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Once the ATLAS instrument transmits six laser beams to Earth,
the telescope receives the photons that bounce back from Earth.
Tyler: Hey there, I'm Tyler Evans, an optical
mechanical engineer on the ICESat-2 mission, working on the ATLAS instrument.
So for the receiver subsystem, the biggest component
is the telescope, which is 0.8 meters in diameter, which is about
31.5 inches in diameter for the primary dish. So the primary
dish is the reflective surface that you see which has curvature to it, and
the way that it works is light goes into the primary dish and is collected by this
big dish and it gets focused up into the secondary mirror, which is at the end
of this tower. And this secondary mirror shines the light and focuses
it down through that tube to the center, to the back
of the telescope where the detectors are. And this telescope is made out of beryllium,
which is kind of a unique material that has a strength-to-weight ratio,
so this whole telescope only weighs 50 pounds.
Now to know which way the telescope is pointing, there's four green laser that are coming
out of the telescope. So if you look into the main dish you can see
the four TAMS spots, the telescope alignment monitoring system,
and those go out of the telescope and get picked off
by this periscope. And the receiver periscope
picks up the light from the telescope and bends back into the
LRS. The LRS is the Laser Reference System camera. So that's really the brains
behind the closed-loop control system that looks at those four spots and knows
as it tracks those four spots how the telescope is pointing.
So now that you've seen the front of the receiver subsystem where the light goes in,
I'll take you around back to be able to show you the fibers
on the other end where the light actually goes into the
detectors. You can see the six black fibers that are plugged into the back of the
telescope. Those actually go out to the detector
where they're able to analyze those photons that came back from Earth and compare them to the
photons that were sent out to check the stopwatch and see
what the distance actually was of that ice that they just measured.
So this is the stellar side of that LRS camera and this
lens here looks out into space and it looks at the stars to see
how the satellite is pointed. So based on what constellations it's seeing and how
the stars' angles are coming into this lens, it knows how the spacecraft
is pointed relative to Earth. Once the photons are filtered
through the telescope and into the fibers, they go into the optical filter assembly
or the etalons. These filters have a really small bandwidth
around 532 nanometers. So that's really specific
to only have the photons that we sent out come back. This is really important because
otherwise the Sun would dominate the signal. So the photons now go to
the PMT detectors, which are the photomultiplier tubes,
where the photons are converted to electricity and timed.
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