Ground-based Telescope as Sharp as Hubble

Midnight Thunder writes: "The BBC has an article describing how the Paranal Observatory has been able to take images that are just as sharp as the Hubble Space Telescope. For a ground based telescope the images are of amazing quality."

Comments

Corrective lenses?

[Liquor]

It's interesting to note that the Paranal telescope modifies the mirror to correct for an imperfect lens (the atmosphere), while the Hubble has a corrective lens (installed in orbit) in it's system to correct for a manufacturing error in the mirror.

It makes me wonder if the Hubble would have been significantly better than Paranal if the mirror had been made correctly in the first place.

Re:Corrective lenses?

[LMCBoy]

It's not a good analogy, because the correction used by Paranal is an adaptive optics system that continually updates the shape of the mirror 500 times per second to correct for the turbulent atmosphere, which causes images to bounce around in the focal plane (a/k/a the "twinkling" of stars). OTOH, the HST correction is static aberration correction, and therefore much simpler, technologically. HST doesn't need adaptive optics, because there is no atmosphere between it and its targets (that was the whole point of putting it in orbit).

Furthermore, even if the HST mirror had been manufactured perfectly, it would be no better than the post-corrected HST. In other words, the HST fix made it "as good as new".

Theoretically, once you have perfect optics, and have corrected for the atmosphere perfectly (if you're on the ground), the sharpest image you can achieve is limited by quantum mechanics; it's known as the diffraction limit. The size of a diffraction-limited point source is inversely proportional to the diameter of the aperture (i.e., the primary mirror).

Since Paranal is a much larger telescope than HST, (8.2 meteres compared to 0.9), it's ideal, diffraction-limited image is much sharper than HST's. The fact that they can "only" get as good as HST shows you how hard adaptive optics is.

Re:Corrective lenses?

[HorsePunchKid]

I did a lame research paper and classroom presentation on the Hubble Space Telescope way back in the day (read: I'm pretty sure of my info, but I'm not sure where to look for verification currently). Anyway, the result of my little grade-school investigations were that the in-orbit modifications they made improved HST significantly beyond its initial design. Actually, a little bit of looking turned up this: HST Servicing Missions. It's rather dumbed-down, but I think it'll help you draw comparisons.

In any case, there are a few things you should keep in mind. First, HST is quite old now, and past its initial proposed service lifetime, IIRC, so technology has come a long way since it went up. Second, things like this are often technologically lagging even before they go up, since it can take literally decades to plan an instrument of that size. Size is the other critical thing to keep in mind. It's (relatively) easy to build huge arrays like VLT on the ground, but try getting that into orbit :). Even if you break it up into many missions and assemble in-orbit (some early plans for NGST considered that, IIRC), it would still be a monumental task (though not impossible).

So I'd say in summary that chances are HST never could have been better than Paranal, even though it's been an awesome instrument throughout its lifetime.