Henry Spencer(!) on Hubble vs. Ground-based Optics (Score 1)

Henry Spencer (yes, that Henry Spencer, famed Usenet figure, recognized space historian and enthusiast, founding member of the Canadian Space Society, Fellow of the British Interplanetary Society, etc.) posted the following commentary to sci.space.policy in July. I hope he will not mind my quoting his informative notes.

> Aren't modern ground-based telescopes with adaptive optics superior to Hubble,
> especially when they are linked up?

[Text by Henry Spencer follows]

No. In principle, they [ground-based telescopes] can do *some things* better... although those are mostly still "Real Soon Now!" promises rather than demonstrated facts.

However, in other areas Hubble retains an inherent advantage that is not going to go away.

Hubble can take much longer exposures, given targets in the right parts of the sky, because its sky isn't full of scattered sunlight half the time.

And it works farther into the infrared, and much farther into the ultraviolet, than any ground-based scope can.

And it has a much darker sky background, which matters when working on very faint objects.

And it can point closer to the Sun, although its cautious operating policies limit that.

And it can observe rapid time variations without a lot of superimposed atmospheric noise.

And -- minor but not entirely insignificant -- it has a clear view of the entire sky, something that is quite difficult to achieve from any single point on Earth.

And, finally, although its high resolution has been exceeded by adaptive optics and interferometry on the ground, its high resolution comes with many fewer ifs, ands, and buts.

Adaptive optics requires either nearby bright guide stars, or still-experimental laser guide stars. Imaging interferometry can observe only bright sources, because you need a fair number of photons per millisecond to detect interference fringes.

[End of text by Henry Spencer]

I'm no authority on this (Spencer is), but I've heard some similar points second-hand from a Hubble scientist @ JHU.

One more thing: The parent comment talks about Hubble 'in its day'. If you think its heyday has ended, I suggest that you read some of the HST Daily Reports. (In the January 5th report for example, you'll find that Hubble continues to study objects which "do not have bright enough stars nearby that can be used for natural guide adaptive optics in ground-based telescopes", which was one of the points Mr. Spencer mentioned above.)

As shown in the daily reports, Hubble is constantly active, doing valuable and interesting experiments that go far beyond what you read in the mainstream news. The HST schedule will remain packed right through the day of its untimely demise (even without the invaluable new instruments which won't be installed). Imagine the painful 'prioritizing' that must be commencing as we speak; many possible experiments will be tossed, to make room for those chosen as the most important.

-- omr