| Subject: Re: Hubble is ancient history |
| From: Keith Wood |
| Date: 19/10/2004, 22:52 |
| Newsgroups: alt.sci.seti,sci.astro.seti,sci.physics |
David Woolley wrote:
In article <2tkff7F20kr42U3@uni-berlin.de>, Knoppix User
<knoppix2004@hotmail.com> wrote:
I have not read anything about new Hubble but I wonder. Is it wise to
send another telescope on space? I mean look at the price of Oil? How
about adding one to the ISS?
I believe the reference was to the use of adaptive optics to compensate
for atmospheric scintillation (star twinkle) and achieve better than
Hubble results with earth based telescopes.
Pure fantasy. Only telescopes in space can produce wide-field
diffraction-limited images every time. The resolution of HST has been
surpassed from the ground at optical wavelengths for some time via
Adaptive Optics, but AO works only over a very small field, still requires
good atmospheric conditions and requires a suitable guide star very nearby
(or an artificial one, which few observatories have). AO doesn't work
through clouds, and can only operate at night. Advances like
multi-conjugate AO and tomography will improve the FOV, but will still be
unable to image the Eagle Nebula in one exposure.
In theory, the enormous collecting power of today's observatories should
allow them to penetrate to magnitude +30 and beyond, and do so faster than
Hubble. However, light pollution notwithstanding, molecules in the
Earth's atmosphere produce a natural skyglow. This reduces the
signal-to-noise ratio of ground-based deep field images. But their
ability to capture faint spectra of high redshift galaxies makes them
complementary to HST (removing skyglow from spectroscopic data is easier
than with images).
Although tremendous progress has been made in ground-based IR
observations, they can only observe a small number of Infrared bands that
are not completely absorbed by Earth's atmosphere, and also have to
contend with the Earth's intense IR skyglow. HST's NICMOS can achieve a
H-band magnitude of +26, about 2 magnitudes deeper than is realistically
possible with Keck or Subaru.
Much of the advances in our understanding of the famous extrasolar planet
HD209458b has come from spectra taken with HST at Ultraviolet wavelengths
(Lyman-alpha). Earth's Ozone layer prevents observations shorter than 310
nm. Ground-based telescopes have tried for the easier Sodium absorption
lines seen during the exoplanet's transit, but even mighty Keck failed to
extract the Sodium lines from Earth's own Sodium spectrum with enough S/N
to provide a reliable measurement. For HST, the Sodium was a cinch, and
went on to detect Hydrogen, Carbon and Oxygen.
Oh, and don't forget about eta Carinae: Since the primary star's dense
wind extinguishes much of its own far-UV luminosity (Hillier et al. 2001),
and because the companion star is probably an O-type star, our only hope
of directly detecting the companion is in the UV with HST.
The cost effectiveness of Hubble's impact is clear: HST's annual
operations cost in 2002 was 2% of NASA's total budget, yet it accounted
for 33% of all NASA discoveries.
The National Academies: "The Hubble Space Telescope is arguably the most
important telescope in history."