(Steve Willner) wrote on Mon, 25 Jun 2018
21:17:06 -0000 (UTC):

In article ,
Fred J. McCall writes:
Oversize fairings are easy.

No doubt the JWST engineers wish you had told them that.


Well, someone should have if mirror size alone was the reason for
making it folding.

[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?


What do you think they hide them behind? I think it's complete
because lots of people put lots of effort into it. And it takes a
telescope and some time to measure their size. This may be rocket
science, but it's EASY rocket science.

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.)


You can only 'mitigate' so far. Are you one of those people who
believes the movies that you can make out faces from orbit and that
you can 'improve' an image with processing beyond the information that
it contains?

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.)


Sources aren't hard to find. Here's one.

https://spaceflightnow.com/2015/05/0...ching-in-2018/

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.


'Temporal resolution' is difficult, given that both the satellite and
the Earth's surface are moving relatively rapidly with regard to each
other. Increased sensitivity means increased image noise. Distance
matters. That's why as newer satellites launch the older ones are
moved to lower orbits.


--
"The reasonable man adapts himself to the world; the unreasonable
man persists in trying to adapt the world to himself. Therefore,
all progress depends on the unreasonable man."
--George Bernard Shaw