ATM On Recommending F-Ratios, or Anything!

["Bill T." <twentiethwave@hotmail.com>]

Oops, I seem to have created a mini controversy about f-ratio's.  Although I
personally like long f-ratio telescopes, I do not recommend that telescopes
should necessarily have long f-ratios, or that there is some "magic"
f-ratio.

What I do recommend above all else is that the total experience of using a
telescope should not add up to a hassle.  The design consequences of this is
that a scope should not just meet the optical requirements of the user, but
also be easy to transport, set up, tear down, maintain, and pleasant to use!
Practical and budgetary issues are no less important than optical ones.

In the case of a recent 8" project, I recommended f8 mainly because it would
give a comfortable standing-up eyepiece height for an adult.  I assumed the
bearing box would be built no higher than needed.  While shorter focal
length scopes can be placed on a table to achieve a comfortable eyepiece
height, simply making the focal length a little longer eliminates the need
for a table.  For me, that's a good design trade-off even knowing that table
support offers some advantages at low tube elevations.  I would never have
recommended f8 for anything larger than an 8", unless the user was pursuing
a "super-planetary" project.

But please don't ask me to look through an 8" f5 sitting in a small bearing
box on the ground!  Snap! Ouch! groan....  You know, it's easier for kids to
climb ladders, than for adults to stoop down.  Personally, I often use a
12.5" f5.4 for eyepiece height reasons.

Long f-ratios do offer some optical advantages over shorter f-ratios
including a larger "sweet spot" size, a smaller secondary, and the ability
to use a more extended, well-shielded focuser tube.  They are also easier to
figure.  But IMHO these considerations hit the diminishing returns point
somewhere around f5.5.  I doubt if casual observers under typical conditions
would be able to see dramatic optical differences between same diameter f8
and f5 scopes, if both scopes were optimally constructed and had good
eyepieces.  But there are those who could...it's all a matter of each
builder self-honestly balancing out performance versus convenience.

THE MORAL: those of us who recommend things on this list need to be very
clear about the limitations of our recommendations.  Novices hungry for
information have a tendency to generalize everything.  We need to be sure
that "special case" recommendations are clearly labeled as such, especially
when they involve basic issues.

Bill T.
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Nice, cozy, easy to use 8" f8, with a 64" high eyepiece:

http://www.kupercontrols.com/dob8.htm