Title text:
I'm trying to buy a gravitational lens for my camera, but I can't tell if the manufacturers are listing comoving focal length or proper focal length.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3182/
Do not drink the liquid mirror!
I repeat. Do. Not. Drink. The liquid mirror!
But I’m so thirsty meow.
But its shiny, and smells interesting.
Instructions unclear, drinking straw stuck in telescope.
Forbidden juice…
You can only do it once