That's a quite common approach for spacecraft, but naturally comes at an
equally higher cost. Thats is also why quite a few star tracker systems
has a central computer which supports multiple cameras. It also has the
added effect of increasing resiliance to Earth, Moon, Sun etc. blindings
of any single camera. However, from personal experience it is often not
the best idea to go for a 3-ortho triad, as it makes on-ground
calibration and spacecraft integration quite difficult. Moreover to get
the full performance benefit of multiple cameras you really need an
optical bench in between them to control the thermoelastics.
/J
Den 12/26/2018 kl. 01:08 AM skrev Jake Anderson:
How much of that could be ameliorated with more than one star tracker and some constraints placed on their mounting?
Say if one had 3 orthogonally mounted trackers could you be relatively assured at least one would be in a place of "happy stars"?
On 26/12/18 10:33 am, Jonas B. Bjarnø wrote:
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