Sounds like your after what I got 150x750 acro with 25 or 30mm ep goes pretty wide !
25 mag on Eta or Orion fits all omega cent great
Pick em up $500 good value
Hi Alex, I picked up a 6 inch F5 Achromat from Andrews a while back and can say that it is absolutely awesome on wide field low power views.
I think they still have them....................worth a try.
If you want an RFT you can use f/7 - there's no real need to go down to f/5 these days unless compact size is what you really want. The benefit of f/7 is that the ED f/7 doublets can also give perfect high powers too, far better than any f/5 can.
Basically, it comes down to being able to
a) reach the upper limits of magnification feasible from a good 4" refractor, when you want to (bright doubles or the planets) with eyepieces that are still comfortable to use, which makes it useful on weekends away.
Faster 4" refractors will be quite inferior at high power (an f/5 is not much good, actually)
b) it can also operate as an RFT at f/7, if you have a 40mm 2" eyepiece that has no field stop (I have one).
By way of example:
High power: at x1 per mm of aperture you should jut be able to discern the maximum detail in images that the scope can provide. If you use more power (say 1.4X per mm) you should clearly see diffraction rings around bright stars, but not really any more detail.
For an f/7 scope, a 7mm eyepiece gives x1 per mm, and a 5mm gives x1.4 per mm. On my scope, 102X and 148X respectively.
Low power: if you assume your exit pupil could be 6mm, for a 4" scope that means the lowest useful magnification is 17X. This corresponds to a 42mm eyepiece, and I happen to have a 40mm (close enough) which means it will work fine as an RFT.
If you want an RFT you can use f/7 - theres no real need to go down to f/5 these days unless compact size is what you really want. The benefit of f/7 is that the ED f/7 doublets can also give perfect high powers too, far better than any f/5 achromat can.
By way of example:
Low power: if you assume your eye pupil could be 6mm, for a 4" scope that means the lowest useful magnification is 17X. This corresponds to a 42mm eyepiece, and I happen to have a 40mm (close enough) which means it will work fine as an RFT.
Note that this lowest power is determined by m = D/d
where D the ratio of the objective of the scope, and
d = the diameter of your eye pupil.
It is not governed by the f/ratio of the scope. If you choose an f/5, it will only need a 30mm eyepiece to give the same exit pupil and the same field I have in mine.
High power: at x1 per mm of aperture you should just be able to discern the maximum detail in images that a scope can provide. If you use more power (say 1.4X per mm) you should clearly see diffraction rings around bright stars, but not really any more detail.
For an f/7 scope, a 7mm eyepiece gives x1 per mm, and a 5mm gives x1.4 per mm. On my scope, 102X and 148X respectively. On an f/5, those eyepieces will be 5mm and 3mm. And I don't like using eyepiece that short.
Generally speaking, scopes faster than F6 won't give pinpoint star images very far out from the center sweet spot unless you use some very expensive glass.
I'm not sure why a visual observer would have F ratio fever? Consider that the field of view with an F8 scope and 30mm eyepiece will be the same as an F4 scope with a 15mm eyepiece. And the view through the F8 scopes will have less noticeable "seagulls" near the edge of the field.
Another thing to consider is exit pupil size. If you're a young fella you might get 7mm. As we age it's more like 5mm. If the ultra fast scope gives an exit pupil larger than our eye is capable of, the extra light is wasted.
To calculate exit pupil size, divide the aperture of the scope's objective lens by the magnification of the eyepiece. For instance if you have an 80mm objective say at F4 with a 40mm eyepiece, first work out focal length 80 x 4 = 320mm. Then work out magnification 320 / 40 = 8. Now that you have a magnification of 8, 80mm / 8 = 10mm exit pupil.
If you're middle age and have a 5mm max eye pupil size, that fast scope is just going to waste with any eyepiece over 20mm. You can still get wider views, but it won't be any brighter.
F ratio fever is for photographers.
EXACTLY!
If I want refractor it is gonna be one of those old style f/15.
