On Feb 26, 10:09=A0am, John Schilling <schil...@[EMAIL PROTECTED]
> wrote:
> On Sat, 23 Feb 2008 08:41:34 -0800 (PST), IsaacKuo <mech...@[EMAIL PROTECTED]
>
> wrote:
> >I recently wondered the same thing, because I'm trying to analyze
> >the performance of an occultation sensor network. =A0The basic
> >idea behind an occultation sensor network is that each sensor
> >drone simply stares at all known pinprick light source stars and
> >looks for any of them briefly winking out. =A0This corresponds to
> >a detection of a target.
> Do be careful to consider diffraction effects here - they are every
> bit as important for light flowing around a small obstacle, as for
> light flowing through a small hole or slit.
Ah, thanks! I hadn't really thought of that. I was thinking in terms
of pretty large targets, partly on the theory that small targets
would be much less likely to cause a "ping" anyway. Besides
the smaller chance of crossing a detection line, it'll cross it for
a shorter amount of time.
> The math to do this right, esp. for complex shapes, gets rather hairy,
> but I think that if both the source-target and target-sensor ranges
> are large compared to d^2/y, d =3D target diameter and y =3D wavelength
> of light being observed, the poisson spot will fill the umbra and
> you won't get an occultation - even if the target completely obscures
> the light source in purely geometric terms.
That sounds right.
So, for example, let's assume the use of a relatively long IR
wavelength of 1 micron. A target diameter of 1m would imply
a limit on the order of some thousands of km. A target
diameter of 10m would imply a limit on the order of some
hundreds of thousands of km.
Going to a smaller wavelength like 100nm boosts the
ranges an order of magnitude. Going even smaller would
boost the range even more, but my gut feeling is that you'll
reach diminishing returns beyond 100nm as there are
fewer photons to detect.
This seems to limit this sort of occultation sensor system
to targets on the order of 1m or larger in diameter.
Isaac Kuo
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