Here is an article about a new SETI telescope that searches light waves rather than radio waves. What kind of light are they looking for?
Today’s laser technology makes it possible for petawatt (10^15W) lasers to emit highly collimated nanosecond optical pulses that briefly outshine the Sun by a factor of 10000. Because no known astrophysical source could put out a bright nanosecond optical pulse, some SETI searchers have concluded that looking for signals from technologically advanced aliens is more promising with optical telescopes than with radio telescopes.
If we find nanosecond pulses, we can’t lose,” says Horowitz. “If it’s not from an alien civilization, at least we will have discovered an astrophysical phenomenon that no one anticipated. Not a bad consolation prize.
Physics Today, June 2006, http://www.physicstoday.org/vol-59/iss-6/p24.shtml
These SETI researchers are therefore using optical telescopes as an explanatory filter.