a puzzling question with dobs / newtonians today

[Dunkster]

I must be being dense now, sorry folks, but why would the higher f/ be better?

250px and 300p both under f/5... why would these be bad?

[Moonshane]

there's an increase in apparent contrast (to my eyes at least) with a smaller secondary (resulting from the longer focal length of the slower scope).

[John]

there's an increase in apparent contrast (to my eyes at least) with a smaller secondary (resulting from the longer focal length of the slower scope).

What Shane said plus that the slower scopes have a larger collimation "sweet spot" so are more likely to be in good or good enough collimation to deliver their full potential.

The collimation tolerances of fast scopes (say faster than F/5) get really critical eg: it's just 2mm for an F/4.5 but a whopping 11mm for an F/8.

[Dunkster]

Think I get it, thanks, at least ball park

Sounds like a tricky balance

[garethmob]

Lol 2mm? Lol that's gonna be fun

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