Inspired by some formulas given in a video about Dragonfly
(
http://www.iceinspace.com.au/forum/s...d.php?t=135928) I am trying to work out the effective aperture and focal length of my 2 telescope system. In Dragonfly the lenses all are the same diameter so the formulas given have the diameter as a constant:
Aperture = square root of Number of lenses x Diameter
Focal ratio: f = f/ square root of Number of lenses
So, for my 2 f7 refractors:
f = 7/1.4142 = 4.95
However, my two lenses have different diameters so the formula doesn't hold. I've worked it out by taking an average but I'm quite unsure if this is sound reasoning.
Aperture = 1.4142 x 140mm = 197mm (TEC140)
1.4142 x 180mm = 254 mm (TEC180)
Average: 451/2 = 225 mm (at f 4.95)
Thanks for any input about this!
Peter