Expert: Tom Whiting - 10/24/2005

Question
Tom,
 Hello, you've helped me many times and I've always been highly pleased.  Thanks for the good work.  
 My current question is= I happened upon the Keck website and the adaptive optics section.  It occured to me, which is a better visual telescope?  The HST or Keck with A/O?  I clicked on a picture of Titan by Keck and then looked at one by HST, well the Keck looks much, much better.  If this is the case, - or am I missing something- is not the future of astronomy (at least optical) the way of powerful ground scopes?  Why is NASA pursuing the TPF?  Why not the same thing on the ground with A/O?  

http://www2.keck.hawaii.edu/news/science/huygens/index.html
http://www.lpl.arizona.edu/~lemmon/titan/titan.html

Tim C  

Answer
Hi Tim,
and thanks for the nice compliments.

Well, I don't think we can compare a device (TPF) that will
use interferometry (with a very long baseline in space) to find extra-terrestrial planets, to Earth-based optical scopes.

Perhaps a better question (is) would have been....Why work
on a lowly "6.5 meter" James Webb Space Telescope
(Hubble's eventual replacement)  when we now have a 10 meter Keck with adaptive optics, right?  So why is NASA pursuing the JWST?

Good question....but I think the answer lies in your title
above....adaptive vs space.  Why not look at it this way...
adaptive....coordinates with...space.

And the only things I can think of are (1) a ground based
scope cannot cover the entire sky; even from Hawaii at
about 20 N Latitude, there is a chuck of southern sky
that is never visible to those scopes....from about -30 S.
Declination to the South Pole.   A spaceborne
scope can see ALL the sky, over time.
Also, (2) a spaceborne scope can take a very long time exposure at very high resolution.....note the Hubble Deep Sky photo's which are a 5 day (120 hour) time exposure.  I'm not sure a ground based scope could ever do that.  Deep space is
the primary interest for any space-borne scope...Titan was
just an extra-plus factor at the time due to either the Huygens
probe or the current Cassini mission.  (Pictures of solar system bodies were really not Hubble's primary mission).

So isn't it a bit unfair to use an image of Titan in your
comparison?.....Has the Keck scope completed a 120 hour time exposure, getting down below 30th magnitude galaxies
near the edge of the visible Universe,  like the Hubble
has done?  Maybe it has, but I doubt it.  In any event,
a spaceborne scope would be more adaptable (no pun intended) to that kind of imaging-one of it's primary
jobs, in fact.
Also, adaptive optics don't work well if the weather is
cloudy, or daytime ;-)  Space scopes don't have the worry of any of these.  And BTW, the JWST way out in space
(not just in orbit around the Earth) won't have to worry
about the ~10 day loss every month for a near-full moon ruining the view, either.

So, in conclusion, perhaps we should think of all these
scopes working in agreement/conjunction with each other,
vs. "Versus" each other.

Money wise, if we Americans are stupid and insane enough to spend $200 billion to rebuild a hurricane high-risk city that's 12 feet BELOW sea level, completely surrounded by water, then we can certainly send up a lowly $10 billion new space telescope!
Hope all this helps,
Clear Skies,
Tom W.
Erie, PA