Reducers & Focal Length

[Coco]

I have a 107mm f/6.5 scope and want a reducer; the scope length is 700mm (695.5) should I be looking for a reducer of that matched length or should I be looking for a reducer matched to f/6.5 ish?

I was looking for post from TMB 102 / Raptor 105 & other APM 107 owners who have scopes in similar design and there is little choice apart from the Riccardi reducer €800+ the TeleVue reducers are either under or over sized on length? That leaves the William Optic MK4 as a potential contender and reasonably priced when new..

Any thoughts?

[ronin]

From what I can recall they are matched to the f number.

So you would need one for an f/6.5 scope.

As you seem to be finding they are not inexpensive items.

[alan potts]

Reducer are matched to the Focal ratio I am sure, there is norammlly a bit of play in the mix. You would probabley buy a reducer for a scope from F6-F8 or something like that. There are many on the market I keep thinking of getting one for my F7.

Alan.

[Coco]

This is the problem, TeleVue & William Optics quote scope length ... its not an exact science.

[Capricorn]

WO seem to throw in focal length for one reducer and f number for another.

Trying to think what the difference would be between either value and really cannot see why one would be better then the other.

I suppose that as the image plane is created curved and as that is a function of focal length then the focal length is the main factor, against that is that for the same focal length a bigger objective will have a wider field and it is the edges that suffer most in curvature. So I would expect an f/5 objective to give more curvature at the edges then an f/8 of the same diameter. Which is what we see/experience.

I suppose there is the possibility that a flattener is designed for either the focal length or the f number primarily and so that is the quoted parameter.

When I bought mine it was dedicated to the scope, WO GT-81, and a reasonable cost so I didn't have to do much thinking (fortunately).