[AR] Re: Star Tracker
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket <arocket@xxxxxxxxxxxxx>
- Date: Thu, 27 Dec 2018 15:19:37 -0500 (EST)
On Thu, 27 Dec 2018, Monroe L. King Jr. wrote:
I found something I don't like about Star Trackers they need a baffle to
block the sun because the sensor can't handle the sun. That sucks! Too
easy to kill one.
Actually, there's two parts to that.
The baffle is there to let the star tracker still work when pointed *near*
the Sun. As soon as the tracker points within about 90deg of the Sun,
some sunlight will fall inside the tracker's outermost opening. It'll hit
the side of the tube rather than going straight down it to the optics, but
the Sun is so bright that even the scattered light from an illuminated
patch of tube wall can wash out the image. The first thing to do,
obviously, is to paint the wall flat black. But even the best flat-black
coating isn't completely non-reflective, especially when light hits it at
a shallow angle. So the next step is to make the tube a little bigger and
have baffles sticking inward from it, so scattered/reflected light hits
them rather than continuing down the tube toward the sensor (and of course
you paint them flat black too). Designing and building a really good
baffle is complicated, but the more effort (and mass and volume) spent on
the baffle, the smaller the region of sky around the Sun where the star
tracker doesn't work.
(Other bright objects, like the Moon or brightly-lit Earth surface, can
also cause difficulties, and a good baffle helps there too.)
Part two is that obviously, no baffle will keep sunlight out of the sensor
if the star tracker is looking straight at the Sun. Clearly the tracker
doesn't work in that orientation, but there is the question of whether the
intense light can damage it. So you may need a shutter to protect it.
The extreme worst case is to have the tracker staring right at the Sun.
It's hard to avoid needing a shutter for that. If you're feeling brave,
you can assume that sun-stare is an unrealistic case, and that the worst
*plausible* failure is to have the field of view briefly sweep over the
Sun as an out-of-control spacecraft tumbles -- a brief Sun exposure is
easier to handle than a steady stare. If you're feeling foolhardy, you
can assume that you will always have the spacecraft under control and will
never point the star tracker at the Sun even briefly; yeah, right. So
it's a question of how much you feel like spending on "insurance" against
various classes of trouble, and how confident you are about guessing their
probabilities. (Cases that have been dismissed as obviously implausible
have a bad habit of really happening...)
FSD (Fast Switchable Diffusor) The polarizer-free FSD switches between
clear and light-scattering states. The open state exhibits over 83%
transmittance while the closed state effectively diffuses light. I
wonder if these Star Trackers could function sufficiently with 17% less
light?
You'd have to do the numbers. Clearly it'll hurt performance -- you'd
need a bigger aperture to gather more light, or longer exposures (which
might mean a lower maximum angular rate), or both. It shouldn't make a
workable star tracker impossible, but it would run up the mass and volume
and complicate the design, and numbers would be needed to decide whether
the penalties are unacceptable.
Henry
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