|
OP, in astronomy the usual measure has been f-stops - focal length/aperture - because there are other variables that may be variable or in-measured:
- transmission losses due to poor antireflective coatings - or none at all, which may be the case on old refractors as well as amateur-made stuff;
- light loss due to the secondary obstruction (newtonians, cassegrains, SCTs and Maks)
- light loss due to reflective surfaces - even new the average mirror coating loses 10% and an old one could lose 40% or more before it is recoated;
- all the additional surfaces before the light reaches your eye - such as star diagonals and eyepieces.
This is one of the reasons why refractors in the range 80-130mm remain popular despite their smallish aperture, you have to step up to a 200-250mm reflector to do significantly better.
And yes there really are big newtonians as fast as f/3.5. They aren't without issues however - field curvature and coma are severe. But for those with aperture fever and don't want high magnification, this is an acceptable compromise.
|