I thought it was like a laser gun for moonlight - with the telescope concentrating and amplifying the moonlight, and the diamond focusing it. ... oh well, makes as much sense as most SF.
And it's quite logical for Doctor Who (see 'Horror on Fang Rock' with Tom Baker and Louisa James as Leela).

(Moonlight it just reflected sunlight by the way - same frequencies as sunlight just greatly reduced. So it amplified moonlight would kill it - sunlight would be even better... oh, right werewolves are superstitions)