Because LCD's block 50% of the incoming light. I admit the first thing I
thought of was my welding helmet. Henry covered the baffling thing
pretty good. I agree with everything he said about baffling.
Monroe
-------- Original Message --------
Subject: [AR] Re: Star Tracker
From: Elliot Robert <elliotr@xxxxxxxxx>
Date: Thu, December 27, 2018 11:38 am
To: arocket@xxxxxxxxxxxxx
I understand someone was talking about switchable mirror to prevent the
camera from over heating. Instead a switchable mirror with high power
consumption and instead of a baffle that takes up space on a cube sat, why
not just use a crystal liquid display, like the ones in cheap calculators.
They very little power, could be set up to turn on by default as long as
there is a least a trickle of power coming in so if the space craft is in a
tumble all cameras get muted.
On Thu, Dec 27, 2018 at 4:52 PM Monroe L. King Jr. <
monroe@xxxxxxxxxxxxxxxxxx> wrote:
Damn yep gotta have a baffle. I'm thinking cubesat here so I've got >
100mm to work with more like 70 or something whatever the body with of
the camera is. The lens could be part of the baffle I suppose not much
choice there. So I guess I need to know what FOV I'm going to be trying
to make work first.
Oh yeah, my luck the system locks up and the damn thing stares right at
the sun. Get the system rebooted and find out the camera's shot. So it
has to have a shutter. I want one independent of the main system too. So
if it crashes the shutter still works.
I was working with the guys at "Eye in the Sky" from Australia and I
came up with a neet 80mm telescope that was one side of the sat. It was
a Gregorian with an extending secondary. It took up almost no interior
space in the sat. I'was trying to get it to double as the radio dish at
the same time. Because if you can point a reaction wheel telescope at a
star surely you could point it at the earth.
Pretty tuff to get enough components in a cubesat to do any real
science. But things get smaller every day!
We need some GEO repeaters so we can use light to them and radio to the
ground lol.
Keep talking Henry I'm listening.
Monroe
-------- Original Message --------to
Subject: [AR] Re: Star Tracker
From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
Date: Thu, December 27, 2018 1:19 pm
To: Arocket <arocket@xxxxxxxxxxxxx>
On Thu, 27 Dec 2018, Monroe L. King Jr. wrote:
I found something I don't like about Star Trackers they need a baffle
*near*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
the Sun. As soon as the tracker points within about 90deg of the Sun,hit
some sunlight will fall inside the tracker's outermost opening. It'll
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 illuminatedflat-black
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
coating isn't completely non-reflective, especially when light hits itat
a shallow angle. So the next step is to make the tube a little biggerand
have baffles sticking inward from it, so scattered/reflected light hitscourse
them rather than continuing down the tube toward the sensor (and of
you paint them flat black too). Designing and building a really goodon
baffle is complicated, but the more effort (and mass and volume) spent
the baffle, the smaller the region of sky around the Sun where the starsensor
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
if the star tracker is looking straight at the Sun. Clearly the trackerthe
doesn't work in that orientation, but there is the question of whether
intense light can damage it. So you may need a shutter to protect it.will
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
never point the star tracker at the Sun even briefly; yeah, right. Soagainst
it's a question of how much you feel like spending on "insurance"
various classes of trouble, and how confident you are about guessingtheir
probabilities. (Cases that have been dismissed as obviously implausibleless
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%
volumelight?
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
and complicate the design, and numbers would be needed to decide whether
the penalties are unacceptable.
Henry