Having designed and built many a star tracker I can certainly attest to
what Henry states. The hardware is indeed one of the most difficult
parts of a star tracker build, as is the database management part of the
software. To do lost-in-space acquisition of attitude robustly you need
~10 stars in the FOV, and accommodating that requirement in the meager
star regions of the night sky drives the FOV size. Typically you end up
in the 20x20deg region with a pretty fast lens to make the photon
statistics pan out. If you in addition need to accommodate high attitude
rates, you need high sensitivity/quantum efficiency of the detector
which pushes you towards CCDs.
The optical part of the design is very elaborate and a science on its
own, but here a lot depends upon the ambition level. If we are talking
arcsecond level performance it gets to an expensive multielement lens
system with low thermoelastic biases very quickly. If its arcminutes,
one can get away with significantly less complexity. If you do an
attitude error budget analysis for an application like the one Monroe
describes, you will find yourselves in the latter category as the star
tracker accuracy will be unlikely to drive the budget.
BR's
Jonas
Den 12/25/2018 kl. 11:28 PM skrev Henry Spencer:
On Tue, 25 Dec 2018, Monroe L. King Jr. wrote:
This "Open Tracker" looks pretty good! I wonder why I have not seen it
before?
Beware of thinking that the software is the big problem and adequate camera hardware is a contemptible minor detail, easily solved by just buying something cheap off the shelf. Not necessarily so.
(Despite my past references to them, and having done work with them, I have no serious inside knowledge of the Sinclair star trackers. I do know that our lab considered doing its own star tracker -- we do build our own sun sensors, our own optical instruments, our own on-board computers, and our own attitude-control software -- and decided to buy the Sinclair ones instead. And I and others lately have been wrestling with a vaguely-related optical problem, and no, the hardware is not at all a contemptible minor detail; just setting the hardware specs requires non-trivial design effort, and meeting them may be a challenge.)
Henry