passes by from time-to-time, the distance could be much smaller than from Earth's surface; tens of km vs ~6,000 km or more. That's a huge effect! – uhoh Sep 14 '17 at 09:39
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    A Humanity Star on steroids. --- @uhoh you might also like this incomplete list: "A Small Compendium Of Shiny Orbiting Balls". --- I'll see if I can find an answer to this question. YT, – Rob Aug 08 '18 at 19:52
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    Found something specific to the 4 reflectors, check back in a couple of hours. – Rob Aug 08 '18 at 20:06
  • I didn't want to keep you waiting even longer, and would like to expand on my answer - but below is what it is, so far. I'll continue later. – Rob Aug 08 '18 at 23:19

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