I'm not sure he's precisely correct, but Hubble's primary mirror was changed from 3 meters to 2.4 meters, which happened to be the same size as the primary mirrors for the KH-11 KENNEN series of spy satellites. The book Power To Explore - History of Marshall Space Flight Center 1960-1990 says of the decision to change the mirror:
> "In addition, changing to a 2.4-meter mirror would lessen fabrication costs by using manufacturing technologies developed for military spy satellites."
There's quite a difference between "used infrastructure built by the military for their needs" and "manufactured parts from intelligence programs that were the rejects from those production lines".
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.
Pointing a telescope at something bright and nearby is easier than something very distant and dim, so the precision requirements are obviously in the opposite direction.
> Pointing a telescope at something bright and nearby is easier than something very distant and dim, so the precision requirements are obviously in the opposite direction.
You're wrong. Between 1979 and 1998 the MMT Observatory used six repurposed spy satellite mirrors that had been donated by the NRO, manufactured for the cancelled KH-10. Furthermore in 2012 the NRO donated two unflown spy satellites to NASA, the mirrors from which may be used for the upcoming Roman Space Telescope. Spy satellite mirrors make great telescope mirrors.
> "manufactured parts from intelligence programs [...]"
As I point out above, the NRO has donated at least eight primary mirrors that were specifically manufactured for cancelled or unused spy satellites. The Hubble mirror may be another example of such, or it may have merely been fabricated by the same people.
> that were the rejects from those production lines".
That's the only dodgy part of his claim, but it's not that strange of a claim when you recall that Hubble's primary mirror infamously had a spherical aberration. I presume NASA didn't know about the aberration before they launched HST, otherwise they should have corrected for it on the ground instead of having astronauts correct it in space. But it doesn't seem completely outlandish that the NRO gave NASA mirrors they had previously deemed unsuitable for their own use.
That people accept mirrors gifted for free is not strong evidence of their superior quality, when you get something for free you can make do with something that's not really what you want.
As an example, the MMT observatory was so happy with the "cobble six telescopes together" design foisted upon them by the donation of the mirrors that within 8 years of completion they were officially[1] reporting they hoped to replace them with a single mirror by 1993. Honestly I think the MMT is the only telescope I've heard of where the main mirror was so bad they opted to replace it with a completely different design within 20 years.
Anyway, the Hubble mirror has a different focal length than the KH-11 so can't have been used as is, it would have to be reground and polished for the new curvature and so it would have been essentially just a mirror blank.
> 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.
> "In addition, changing to a 2.4-meter mirror would lessen fabrication costs by using manufacturing technologies developed for military spy satellites."
https://web.archive.org/web/20110615091530/http://history.ms...