clements@vax.oxford.ac.uk (06/14/91)
>>While it is true that HST does make *some* improvements on the resolution >>achievable by normal ground based telescopes, I would expect that we could do >>a *lot* better by putting the 1.5 billion dollars HST cost into adaptive and >>active optics. These techniques actively correct for the distortions introduced >>by the atmosphere. > But work only in the Near Infrared! The farther towards the visual, let alone > UV you go, the worse the seeing gets and you run out of photons and cannot > analyze the wavefront errors anymore. [I detailed that two years ago in a > letter to 'Nature' which was *accepted* (I even got a proof sheet) but then > never made it into print (or has someone seen it?) - strange...] This is true if you use the photons from you (faint) astronomical object to provide the correction information. Recently published papers (AAS meeting Seatle, 1991) from the declassified SDIO studies in adaptive optics use a laser to generate a fake bright 'star' in the ionosphere which is then used to calculate the necessary wavefront corrections. This, I am lead to believe, has proved very successful, and is *still* much cheaper than HST even at its first guess price. > >>The NTT (New Technology Telescope) at ESO regularly gets >>resolutions well below 1 arcsecond, by using only relatively simple correction >>techniques. The CFHT on Mauna Kea has now been sed to image stars in Virgo galaxies, a project originally slated for HST but shot down by the mirror flaw. > Which have nothing at all to do with Adaptive Optics: the NTT uses Active > Optics which is a *much* slower analyzing & correcting process which ignores > the Seeing completely. [See my article in Sky&Tel Sept.1989 for details.] > This Adaptive Optics plus an excellent site plus an ingenious seeing-reducing > pseudo-dome give the NTT 0.5" regularly and 0.3" at times. HST, BTW, would > have been some 5 to 10 times better, not to speak of its UV capabilities > beyond the atmospheric barrier. > OK. I always get confused between active and adaptive optics. The points remains the same. The UV stuff that HST is still very capable of doing is a unique facility, which should yield some great science. But HST is a very expensive 'super IUE' in that case... >>Other possibilities for beating HSTs resolution on the ground >>include optical interferometry (like the radio mapping that the VLA does but >>in the optical). The ESO VLT will use 4 linked 8 metre optical telescopes to >>get resolutions better than HST will achieve *even if it was built right*, and >>it will cost about 1/10th as much. > The VLTI (VLT Interferometer) is indeed a tremendously exciting prospect for > the early years of the 21st century, but getting actual *images* with 1/10000" > resolution this way will be a *BIG* task. ESO's press people love to show > around the sharpest Neptune images by Voyager 2 and promise that the VLTI will > be able to repeat that (with Adaptive Optics) - unlikely, to say the least. I think you're being a bit doctrinaire in what you call an image. Interferometry is certainly *the* way to get really great resolution. VLTI is not the only way to do it either. A group at Cambridge have got a map of Arcturus using the Hershel in an interferometer mode (mask off most of the 4m reflector and use a few holes in the mask to do interferometry and achieve diffraction limited resolution). The same group are also working on COAST (Cambridge Optical Aperture Synthesis Telescope) whichj they hope to ave working with baslines of 50 to 100 m. This will be streets ahead of HST in rsolution... (when it works). Dave I'll do the .sig later.