h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

http://www.bbc.co.uk/h2g2/guide/A601129

I noticed there was a picture of the Hubble Telescope, but no entry to go with it. So I wrote this.

I've heard that Hubble was 'developed' from one of the CIA/NSA 'keyhole' spy... err, optical reconnaissance satellites rather than having been something really new. Only after it was deployed in orbit, NASA realised that the 'keyhole' optics were designed with earth's atmosphere in mind, which wouldn't be in the way in the Hubble case because Hubble is looking into outer space rather than down to earth. Hence arose the need for the correction lenses (or call them eye-glasses) which were applied later. This could be an urban myth, but who knows?

This is an interesting article, but it appears to completely gloss over the history of the Hubble telescope, in which a design fault led to the telescope being launched with a flawed main mirror, which is impossible to replace without grounding the telescope. The telescope was fitted with corrective lenses to overcome the problem, but the resolution, while still the best in the world, is nothing like as good as was expected. This must be one of the most expensive mistakes in history and deserves at least a passing mention.

I posted my note before I had read Bossel's one. The problem with Hubble was as follows. In grinding the main mirror to be exactly right, a small optical device was used to exam a test pattern reflected off the main mirror. The optical device itself was flawed. One of the components in it had been created the wrong size due to a transcription error in the design (for example, one component was made with a radius of 3.92 instead of 3.62 mm or something like that). The mirror was one of the most precisely made pieces of optics ever made, but it was made the wrong shape. The error was only discovered after the telescope was launched. They knew exactly the shape the mirror was, despite it being the wrong shape, because it had been done to match the optical checking device, which they still had in the lab. Thus, they were able to make correcting lenses. But unfortunately, the lenses reduce the amount of light, because the light must travel through an extra layer of glass.

Thanks Gnomon, congratulations on reaching 1%.
I'll stick in a bit more on the fault and correction. If you talk to astronomers today, they get excited about the results from HDF and plans for the NGST. They've all but forgotten about the early problems. But you're right, it deserves a mention.

Also, you might want to point out that with laser-adaptive optics at an observatory with multiple-alligned optical telescopes, the seeing is just as good (if not better) than the HST. You go into adaptive optics a bit, but do see if you can glean any more information about the new earth-bound observatories that are opening.

However the HST is still among the top of the class. Very nice entry!



- X

I'm not sure if that's entirely true, I worked on adaptive optics for a while and the HST was always much better than anything we could achieve. At long infrared wavelengths the best AO systems may just compete with Hubble, but for visible or UV light they have no chance. It may be computer simulations of a multiple alligned AO system show it could rival Hubble, but simulating it is one thing, building a working system is another. Such systems are still just experimental projects.
The NGST will need to use adaptive optics technology to keep its segmented mirror steady, so it may be in the future ground observatories will reach the limits of space telescopes. But that will probably not happen for decades.
I'll check out the lastest news in case there's been any major developments lately.

Glad you like the entry
MP

A small typo: "they done waste time" should be "they don't waste time".

Editorial Note: This conversation has been moved from 'Peer Review' to 'The Hubble Space Telescope'.

This thread has been moved out of the Peer Review Forum because your entry has now been recommended for the Edited Guide.

You can find out what will happen to your entry here: http://www.h2g2.com/SubEditors-Process

Congratulations!

Great entry.

I do like your entries, wspecially this one and the Adaptive Optics one


Dancer

cheers Dancer

astronomy entries always seem to be popular

MP

Congrats!