Quote:
Originally Posted by brian nordstrom
Another mis-conception is that a long fl will show a dimmer image to a short fl telescope of the same size ,, wrong  , for example a 4 inch refractor at f15 would be exactly as bright on all objects as a 4 inch f6 , 4 inches of light gathering is 4 inches of light gathering , law's of physics cant be broken .
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In practise that's not quite true.
A typical old F15 refractor probably came with 1" fittings , and might have a 40mm eyepiece if you were lucky . The user - seeking the lowest power and brightest image will reach for the 1" 40mm eyepiece which will give a narrow true field with a lousy 35 degree apparent field and a rather dim 2.8mm exit pupil . The owner of the modern F6 apo rafractor may reach in for a 2" 40 mm eyepeice and get a much brighter nearly 7mm pupil and a tasty 70 degree apparent field .
I used to own a 4" F15 Polarex rafracter and I am certainly not going to waste any time defending its qualities
