[amsat-bb] Re: On the possibility of imaging AO-40
antonio at qualcomm.com
Fri Oct 16 22:20:24 PDT 2009
At 04:56 AM 10/16/2009, Alan P. Biddle wrote:
>Getting an image is clearly possible, but only if you can get a certain
>agency known by its initials to do it.
I offer a different opinion. We know how large the primary mirror
(optical aperture) of these spy satellites are, because we know how
big the launch vehicles are, and the satellites had to fit into the
launch vehicles. Therefore, I believe they are no larger than
Hubble. Therefore, I figure the spook agencies can't actually help us.
Someone earlier offered a calculation of the resolution Hubble could
achieve when imaging AO40, based on the aperture, and the minimum
possible distance, given the orbits of Hubble and AO40. The answer
was 5", which just isn't really good enough to see what may have happened.
Because the distances are large, and because we have no hope of any
telescope that's gonna fly up next to AO40 to take a close look
(unless we build & launch it ourselves!), all existing
single-aperture telescopes seem to be unable to do the job.
Seems to me that the only technique which could have a chance is
interferometry. At radio frequencies we're familiar with phased
array antennas where signals from multiple antennas are combined
coherently. Doing this at optical frequencies always seemed
impossibly difficult to me, so I figured I would never see it.
In recent years however those darn astronomers have started doing
optical interferometry. They've imaged the surface of stars,
measured the sizes of extrasolar planets, etc. Some of their images
appear to indicate sub-milli-arcsecond resolution.
Two of these instruments are CHARA and COAST.
See the two detailed documents on CHARA by clicking on the links
labelled "here and here" about half way down the page.
I wonder if this sort of instrument could image a satellite?
There are of course a large number of practical engineering
considerations. These machines require bright objects. A satellite
in sunlight is pretty bright. Enough? Satellites require rapid
tracking. I doubt they designed these machines to do that. The
satellite is likely tumbling, so maybe you can't correlate or average
data taken over a few minutes or even seconds apart. I don't know
what exposure times would be required. Etc etc. I don't know the
answer to any of these questions, or even how to go about figuring
out the answers.
Having said all this, I suppose it is possible that the spooks have
done the engineering to make multiple optical satellites work
together as an optical interferometer, and therefore have the high
resolution capability that we need, but I am guessing that they have
not (yet). (Just look at all the work the ground-based astronomers
have done to make this work with a huge machine nailed down to the
ground. Just doesn't look ready for space yet!) And even if the
spooks had done this work, they wouldn't give it away (yet) by
showing us pictures.
So, back to the non-spook world, could a machine like CHARA or COAST
image a satellite?
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