[Satchmo]

Wavy,

My point was that you stated that an F3 to F3.5 telescope was totally unsuitable for a heavily light polluted city sky. Why?

An exit pupil of sub 1mm is achievable with a 3.5mm Televue Delos or Ethos with good eye releif. A Televue Paracor gives about 15% increase in magnification , making an F3.5 an F4 focal ratio. If you want to acheive 1/2 mm pupil which you would rarely use with any scope, the Teveue Powermate amplifiers are optically beyond reproach ( they are not a Barlow design. )

Generalities that long f ratio scopes were better for city skies existed when the best high quality short focal length eyepieces were like a 4 mm Orth with 40 degree field and zero eye relief. Fast scopes often had massive secondaries, which are now obviated by low profile focussers and awareness of true needs for visual observing

Modern focussers like the Feathertouch with 10:1 microfocus make it easy to focus a fast F ratio scope.

I have had no problem using 600X with 20" scopes in good seeing .

Central obstructions can be kept below 25% for visual use even at F3.

I'm not entering into discussion about suitability of various aperture sizes to for general viewing vs average seeing conditions- its not relevant here. I'm simply pointing out that , provided you source a good primary , a faster F ratio can be just as flexible as a long one . If you can achieve a full range of exit pupil that you would use with a longer F ratio scope then the F ratio is irrelevant.

I've spent a few nights with a quality 20" F3.6 with Paracor and good eyepieces and found the images indistinguishable from a 20" F5 ( which I have a lot more experience with ) . The only difference was that I spent a lot more time looking at each object because I was not hanging off a ladder. I have a friend in Perth who had a 20" F5 and converted to an F3.6 and reports no loss of detail on the planets in good conditions.