[Wavytone]

Mark, I'm not repeating this from Patrick Moore (gawd)...

I suggest this because:

1. an f/7 scope permits a range of magnification from highest to lowest of 10:1 utilising entirely reasonable eyepieces.

With longer focal ratio scopes, notably from f/10 upwards, there is not much point in eyepieces under 8mm, whereas achieving a maximum 6mm exit pupil requires huge eyepieces.

Conversely, scopes f/5 or less can't really utilise long focal length eyepieces because the eye pupil would be too great - as you may recall I had an extreme example of a f/3.7 Newtonian in which anything longer than 15mm was pointless.

2. On the occasions you want high power an f/7 can deliver, with eyepieces in the range 7mm down to 4 or so. Plenty of good choices here and they don't have to be expensive, either.

3. When you want wide fields an f/7 can fill a 2" barrel eyepiece with a glorious field of view edge to edge.

4. The consequences of having a very large secondary obstruction (30-33%) vs a small one...

5. Aberrations. I suggest that you of all people know full well the mathematics of coma, astigmatism and field curvature of short focal ratio Newtonians. At f/4 the diffraction-limited field of view is just a few minutes of arc across.