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Maybe someone who knows satellites can help here. I assume with one gyro the satellite still has pointing function because one gyro senses in 3 axis, but that i
by bdamm 8y ago
Maybe someone who knows satellites can help here. I assume with one gyro the satellite still has pointing function because one gyro senses in 3 axis, but that if that one gyro were to go it would no longer know where it was pointing or perhaps without enough accuracy for science.
But... it’s a telescope. Couldn’t it determine direction from images received from a spotting or wide angle lens? Does it not have one?
- deleted 8y ago[deleted]
- sandworm101 8y agoNo. The gyros are more accurate. If it were to use reference stars, through the big camera, any detected motion would be at least one pixel. So every image would be blurry. It has to react before any drift results in movement more than a pixel.
- frgewut 8y agoCouldn't you measure star motion over longer periods of time, then even minuscule amounts of motion could be detected.
- PeterisP 8y agoBy the time you can see the motion on your image sensors it's too late - you want to detect and correct the motion you need to do it while you're imaging stuff, before it smears all your measurements.
- PopePompus 8y agoActually, you can do substantially better than one pixel, because the point spread function is bigger than one pixel. Even a technique as simple as centroiding will get you sub-pixel accuracy.
- sandworm101 8y agoYes but to get there you need detectable movement, a change in the image because of motion. A pixel has to change somewhere, the definition of blur. Sub-pixel blur is still blur.
- torrance 8y agoGP is right. The point spread function will spread the flux of a point source (eg. a star) across multiple pixels. You can identify the location of the actual star with sub pixel accuracy if you have a good PSF model.
- rcxdude 8y agoNo, you can get sub-pixel accuracy with a static image. The point is the star takes up multiple pixels, and when it moves by less than a pixel, some pixels get brighter while others get dimmer. This is a common fallacy in computer vision. Often people get to a within a pixel and stop because they assume they can't get better. Most times you can get substantially better. I worked on a qr-code like system where the scanner could reconstruct the code from an image with (slightly) less pixels than 'pixels' in the code.
- FabHK 8y agoInteresting observation. This works presumably because the output of a pixel is not just one bit, but some sort of continuous brightness reading (discretised, of course, sooner or later)? Not obvious a priori, but makes sense.
- dekhn 8y agohttps://en.wikipedia.org/wiki/Point_spread_function https://en.wikipedia.org/wiki/Point_spread_function The light is blurred, with a fairly predictable pattern, so if you fit a function to the shape, you can find the peak of the function and that is the most likely center position for the point source.
- sandworm101 8y ago
- freerobby 8y agoIn addition to what others have said, the concern about pixel-imperfect star tracking is somewhat moot, as we're talking about two different cameras. The effect on the imaging camera is very small. There is the star tracker camera (widefield, fast), and the imaging camera (narrowfield, slow). Star trackers use a fast widefield camera because it's easy to get a good signal:noise ratio (there is a lot of contrast between the stars and the background). The imaging camera, on the other hand, generally takes much longer exposures (HST subjects are generally very dim, relative to stars). All those beautiful, nebulous HST photographs you see? Those have exposure times on the order of hours or days. In practice, being off by a pixel momentarily is not a big concern -- the amount of "bad" photons you collect during that time is very small.
- InclinedPlane 8y agoThis is nonsense. The gyros are only used for coarse pointing, guide stars are used for much more accurate and precise position sensing during exposures (which is why they are called Fine Guidance Sensors).
- AtomicOrbital 8y agogyros can be used for both sensing motion as well as creating motion (is this true for Hubble's gyros ?), using spinning spheres on each axis of a high enough mass ratio in comparison to the enclosing object similar to haptic ... if so, I wonder if the issue is a degraded gyro is impacting ability to steer the telescope not just to do the sensing
- InclinedPlane 8y agoThere's no good reason to combine jobs for such devices on spacecraft since the requirements for attitude control and attitude sensing are very different. The gyros on the HST are only for sensing, it uses a combination of reaction wheels and magnetic torque rods for attitude control, which are well suited to orbital observatory application due to their high precision.
