From: inr@zardoz.caltech.edu (I. Neill Reid)
Newsgroups: sci.astro
Subject: re: Hubble's mirror
Date: 11 Nov 1992 20:42 PST
Organization: California Institute of Technology
Distribution: world
NNTP-Posting-Host: zardoz.caltech.edu
News-Software: VAX/VMS VNEWS 1.41    

From: hathaway@stsci.edu

>In article <10NOV199220455982@eccles.caltech.edu>, inr@eccles.caltech.edu (I. Neill Reid) writes:
>> From: dj@ekcolor.ssd.kodak.com (Dave Jones)
>> 
>>>Brian Stuart Thorn (BrianT@cup.portal.com) wrote:
..
>> 
>>>Hubble is doing valuable work right now.  Removing it from orbit, probably
>>>for months, would damage astronomy far more than a mirror whose defects are
>>>known and correctable by image-processing techniques.
>>            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>> --
>>>||Dave Jones (dj@ekcolor.ssd.kodak.com)|Eastman Kodak Co. Rochester, NY  |
>> 
>>  Agreed to all except the last sentence. Let's be under no illusions - 
>> the Hubble telescope is SERIOUSLY harmed for imaging programmes. It is 
.. 
                          ^^^^^^^^^ ^^^^^^
>   Compared to what?   You got anything on earth that can touch it??? 

 Actually, yes - we can get a resolution of 0".07 (that's about twice
the HST) using speckle interferometry on the 200-inch. Image-sharpening
techniques have been used on many other 4-5 metre telescopes to 
recover images near the diffraction limit of those telescopes (which,
at twice the diameter of HST, give twice the resolution). The catch,
of course, is that most of the current techniques can only recover
the diffraction limit with lots of signal (we can't go much fainter
than K = 9 (2.2 microns) on the 200-inch with speckle). But there are
many experiments going on at present with more sophisticated 
techniques using laser guide stars, `rubber mirrors', tip-tilt
systems (Laird Thompson was one of the first with such a system and
has achieved image-sizes of 0.4 arcsecs (I think) from Mauna Kea).
These systems can go much (10+ magnitudes) fainter than speckle.
There will probably always be atmospheric limitations as to how large 
a solid angle these systems can cover, but against that there is
the advantage of more time on a larger telescope. 

 A _working_ HST with diffraction limited images would certainly beat
the current ground-based activity in depth. But the main problem with 
Hubble is that it spreads about 85 % of the light of a given point source 
over an enormous area - take a relatively crowded field (say, the core of 
a globular cluster; a region in the Magellanic clouds or M31; or a field 
towards the Galactic Bulge) and these overlapping halos around the individual 
stars combine to wash out the fainter objects. That's where you lose
the sensitivity, and that's where you lose at least 2 magnitudes in
depth, and that's what hampers many of the projects which were the
main justifications for ST - and that's what makes it such a
disappointment.

Yes - you can go some way towards reconstructing the distribution of the
brighter images, where the central core of the image is identifiable 
(you can even get spectacular pictures of something really bright, like
Jupiter, as Bill Gawne pointed out) - but the fainter stars in a
complicated image are lost. That's why you lose so much in limiting magnitude,
and that's also why the magnitudes that are derived from HST pictures are
much wobblier than you'd like. One of the key projects was monitoring 
Cepheid variables in various galaxies to get a better estimate of the
extragalactic distance scale. While the project is still feasible, it
requires more integration time, which means fewer (and more sparsely
populated) fields, which means fewer Cepheids, which means greater
uncertainties in any distances derived. Similarly, resolving the stellar
populations of Virgo galaxies is that much more difficult (shading
towards impossible).

Obviously, there are still areas where ST is ahead of ground-based
telescopes - mainly involving UV work or imaging sparse fields - but 
the incorrectly-figured mirror means that the imaging performance has 
been reduced to a state where (larger) ground-based telescopes are 
getting close.

 Neill Reid - inr@eccles.caltech.edu

>Wm. Hathaway 

Newsgroups: sci.astro