Re: Observatory Mounts

Jan Owen <janowen1@xxxxxxx> wrote:

Today's "Dobsonian" telescopes have EVOLVED into relatively short focal
length instruments. But most of the originals were far from Rich Field,
short focal ratio scopes... Even in large aperture Newtonians, f/8 was not
an unusual focal ratio in the dim times...

Hi, Jan, and given the general topic "Observatory Mounts," maybe it would be
interesting, as your analysis suggests, to consider some of the trends quite
apart from the question of defining a "Dobsonian."

An interesting question is how far back the "dim times" go.. or up until
how recently? Certainly I'd agree that large aperture Newtonians
traditionally could be f/8 or quite slower. Some of William Herschel's
Newts are the first examples that occur to me. Thus his famous 18.7"
scope had a focal length of 20 feet (by which it was mainly known at
that time), or about f/12.8, compare to a typical SCT nowadays. His
48" aperture scope, really pushing the envelope on aperture fever,
had a 40-foot focal length, or f/10.

But improvements in mirror making and mirror mounting, have resulted in
evolving mount designs that have drifted toward shorter and shorter focal
ratios to allow more and more aperture in smaller and smaller packages,
without sacrificing structural rigidity or operational smoothness, without
backlash, in both axes... In some respects, the newer scopes are better,
but in some cases they are not. Yet, most of them ARE different, while
sharing some common basics...

This is what might called an interesting "synergy" in design: the
practicality of faster optics, as you explain, which could make the
Dobsonian type of alt-az mount indeed a way of getting "more and more
aperture in smaller and smaller packages." Maybe it might be interesting
to survey how f-ratios for Newts of different apertures evolved, say,
over the period of 1960-1985 as reflected in telescope ads in the
leading amateur magazines.

Personally, I think we spend WAY too much time worrying about the minute
details of what to call them and why. It's a design many folks have found
to be very useful, and many of THOSE folks have found things they can do to
make it work for THEM, even better. That's evolution... The fact that it's
based on the mechanics of old cannons just further emphasizes the evolution
of the design...

Please let me admit that I have a certain weakness for semantic and
specifically definitional minutiae <grin>, what are sometimes called
"goat's wool matters" -- not inappropriate, now that Capricorn is so
prominent in the night sky. Does this imply that s.a.a., or Usenet
generally, is mostly woven out of mohair?

However, if these discussions can lead to a broader and better understanding
of design and how it evolves, they have enhanced value. Thanks for inviting
me to consider this larger perspective.

Not to worry... We'll likely still be calling scopes of this basic design
Dobsonians long after most folks have ANY idea where the name came from...

By the way, this also raises a question for me as to whether the term
"Dobsonian" might now be associated, certainly with the kind of design
we're discussing, but also a kind of "observational lifestyle" not necessarily
specific to Newts. I might describe that lifestyle as mostly DSO observing
with free manual pointing of the scope and starhopping as the strategy.

For example, a rich-field refractor around f/6 on an alt-az mount might
favor a similar lifestyle, albeit with more weight for the same amount
of aperture.

Most appreciatively,

Margo Schulter
mschulter@xxxxxxxxxx
Lat. 38.566 Long. -121.430

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