- foobarbecue 8y agoActually it does have a dedicated star tracker, and (the system)[https://www.nasa.gov/content/goddard/hubble-space-telescope-pointing-control-system https://www.nasa.gov/content/goddard/hubble-space-telescope-...] is designed to combine gyro measurements with star tracker measurements. Both the Lunar Reconnaissance Orbiter and Mars Reconnaissance Orbiter are dealing with or expecting gyro issues and are currently coming up with and testing control paradigms that only use the star trackers. I think part of the problem with the star trackers is that if you "lose your place" then it's very difficult to get back on track and this could lead to serious problems.
- teraflop 8y ago> I assume with one gyro the satellite still has pointing function because one gyro senses in 3 axis, Each of the Hubble's gyroscopes only measures rotation around a single axis. > But... it’s a telescope. Couldn’t it determine direction from images received from a spotting or wide angle lens? Does it not have one? Hubble does have imaging sensors to allow it to track the stars and orient itself. But the orientation needs to be not only measured very accurately (to within a few millionths of a degree), but also kept stable over long periods of time, in order to avoid motion blur. So the attitude control system needs to have low latency. In principle it would probably be possible to do this using only image sensors, but it would have been very challenging when that Hubble was designed in the 1980s. It has a number of CPUs, the fastest of which is an 80486 running at 25 MHz.
- therein 8y ago> It has a number of CPUs, the fastest of which is an 80486 running at 25 MHz. Not unexpected but reading that now... Absolutely remarkable.
- Lammy 8y agoSpecifically, a radiation-hardened 486. The type of equipment that can survive in space lags significantly behind what you would consider modern on a desktop computer. For example, the radiation-hardened PowerPC G3 is a popular choice. It will power the 2020 Mars rover too: https://en.m.wikipedia.org/wiki/RAD750 https://en.m.wikipedia.org/wiki/RAD750
- bluedino 8y agoThe 80486 was state of the art in 1990 when the Hubble launched, as it was released in 1989. Was it installed as a later update? The PowerPC G3 was released in Apple computers in 1997, just a few years later.
- Sharlin 8y agoYep, the original main computer was a 1.25 MHz (!) DF-224 [1]. It was augmented on Servicing Mission 1 in 1993 with a 15 Mhz 80386 co-processor [2]. The 80486 was installed in 1999 on Servicing Mission 3A (STS-103) [2]. [1] https://en.wikipedia.org/wiki/DF-224 https://en.wikipedia.org/wiki/DF-224 [2] https://en.wikipedia.org/wiki/STS-61 https://en.wikipedia.org/wiki/STS-61 [3] https://en.wikipedia.org/wiki/STS-103 https://en.wikipedia.org/wiki/STS-103
- Animats 8y agoThe amazing thing is that back when the US had a manned space capability, those gyros were replaced. Twice. There were five servicing missions to the Hubble telescope.
- adrianN 8y agoIMHO we should let Hubble die and take the funds that repair missions would suck up to deploy an improved replacement (i.e. the James Webb telescope). When Hubble was launched it could take better photos than any Earth-based telescope, but with improvements in technology that is no longer the case. We've become really good at filtering out atmospheric noise and the telescopes we can build on Earth are much bigger than Hubble.
- zaarn 8y agoI don't think anyone is planning to send up a repair mission, Hubble is merely being kept alive as long as possible.
- SmellyGeekBoy 8y agoWithout the shuttle servicing missions are not even possible anymore anyway.
- moonbug 8y agoThere will be no repair missions because the US has not retained manned access to space for that sort of activity.
- frankharv 8y agoBut what happens when the folding space blanket fails on Webb? That system seems incredibly flimsy and will be the downfall.
- WalterBright 8y ago> those gyros were replaced. Twice. Hmm, I think the second time the gyros needed replacing, and the gyros were the limiting factor on the telescope's lifetime, I'd have installed a redundant set.
- dredds 8y agoAs i understood it the gyros provide Hubble's axial rotation, not merely sensing for positioning (ie there are no fuel jets for orientation) so a single gyro will only allow them to rotate Hubble in 1 axis, regardless of image orientation data or latency issues.
- foobarbecue 8y agoAren't you confusing gyros with reaction wheels?
- deleted 8y ago[deleted]