XPost: sci.astro, sci.physics, sci.space.policy
From: willner@cfa.harvard.edu
In article <2frgidh5cfamkc9f50i79cts222ho7t09j@4ax.com>,
Fred J. McCall writes:
> Oversize fairings are easy.
No doubt the JWST engineers wish you had told them that.
>>[mirror size of reconnaissance satellites]
> You can see the bloody things from Earth, after all.
See some of them, sure. Why do you think the census is complete?
What observations (equipment) are needed to measure their sizes?
> Past a certain point a bigger mirror doesn't help
> you for Earth observation. Atmosphere speckle becomes the driving
> parameter and a bigger mirror doesn't help that.
Are you assuming LEO and visible wavelengths? I don't see why either
one necessarily represents all reconnaissance satellites. And even
with those assumptions, what about temporal resolution? Taking short
exposures most certainly helps mitigate seeing effects.
("Atmospheric speckle" is only one of those effects.)
> No, they wouldn't. The next generation of recce satellites will use a
> mirror right around 2.4 meters; the same size used since KH-11.
Source? All of them or only some? Other than better sensors and
onboard processing, how do newer telescopes differ from the older
generation? (As you no doubt know but some readers may not, two of
those were declared surplus and delivered to NASA.)
> Distance has a lot to do with everything when
> it comes to telescopes.
What did you have in mind? I'd have said the key parameters are
angular resolution, temporal resolution, and sensitivity. Distance
affects requirements on those parameters, but I don't see that
distance _per se_ matters.
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Steve Willner Phone 617-495-7123 swillner@cfa.harvard.edu
Cambridge, MA 02138 USA
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