Ok, I need a sanity check...
I'm convinced I understand focal length & focal ratio correctly but often people just don't believe me, largely due to the mentality that "bigger scope gives brighter image". I have had this debate with many amateur astronomers over the years. So, here goes and please tell me if I'm wrong...
I am talking photographic only - not visual.
Take a 8" SCT:
- focal length: 2000mm
- focal ratio: f/10
Take a 12" SCT:
- focal length: 3048mm
- focal ratio: f/10
If the same star were photographed in both, with all other things being equal (same camera, same viewing conditions, etc) then the same star would appear the same brightness (pixel brightness values would be the same) between the two scopes for the same star photographed. The magnification/resolution/field-of-view would be different (the 12" would show a smaller FOV due to the longer focal length) but the same star would in fact have the same pixel brightness. This is because they are both F/10.
Then, let's add a reducer to the 8":
Take a 8" SCT @ F/4:
- focal length: 800mm
- focal ratio: f/4
Take a 12" SCT:
- focal length: 3048mm
- focal ratio: f/10
Now in the above case, the same star photographed in both will have a higher pixel brightness in the 8" because the focal ratio is F/4 vs the F/10 of the 12". So, even though the 12" is a "bigger scope" if the resolution of 800mm is sufficient to identify the given star the 8" at F/4 will actually provide a brighter image of the given star.
If I'm wrong, then my whole understanding of my camera lenses and telescopes is potentially wrong and I should give up photography right now
But... just checking?!???
Thanks,
Roger.