I've started this thread for anyone who wants to continue the debate started re Newtonians in the Sydney Observatory needs your help thread.
In response to Wavytone :
Paracors and modern eyepieces with short focal length and long eye relief solve all these objections and you know that exit pupil is a function of magnification and has nothing to do with F ratio. It is possible with modern eyepieces to obtain the full range of useful exit pupil from 7 to 0.5mm with modern correctors and eyepieces. Computer optimised central obstructions do not exceed 25% for visual use on an F3 compared to 35% for a SCT.
I've observed with a 20" F3.5 over full range of exit pupils and couldn't fault the images. Stars looking almost perfect to the edge of a 100 degree apparent field.
Quote:
Originally Posted by Wavytone
Actually, no. The mirror had an excellent figure. The problems were:
a) huge secondary mirror - so big that in many eyepieces you see a big black blob swimming in the FoV;
b) range of useful magnification was very limited, largely due to the limitations with suitable eyepieces - many eyepieces won't work well with mirrors this fast, even now.
c) coma and field curvature (at the time I had this, correctors didn't exist) which were a big problem with most eyepieces except the Pretoria.
d) the fact that the fast f/ratio raises the background sky brightness, which basically defeats the point of a larger scope for looking at extended objects (nebulae, galaxies) in average skies. In really good sites with the darkest of sky, this might not be a problem but even in the Blue Mountains west of Sydney it was evident that f/5 - f/7 would have been a much better choice.
In the heart of Sydney the sky glow is much worse - and at Observatory Hill the naked-eye visual limiting magnitude is about 2.5. An f/3 mirror is about the worst possible choice.
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