[amsat-bb] Re: On the possibility of imaging AO-40

[Franklin Antonio]

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.

CHARA...
http://www.chara.gsu.edu/CHARA/
See the two detailed documents on CHARA by clicking on the links 
labelled "here and here" about half way down the page.

COAST...
http://www.mrao.cam.ac.uk/telescopes/coast/handout.html

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?