From: "Phil Payne" 

> If the shade is any distance at all from the telescope it wouldn't need to
> be much larger than the lense on the scope.

It's a shitload cleverer than that.  A distant enough object produces light
that can be treated in some ways as coherent.  The suggested bafflle lets
the star's light through, but creates interference patterns.  The
plant-seeking sensors are placed in an interference "trough".

If my slipstick is correct, you could even tune for star diameter.  That
would back up a whole load of other measurements.

I haven't messed with interference patterns since I was the first person in
my school's history to show the split in the sodium line.

--- BBBS/NT v4.01 Flag-5