Originally posted by colonel00 Sorry, now you've lost me. How does a 24mm at f3.5 gather more light that a 14mm at f2.8? f3.5 is 2/3rds of a stop slower that f2.8. On top of that, a 24mm lens only allows for roughly a 13s exposure before star trailing versus 20s-25s (23s really) on the 14mm.
No worries... this is the biggest and most important item to learn for astro, well one of them!
Now for the maths:
((24/3.5)/(14/2.8))^2= 1.88x the aperture area, and hence 1.88x the amount of light gathered through the larger aperture area.
(24mm aperture diameter/14mm aperture diameter) squared = a 1.88 times bigger hole to let the light through coming from a smaller and more concentrated field of sky view.
Right, so we are gathering light through a bigger hole for a smaller area of sky, this allows you in Yosemite to gather more light, image fainter stars with a larger magnitude (larger means fainter), thereby you can record more stars/galaxies/nebulae etc. etc.
Yes, on an untracked mount you have less exposure time with longer focal lengths but don't you have astrotracer?
Untracked star trailing happens with all exposure times, it depends on how much you are zooming in (pixel-peeping), your sensor pixel count or how large your prints are. Ignore that 300/500 rule, that is only an acceptance for a small image or print, but like I said before, it depends on what you want, for example, a wall size print or a facebook photo. 😉
With say a 24mm or 50mm to image the full Milky Way, yes you will have to track and mosaic, but what you end up with and with practice/hard work will be superior imagery and a huge mega-pixel file to rival or beat digital medium format.
As to your lens, lenstip tested it and it wasn't good until f2.8/f4 and hence my earlier statements, you can read the analysis yourself...
Samyang 24 mm f/1.4 ED AS UMC review - Image resolution - LensTip.com
Maybe you should swap for a Samyang 14mm f2.8, that is what I've owned for a few years, and is excellent at f2.8 😁
Hopefully all this has helped you out even if you are still scratching your head, you certainly aren't the only one 😉