Re: [ATM] equatorial platform and best scope for uses

[Mark Holm <mdholm@telerama.com>]

Kevin Williams wrote:

> if I wanted one scope for planetary observation, and one scope for deep sky,
> what size f/ratio is best for each


I am sure this reply will get some disagreement going, but here goes anyhow.

Your f/5, if the mirror figures are good (both primary and secondary)
and if it is well collimated, is going to give good performance for both
deep sky and planetary observing.   For an 8 inch, I think f/6 to f/7 is
more nearly the sweet spot for a "compromise" scope between deep sky and
planetary.  You want to keep tube length in mind.  If you mount on a
platform, you add ~6 inches of height.  One of the real advantages of an
8 - 10 inch scope of moderate focal ratio is you need no ladder, or even
step stool to reach the eyepiece.  Believe me, this is a bigger factor
than you think.  This limits the upper range of f-ratio.  I have an
8-inch f/8.1 that I have to stand on tip toe when pointed at the zenith.
I am 5' 10" tall.

I honestly don't see the reason for going below about f/6 in an 8 inch
mirror, unless you have a more restrictive tube length requirement, such
as what will fit in your car.

At f/5, a 25mm eyepiece is going to give 40x which gives an exit pupil
of 5mm.  By the time you get to be my age (0.05 millenia), it is likely
your entrance pupil isn't much bigger than this, so any less
magnification just throws away light.  Boost that to f/6 and you get to
the same exit pupil at about 32mm eyepiece focal length.

-- 
Mark Holm
mdholm@telerama.com

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