Oilinky, Twitter asked:
With the best laser, how small a spot could you focus from earth to the moon?
We posed this question to Graeme Hirst from the STFC Central Laser Facility...
Graeme - You're trying to fight a thing called diffraction and that depends on how big you can make your laser beam down at the earth. A back of the envelope calculation - laser light spreads out about one wavelength in 10 metres. It’s 400,000 kilometres to the moon. You’ll probably get something about 40 metres across [on the moon]. As a football metaphor, we’d be looking at something about twice the size of the 18 yard box.
I always understood that the ranging laser that utilises the reflectors placed on the moon by the Apollo astronauts has a spot size of about 4 Km but I am sure this is not the ultimate that can be achieved. syhprum, Tue, 15th Jun 2010