In article ,
Richard Alexander wrote:
...In addition, I don't think you could
focus a beam at interplanetary distances (without a REALLY big lens).

My rough estimate shows me that a 10 meter telescope could focus a
beam from Earth to Mars down to less-than a one-meter spot at 400 nm
wavelength...

I fear you've dropped a decimal point somewhere. Spot size is very
approximately distance*wavelength/diameter. Mars's distance varies
depending on where it is in its orbit, but let's take a couple of
hundred million kilometers as typical. 200e9 * 400e-9 / 10 = 8000.
A spot size of 8km is just not practical.

A factor of 100 improvement would bring it down within reach of reason,
but a 1km mirror is beyond what's reasonably practical in the near future.
Eventually, yes.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |