I inferred from a show I saw that through the use of adaptive optics and correlating images with other observatories in real time, space-based telescopes either are or will be obsolete. What’s more larger lenses (or at least large effective lenses) could easily be used. Is that true, or did I misunderstand?
Thanks,
Rob
I still think there’s a place for space-based telescopes. Adaptive optics is certainly possible, but at present it’s certainly not trivial. It requires state-of-the-art MEMS devices and pretty complex, extremely fast software for real-time correction of atmospheric errors. I worked with such adaptive optics loops on my last job, and it ain’t easy.
You can get large effective synthetic apertures by correlating data from widely-separated telescopes, but there are limits on that, too. You only get high resolution long the direction joining the stations, for one thing. You ideally want a lot of them covering an area. and i’m not sure how well you can do this at various wavelengths – you really would like to optically interfere the signals directly, measuring the phase between them. as i noted in another thread, the vast majority of optical detectors don’t measure phase.
Based on how many astronomers are trying to get observing time on the Hubble (Mr. Neville periodically applies for it, and the competition is quite stiff AIUI), and from the stentorian outcry in the astronomical community when it looked like there would be no more servicing missions to the Hubble, I’d say adaptive optics certainly haven’t made space-based telescopes obsolete yet.
One problem with adaptive optics, as opposed to space telescopes, is that adaptive optics can’t work everywhere on the sky. To use adaptive optics, you need a sufficiently bright guide star, which just isn’t available everywhere. Some adaptive optics systems use lasers to get around this, but there are problems with that (someone has to sit near the telescope at all times when the laser is being used and watch for passing airplanes, for one).
Another problem with all ground-based observatories is that the atmosphere absorbs some wavelengths of light that astronomers would like to study. Adaptive optics can’t fix that problem. There are two space telescopes up there now observing in wavelengths that are absorbed by the atmosphere- the Chandra X-ray Observatory in the X-ray range, and the Spitzer Space Telescope in the infrared.
Ground-based observatories can use larger mirrors (very few telescopes used for actual astronomical research use lenses these days, and the Hubble uses mirrors) than space telescopes, because of the practical problems of sending a large mirror into space. Large mirrors tend to be heavier than small ones, for one thing, and that makes it harder and more expensive to send them into space. You also have size constraints if you want to use the Space Shuttle to put your telescope into space.
3 actually – there is also XMM-Newton – an ESA led X-ray telescope launched around the same time as Chandra was. 
I’d probably be killed at work if I’d let you get away with that.
Also, consider things like LISA and the search for gravitational waves – its just physically impossible to get the sensitivity that the LISA people are aiming for with ground based detectors, due to both size limits and the fact that the Earth is not ‘stable’ enough for such accuracy (things like earthquakes and heck, even cars driving near the detector will drown out any signal).
I think space based telescopes will be around for a long time yet; astronomy is far more wide ranging than just the optical waveband, and our knowledge of the universe has increased greatly once we were able to get telescopes above our atmosphere.
Mille pardons. Mr. Neville hasn’t mentioned that one, and I hadn’t heard about it in my days in astronomy. Not too surprising, as neither of us did X-ray work.
There are actually quite a few telescopes in space observing in bands where the atmosphere is opaque. Many of them are pointed at the Sun. Several of the professors here work primarily in the extreme ultraviolet, which has an optical depth of only a few centimeters in air (which makes calibration of instruments an interesting challenge)
I’d suggest that it’s very telling that the best pictures of stars and galaxies on NASA’s Astronomy Picture of the Day come from Hubble.
I’m vastly disappointed that we don’t have a fleet of Hubbles up there.
Its OK. Hey, I forgot all the Sun-aimed telescopes.
They are the most publicized, at least in the US.
Take a look at a few of the shots from Paranal. To the layman, they appear to be about as good as the Hubble images, and a lot of them were taken with only one out of possible four 8m telescopes. It has the capability of using all four in an array.
-Obligatory link to Paranal shot of Uranus.
It’s quite detailed!
I’d better have that looked at :eek: