Active scanning across interstellar distances...

[marsbug]

This is an odd thought, that I suspect I already know the answer to, but it lies well outside my expertise.

The solar gravitational focus is often touted as the ultimate natural telescope (http://www.centauri-dreams.org/?p=785), waiting to be accessed if we can ever get a probe out that far. However, as powerfull as that gravitational lens is, it would be a passive scanning system.

My question is: is there any way it might be used as part of an active scanning system, in the same way that the Arecibo radio telescope can be used as a radar (http://www.naic.edu/~nolan/radar/AUSAC.html) to probe the poles of mercury or passing aseroids?

After all, if it can be seriously proposed to build a laser that can reach across interstellar distances to propel a lightsail (http://www.centauri-dreams.org/?p=3493), why not use a similar scale transmitter of some wavelength to scan another star system?

[a1call]

It is very interesting that Google reports only 141 documents which contain the term "active planetary radar" from all the billions of documents that it spiders (or is that trillions).
I can see the significance of the receiving collector being as large as the immediate area around the
circumference of the sun for increased accuracy, range and signal strength (in active planetary radar). I am not so clear why transmission/sending of the scan beam would significantly benefit from a large lens as the sun. Keeping in mind that a transmission originating at the focal point of Sun's gravitational lens will end up not concentrated but scattered parallel at an area as wide as sun. To concentrate the beam to a small area it would have to be cast at a much further and probably impractical distance than the receiving focal point.

Additionally only a very marginal amount of such cast beam would be of use as a percentage of small area around the circumference related to the total area (cross-sectional) of the sun. So you would end up with a very very small fraction of the cast beam reaching the target.

ETA: So I did some more thinking. One correction is due. Any casting point beyond the focal point will have to converge at some very distant point. but I still figure the useful beam will be astronomically weaker than if you didn't use the sun and instead directed as much as you could with a smaller lens.

[marsbug]

I was thinking of only using the suns gravitational lens as a reciever, and wondering how large/powerfull a transmitter based in our solar system would need to be to send a signal to say, bernards star, that could reach it with enouh power to be reflected and picked up by a probe at the solar focus lens. It din't occur to me to try using the solar focus as part of the trnasmitter, and as you say it probably wouldn't work.