> Especially the strong implication that Hubble is made out of lower tolerance parts, since they are rejects, is a claim that need significantly more evidence than what you provided here.
Well, it did ship with a faulty mirror after all :)
Just joking, I know this was a manufacturing error specific to Hubble which kinda proves it wasn't a reused part. I think the mirror would have a slightly different focal length anyway? But maybe 400k is far enough to be "infinity" even at that scale.
Spy satellites don't have a fixed distance to their target; they have elliptical orbits and take take pictures of earth at different angles, different distances to the target. So both spy satellites and telescopes use mirrors focused to infinity.
> I know this was a manufacturing error specific to Hubble which kinda proves it wasn't a reused part.
It was ground very precisely into the wrong shape during the final stage of manufacturing. But I don't think that proves anything one way or the other. Even if the mirrors were finished in unique ways, they may have started identical earlier in manufacturing.
> It was ground very precisely into the wrong shape
It was said at the time that the aberration made the Hubble near-sighted and, indeed, was corrected with lenses. I had suspected, apparently incorrectly according to yours and others comments indicating that spy satellite mirrors are interchangeable with and used for astronomy without alteration, that the mistake was one of habit, because I figured they were ordinarily making near-sighted mirrors to specification to focus up to a few hundred miles rather than stellar or galactic distances. I am honestly am still having trouble accepting that a mirror designed for a telescope to look no further than a few hundred miles is identically focused to infinity precisely like similar telescopes that are designed to peer with a lower bound of at least millions of miles. But if you say so.
> I am honestly am still having trouble accepting that a mirror designed for a telescope to look no further than a few hundred miles is identically focused to infinity precisely like similar telescopes that are designed to peer with a lower bound of at least millions of miles.
Consider the 2.4m lens to be the base of an isosceles triangle with a height of 100km, the edge of space and much lower than the satellite's orbit. This triangle has two angles of 89.9993 degrees. If the lens was 10cm thick and you wanted to taper the edges to match this angle, you'd need to bring in the near edge one micron, or about 1/50 of a human hair.
100km or 100,000 lightyears, they're still effectively straight away (focused to infinity).
> So both spy satellites and telescopes use mirrors focused to infinity.
That's just not what focal length means in the context of mirrors, the focussing happens by moving the secondary not by re-grinding the entire primary mirror or whatever it is you are imagining.
Well, it did ship with a faulty mirror after all :)
Just joking, I know this was a manufacturing error specific to Hubble which kinda proves it wasn't a reused part. I think the mirror would have a slightly different focal length anyway? But maybe 400k is far enough to be "infinity" even at that scale.