Re: [ATM] F/10 or F/8?

[Woodchuck <djv@bedford.net>]

On Mon, 13 Jun 2005, Curtis Spivey wrote:

> Can anyone tell me if there would be any advantage in making a mirror
> in an F/10 vs. a F/8?  Seems like everything is a an F8.  This will be
> for a 6" mirror for primary use in a planetary scope.  My thinking is
> that with the longer focal length it will help with higher
> magnification for planetary viewing and may be a little more forgiving
> for a "first" mirror.  I also wear glasses and understand that you can
> use longer focus eyepieces with the longer F/R.
> Since this will be my first, I need all the help I can get.  Not, that
> I intend to do a poor job.  I just don't want to make this more
> difficult than it has to be.

Your instincts seem sound.  The big deal with the longer scopes is
mechanical, not optical, for planetary and other high-power viewing.
If you can make a mount to carry the longer tube, do it.  Go to f:15
if you dare.  That'll be a 90" tube, which may be inconvenient, involving
ladders and other impedimenta to view the zenith, although at your/our
latitude there won't be any planets there. 90" just barely fits in a
full-size pickup bed.  f:10 or f:12 is more practical for a 6", for sure.

An alternative is to use a high-grade Barlow lens.  All the advantages
of good eye relief, better match of EP to primary, with 1/2 the tube
length.  Barlow lenses are better than they were 100 yrs ago.

Another advantage of the long mirror is that you can reduce the size
of the diagonal, reducing its contribution to diffraction and illumination
and contrast loss.

The "simple" (spherical "diopeter") part of your eyeglasses can be
compensated for simply by refocussing.  Only astigmatism (cylinder
correction) is a bother.  You can get your optometrist to prescribe
you a corrector which you can mount on your eyepiece, too.

Dave
-- 
        In each of us, there burns a soul of a woodchuck.
        In every generation a few are chosen to prove it.

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