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/
Anyone want to tell me how the telescopes where the mirror is in the middle of the aperture sometimes still show the image without a big dot/wires holding the mirror in what you see? It's smack in the middle you'd think it would block the view.
The wire will cause the entire image to become a little bit darker.
in a telescope light travels in many paths from start to finish. so a single wire will have a very soft shadow, which stretches over the entire image. This works because the wire is well within the focal length. If the wire was exactly at the focal length, it's shadow would be sharp, but the farther away it is from the focal length, the softer the shadow will become.
edit: when the object is exactly at the center of the image, then I think it will still cast a sharp shadow, because all the light-paths that go through the center, stay close to the center. Not sure though
The wires also bend the light which makes the stars not look like points but gives them prongs.
for similar reasons cracked camera lenses take perfectly normal pictures
definitely a bit counter intuitive at first