wrote in message
m...
1. Hubble's Telescope has 0.1 arcsec resolution in visible and
ultraviolet ranges. What is the next resolution level that can bring
substantially new knowledge about the Universe ? Of course the higher
resolution the better, but what is a thershold to gain new knowledge ?

In article ,
"Mike Dworetsky" writes:
Many astronomers would settle for 0.1 arcsec with 100 times the light
gathering power...the great advances will come from being able to do
spectroscopy, not just from resolving things. Resolution will help in
specific problems, including the search for direct imaging of extrasolar
planets, and examining starbursts in the very early universe.

I'm afraid I have to disagree with Mike in some respects. What that
should tell you is that the answer is by no means clear!

While I agree that spectroscopy is important (and there is no
substitute for collecting area), a factor of two to three improvement
in resolution would be quite useful. In particular, it would allow
much greater sensitivity because of the decrease in the area of
background accompanying each object. For background-limited
observations, the observing time to reach a given point-source
sensitivity goes as the fourth power of the angular resolution, and
better sensitivity would allow us to detect intrinsically fainter
objects in the early Universe. Higher resolution would also give
much more information about the morphologies of very distant
galaxies.

2. As far as I know there is deep interest in using far infrared to
peek into the centre of our Galaxy. What is the current limiting
factor here - resolution (mirror size), or a need to cool the
telescope down ? In case of cooling - is it enough to shield the
telescope in space to bring the temperature its elements close to 0K,
or it has to be launched far off Earth orbit (far from the Sun), or
some active cooling can be used ?

You always want the biggest mirror you can afford, mainly for
light-gathering power rather than resolution, though resolving power is
helpful.

In the far infrared, resolution is critical. For all the objects we
can study now, there are plenty of photons, but existing telescopes
are very resolution-challenged. Also see above about background-
limited observations, which is always the case in the far IR.

All such IR telescopes have to be cooled, if possible by liquid helium. It
is not sufficient to shield such a telescope, active cooling is required.

This depends very much on what you are trying to achieve and what
wavelength you are working at. SOFIA will be uncooled (except by
ambient atmosphere, say -50 deg_C). The JWST optics are expected to
be passively cooled to 30 K or so; the instruments will need active
cooling. I'm not sure what is planned for Herschel, but there has
been at least one proposal for a passively cooled telescope operating
in the sub-millimeter.

The main reason for moving such a telescope away from Earth (say to L2) is
that Earth is a huge source of IR radiation, and this makes the coolant run
out much faster. The Sun is a problem, but not as big a problem as Earth,
because a reflecting shield that works against visible light is relatively
easy to make, but it is hard to build an efficient reflector of far IR
wavelengths.

On the contrary, most metals are more reflective in the IR than in
the visible. It would be no problem to shield the telescope from the
Earth if the Earth were the only relevant source of radiation. The
problem is shielding from _both_ Sun and Earth at the same time.
While this has been done (IRAS, COBE), designing an observatory is a
lot easier if you can choose an orbit far away from Earth and just
worry about the Sun.

In space, you can cool the optics as well as the detector; on Earth, this is
n't possible due to the atmosphere and ice condensation.

Indeed, although stratospheric and Antarctic observatories are
somewhat better than typical mountaintops.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
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