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[ music ]

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[ music, buzzing sound effects ]

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Once the ATLAS instrument transmits six laser beams to Earth,

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the telescope receives the photons that bounce back from Earth.

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Tyler: Hey there, I'm Tyler Evans, an optical

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mechanical engineer on the ICESat-2 mission, working on the ATLAS instrument.

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So for the receiver subsystem, the biggest component

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is the telescope, which is 0.8 meters in diameter, which is about

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31.5 inches in diameter for the primary dish. So the primary

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dish is the reflective surface that you see which has curvature to it, and

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the way that it works is light goes into the primary dish and is collected by this

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big dish and it gets focused up into the secondary mirror, which is at the end

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of this tower. And this secondary mirror shines the light and focuses

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it down through that tube to the center, to the back

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of the telescope where the detectors are. And this telescope is made out of beryllium, 

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which is kind of a unique material that has a strength-to-weight ratio,

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so this whole telescope only weighs 50 pounds.

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Now to know which way the telescope is pointing, there's four green laser that are coming

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out of the telescope. So if you look into the main dish you can see

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the four TAMS spots, the telescope alignment monitoring system,

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and those go out of the telescope and get picked off

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by this periscope. And the receiver periscope

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picks up the light from the telescope and bends back into the

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LRS. The LRS is the Laser Reference System camera. So that's really the brains

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behind the closed-loop control system that looks at those four spots and knows

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as it tracks those four spots how the telescope is pointing.

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So now that you've seen the front of the receiver subsystem where the light goes in,

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I'll take you around back to be able to show you the fibers

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on the other end where the light actually goes into the

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detectors. You can see the six black fibers that are plugged into the back of the

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telescope. Those actually go out to the detector

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where they're able to analyze those photons that came back from Earth and compare them to the

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photons that were sent out to check the stopwatch and see

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what the distance actually was of that ice that they just measured.

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So this is the stellar side of that LRS camera and this

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lens here looks out into space and it looks at the stars to see

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how the satellite is pointed. So based on what constellations it's seeing and how

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the stars' angles are coming into this lens, it knows how the spacecraft

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is pointed relative to Earth. Once the photons are filtered

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through the telescope and into the fibers, they go into the optical filter assembly

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or the etalons. These filters have a really small bandwidth

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around 532 nanometers. So that's really specific 

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to only have the photons that we sent out come back. This is really important because

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otherwise the Sun would dominate the signal. So the photons now go to

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the PMT detectors, which are the photomultiplier tubes,

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where the photons are converted to electricity and timed.

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[ music ]

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[ beeping sound effects ]

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