Focus for meteors

[JohnD]

Today's EPOD (2/1/2020)shows a group of Geminid meteors https://epod.usra.edu/blog/2020/02/enco ... tugal.html

They seem to radiate from more than one focus. Is this an artefact of a prolonged exposure? Or what?
John

[Chris Peterson]

Today's EPOD (2/1/2020)shows a group of Geminid meteors https://epod.usra.edu/blog/2020/02/enco ... tugal.html

They seem to radiate from more than one focus. Is this an artefact of a prolonged exposure? Or what?
John

You always have to be careful when projecting lines on any wide angle shot, due to the sort of distortions caused by projecting the sky onto a plane. However, in this case, consider that the image is a composite collected over four hours. In that time, the Geminid radiant moved from an altitude of 80° in the east (close to the zenith) to an altitude of 48° in the west- crossing nearly 50° of sky. So it's hardly surprising that the captured meteors appear unrelated.

Images which show a strong radiant are either taken during very active showers over a short period, or taken with a camera that is tracking the sky, and then composited onto a single background image.

[JohnD]

Thank you, Chris! I can see that, except that if the sky "moved" during the exposure, then the stars would appear as trails. Meteors are bright, so stars won't register in a tracking shot?
John

[Chris Peterson]

Thank you, Chris! I can see that, except that if the sky "moved" during the exposure, then the stars would appear as trails. Meteors are bright, so stars won't register in a tracking shot?
John

I don't know the finer details of how the image was processed. The meteors may have been masked out of the individual frames (which might typically be on the order of 30-second exposures). The stars we see here may have been from just one frame, or a longer frame. Obviously, when we're dealing with any image made over hours, where some things are moving and some things are not, where there is a range of brightness of many orders of magnitude, creative choices have to be made.