Just curious. If there that embarrassing error in the optics never happened would it be sharper and better (if done correctly originally) than after the fix to the grinding error was implemented in orbit?
Probably very slightly. Each optical element adds a certain amount of diffractive error, and lenses add some distortion from surface reflections and other defects. All-in-all, it’s probably a measurable amount of additional distortion, but not so much that you’d be able to tell the difference visually.
IIRC, it was a first-surface mirror, not a lens, that was originally misshapen - so no unwanted surface reflections. If the replacement mirror was manufactured to the correct specification then it should in theory perform the same as the original would have had it been done right in the first place.
Yes, this is correct, however, in order to bring the mirror into proper focus, they fitted hubble with a special system of mirrors called COSTAR, which intercepted the reflected light from the misshappen primary and made corrections before passing it along to the instrument packages. Additionally, new instruments installed on Hubble have their own corrective lenses built in, to obviate the need for COSTAR–lenses which otherwise would not have been required. As I said, the original would have been better had it been ground correctly, but the corrections don’t detract enough from the image quality to be generally noticeable. Read about COSTAR here.
If they’re not perfect mirrors, you’re going to lose some light. Any extra lenses will do the same.
I don’t know if this matters on the Hubble though, since I presume the exposure times are long anyway, and this would just make them longer.
This is true. For visible light, about the best reflector we know of is aluminum, which gives first-surface mirrors about a 92% reflectivity, which means you lose about 8% of the incident light for each mirror. Even worse is the diffractive error. Diffraction is an edge effect which results in a scattering of light at the edges of objects, including mirrors. In general, small mirrors and lenses will suffer more from diffraction than large ones, since the ratio of edge length to surface area is smaller.
In order to install the “phone booth sized” COSTAR, they had to remove the High Speed Photometer instrument package, one of the four instrument packages on Hubble. So, optics aside, the telescope is operating at less than its designed functionality.
I thought the thread title read The Fixed Hubbie and that this was a vasectomy thread.
For broad-band visible light, at least. If you’re only interested in one particular wavelength, you can get well over 99% efficiency with a multi-coating mirror.
And currently, every instrument on the Hubble has its own corrective optics, so the COSTAR is no longer necessary, and every instrument bay is back in use.
I had thought that because of other refinements made at the time of the fix that the Hubble was better than when originally flown. I got that notion from a Nova program, IIRC.
Those are some bad-assed pictures, however they were made.
Peace,
mangeorge
There have been several upgrades since the 1993 emergancy repair, some of which have involved instruments being replaced by more advanced ones. The capabilities of the system as a whole are indeed significantly better now than they’d have been in the original design.
Of course, such instrument upgrades were always part of the plan and there’d have been similar improvements had the repair never been needed.