With the appropriate eyepiece, an f6 system will give you the widest possible maximum field for a
given aperture, for visual use, assuming you want a 7mm exit pupil (the largest a youthful eye will
accept, usually). It used to be that f ratios smaller than 6, such as 5 or 4, would give you bigger
maximum fields, but with modern ultrawide apparent field eyepieces, such as the 35 and 41 Panoptics
and the 31 Nagler, this is no longer the case. It is a matter of achieving a about a 7 mm exit
pupil, which can be calculated by dividing the focal length of the eyepiece, such as 31, or 35 or
41, in this example, by the focal ration, which is 6 in this example. The 31 Nagler will thus give
only about a 5 mm exit pupil on an f6 system, but it has an 82 degree apparent field, which a lot of
us like. If you want a 7 mm exit pupil with a 31 Nagler, you need about and f.4.5 focal ratio.
As you get older, the maximum exit pupil your eye will accept gets smaller, and so by the time
you are 60, you will be better off with a 5 mm exit pupil.
If you have substantial light pollution where you observe, you will also prefer an exit pupil
smaller than 7, because a smaller exit pupil will darken the sky more.
At least this is my undersanding of the matter.
On a telescope with a mirror larger than 10 inches I wouild go with a focal ratio smaller than
6, and use a Tele Vue Paracorr to correct the coma.
Bill Meyers

"Alan W. Craft" wrote:

On Thu, 24 Jul 2003 08:40:04 +0000 (UTC), William Mc Hale ...reflected:

LarryG wrote:
"Alan W. Craft" wrote in message
...

I'm considering a Parks classic Newtonian,
and to mount on a Vixen GP-DX equatorial
mount. While their 8" f3.5 seems to be just
a wee bit too fast, and their 8" f6 a tad too
slow for DSO's and the like(not to mention
the tube length), I've looked into the
possibility of an 8" f5 custom-made by
Parks and sold via Scope City...

Check out the recent issue of Sky and Telescope about observing DSOs
and the merits of high or low magnification. "Slow" scopes are fine for
many such objects. The conventional wisdom dictating fast scopes simply
doesn't hold up in practice, once the other variable of vision and observing
are factored in.

Not to mention that f/6 is not really all that slow. On an 8" one could
get about 2 degrees out of a 35 mm Panoptic.

Correct me if I'm wrong, but shouldn't the focal length of the telescope
in conjunction with the f.l. of the eyepiece, that is, the magnification,
determine whether or not a telescope is fast or slow?

While an 8" f6 would be considered fast when compared to an 8" f10,
the same 8" f6 would at the same time be equitable in "speed" to a 4" f12,
and therefore considered slow.

Such slowness is precisely what I am trying so desperately to avoid,
and in order to use the telescope for comet-hunting and widefield DSO
observation, while at the same avoiding glaring instances of coma
and exacting collimations.

Please excuse my ignorance on the matter if I've overlooked something.

Alan