So, when we last left our poor afflicted orbiting telescope, it had lost side A of its CU/SDF (Control Unit/Science Data Formatter), which is responsible for translating the data taken by an instrument into bits that can be readily transferred down to the ground. Luckily, one of the things that NASA does really, really well is redundancy, so there is a side B that is ready and waiting to be turned on. Before doing so, however, the Hubble folks need to make sure that they understand what happened to side A, and that they know how to safely turn on side B without fragging anything else. The latest news is that Goddard completed an independent review last week, and they think they understand what happened, and how to safely turn on side B. The staff at Goddard has been practicing with a spare SIC&DH (Science Instrument Control and Data Handling System, which contains the CU/SDF) on the recplica HST that’s been in cold storage for the past 18 years or so (see what I mean about redundancy?). A final Transitional Readiness Review was held, and they’re recommended starting the switch to side B. If this is approved, the switch should take place in the middle of next week.
The cool thing is that Hubble has been keeping itself scientifically busy doing astrometry (high accuracy positional measurements) with the Fine Guidance Sensor. Past papers that have come out using FGS data are some of the coolest and most underpublicized Hubble results, so I’m jazzed to see that they’re cleaning up while everyone else is idle!
October 10th, 2008 at 4:04 pm
Very cool.
and…
Fingers crossed!
October 11th, 2008 at 9:08 am
Redundancy is the key to biology. It seems to work for space telescopes as well. Keep fingers crossed that switch from A to B works. As for astrometry (measuring distances to objects etc), here is a recent example more close to home. An asteroid was found, its ephemeris computed and its impact on Earth recorded from space.
http://www.eumetsat.int/Home/Main/Media/Features/707785?l=en
Lawrence B. Crowell
October 11th, 2008 at 2:29 pm
Once again, I’ve overjoyed to see how scientists manage to get something for nothing. Much like the many innovative reüses of satellites that have completed their primary missions.
Here’s to Hubble! What a lovely old gal!
October 12th, 2008 at 4:24 am
Considering Hubble’s successor now the grand old dame is in her twilight years, instead of one great big telescope, has any thought been given to having several smaller telescopes working in concert, a brace of Hubblets if you like?
I presume they’d be cheaper and less demanding to make (if the lenses are the trickiest aspect), and to launch. They’d also spread the risk of instrument failure and debris collision.
Of course the main benefit would be a huge effective aperture size, assuming they could be steered and coordinated with the required accuracy and would not be limited (by optical principles) in the directions they could collectively view objects.
October 12th, 2008 at 10:46 am
Wait… There’s a replica HST in cold storage? Wow. Now that’s how to do redundancy!
October 12th, 2008 at 10:58 am
I’ve actually looked into putting together a proposal for something like this, and the problem is that you can’t really get a pointing system as good as Hubble’s on a low-cost satellite bus. You’d still get good UV sensitivity, so the UV folks have been considering small satellites in the latest round of SMEX proposals. But, you can’t get the same level of spatial resolution, which is the science I’m most interested in at the moment.
October 12th, 2008 at 12:34 pm
A system of small telescopes in consort is the basis for the optical interferometer. For radio telescopes this is fairly easily arranged since the radio signal from each dish is sent into a wave guide and the EM waves from each dish then interfers with each other. Lots of nice Maxwell equation E.E. stuff makes it work as one huge radio telescope. With optics things are more difficult. An array of optical telescopes is a sort of optical beam splitter. The quantum interference of photons from each scope could construct the equivalent image of a large scope. An optical interferometer puts the output of each scope into an optical fiber, while preserving the phases of the photons, and the wave interference of the photons from the array of fibers is used to construct the equivalent large image. In principle it is the same as with the radio interferometer, but is more difficult to make work.
Lawrence B. Crowell
October 13th, 2008 at 2:40 am
HST’s follow-on, the larger James Webb Space Telescope, is already well under construction. The lead time on these projects is enormous.
For deep imaging from space, if everything else is equal, large telescopes tend to win over small because of the improvement from diffraction-limited imaging. Comparing ground-based telescopes of diameter D with equal atmospheric seeing, the signal collected in a given exposure time goes as D^2, while the cost of construction goes as some power of D (2.5 or so). But for space-based telescopes, the diameter of a diffraction limited image goes as 1/D. This means that the solid angle occupied by a point source goes as 1/D^2. The smaller size of a point source means there is less sky flux under it. For deep observations longward of about 500 nm (depending on the detector, etc), observations are limited by Poisson noise in the background from the zodiacal light, so for diffraction limited imaging the S/N can go as D^3 rather than D^2. (But you need to spend a lot on a big detector to get a decent field of view because the pixels are smaller. Detectors often drive the size and cost of instruments for telescopes, more than the optics do.)
Of course, everything else is not equal, the cost of putting large things in space doesn’t necessarily go as D^2.5 (it’s probably steeper) and smaller projects can come to fruition faster, take advantage of advances in detector technology, are better suited to mapping large areas of sky, and so on. So there is a place for both small and large satellite observatory projects.
October 13th, 2008 at 7:57 am
I admit to knowing very little about the logistics, costs, and bureaucratic processes involved, but if we have a spare Hubble in cold storage, why not send it up, now that the first one is winding down?
October 13th, 2008 at 11:54 am
John, there are a host of reasons why not to do that: the larger successor to Hubble is under construction; the original Hubble was designed in the 1980s and is full of “vintage” electronics; Hubble was designed to be serviced by the space shuttle, which is soon to stop flying; and so on.
Let me amend the S/N calculation I gave for seeing-limited vs. diffraction-limited telescopes above (it was late). For a seeing-limited telescope of diameter D, the signal from a point source goes as D^2, and the sky background flux in an aperture also goes as D^2, Poisson noise is sqrt(N), and so the S/N improves as D^1. For a diffraction-limited telescope, the signal from a point source goes as D^2, the image diameter goes as 1/D and so the aperture area goes as 1/D^2 and the sky background flux in the aperture is constant, so the S/N improves as D^2.
October 14th, 2008 at 9:50 pm
[...] come down with a bad case of broken electronics. In the meantime, they’ve still been able to do useful science by obtaining ever-more accurate positions of [...]
October 14th, 2008 at 11:53 pm
Dude, they can weigh exoplanets with Astrometry. That’s awesome.
October 15th, 2008 at 7:46 am
Thanks, Ben. So the backup Hubble will eventually end up in the Smithsonian, I guess. That’s certainly fitting, but I’d rather have it in orbit, collecting data.
October 15th, 2008 at 9:34 am
With all due respect, the James Webb Space Telescope is rather a successor of the Spitzer Space Telescope than to the Hubble: solely the IR wavelengths, and with no possibility of a servicing mission (except of minor repair work via the docking ring).