Quote:
Originally Posted by kevy02
OK guys, why are there so many 10" and 12" dobs for resale at the moment.I have only owned equatorial and fork mounted scopes in the past,but intended buying a large dob in the near future.Is there a problem with manuevoring the scope around the heavens,particularly if no drives? I need a bit of positive input before i commit.Thanks. Kevin.
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Hi Kevin,
Great to hear you have aperture fever.
One advantage the Dobsonian concept has is that the mirror size can be scaled up to
large apertures whilst keeping the cost of the mount itself low. Whereas there will
be many readers of this forum who will remember the not so distant past when a
scope of 12" aperture would be the envy of many others in their club, these days
Dobs with apertures of 10" and 12" are highly affordable and have become
ubiquitous. Their ease of portability and the "bang for the buck" they provide mean
they have become commodity items in the telescope world.
As a far as ergonomics are concerned, many people tend to find Dobsonian
mounts easier to push around the sky compared to Forks and GEM's.
Though the low cost, mass produced Dobs coming out of the Far East do the
job as far as pushing them around goes, when one grabs hold of a premium
commercial mount or a well designed and constructed home-built mount and
pushes them around, the difference in "feel" can be stunning. If you have
ever driven a performance motor car and compare its handling to the commodity
family sedan, you will know what I mean. A good Dobsonian mount does
not come about by accident. It comes from good engineering, combined with
the right combination of materials and careful construction.
By way of example, if you get the opportunity to go to a star party, ask a owner
of a premium large aperture Dob whether you may be permitted to give it a bit
of a push around. David Kriege of Obsession Telescopes uses the expression
"buttery smooth". The bearing surfaces have just the right combination of
friction and stiction so that when you push the scope, it does so without
hesitation and when you stop pushing the scope, it stops moving. When the
apertures of the scopes become large enough that you require a step ladder,
say in the 18" to 48" class, this becomes a highly desirable attribute.
You want to be able to nudge the scope whilst at the eyepiece almost without
being conscious of it. Another premium scope builder is SDM in Victoria and
there are many other excellent commercial premium scope builders.
As I mentioned, some users go the hard yards and build their own. "The Dobsonian
Telescope" is a title of a book by Kriege and Berry which is a must-read for any
prospective Dob builder. Dobsonian design has evolved over the years and
drawing upon the experience of those who have gone before is recommended.
As Eric mentioned, when Dobs point toward the zenith, they are getting close
to a singularity that makes them awkward to push - the so called "Dobson's Hole".
You will know from your experience of other mount types that ergonomically they
have a region of the sky that makes them awkward in some way to use. For
example, an equatorial mounted fork can be troublesome to use near the
celestial pole.
Beyond maneuvering a scope, the other consideration is whether you need to
physically transport it, namely in a car. Whereas a 10" or 12" solid tube scope
can fit across the back seat of most cars, for larger apertures, a Dob that
is designed to be easily transported enables it to be taken to dark sky locations.
Best regards
Gary
