Re: [ATM] Newtonian Alt-Az with Level Focuser Position

["The Alarm Co." <tac@i1.net>]

This is feasible for newts with a focal length under 6'.  Make a standard
Dob, then raise the rocker box until the altitude bearings' centers coincide
with the axis of the focuser.  That gives you your level, nearly stationary
eyepiece position, without any extra reflections.  Now, the scope is WAY out
of balance, so you'll need to build another tube (truss assembly, whatever)
ahead of the focuser cage and weight it to make the balance point also
coincide with the focuser axis.  Think of it as a really big, heavy dew
shield.  There would be many structural problems to overcome if the scope
were very large (say, over 6").  Trying a Dobsonian/Nasmyth just won't work.
A quick sketch showed that using a 16" f/5.4 primary and stipulating a
maximum obstruction of 25%, the tube would be shortened only 7", and the
tertiary wouldbe 54" from the primary.  That's so close to the secondary, it
would shade something like 3/4 of it!  Vignette City!
The Cassegrain/Nasmyth configuration looks a lot better.  Keeping the
secondary at the same location (65", 25% obstruction), the tertiary can now
be moved to about 30" from the primary.  That puts it at a point where the
tube is easy to balance, and the eyepiece height is about 36", a convenient
seated height.  The focal length changes to about 112", for a secondary
amplification factor of about 1.4.  That's an easy 4" mirror to make, even
considering it's convex.  It's not even out of the question to make the
whole thing a solid tube.  It would be about 18-20" in diameter and six feet
long.  No problem for an Explorer or a minivan.
The Gregorian configuration is attractive because its secondary mirror is
concave, and therefor easily tested.  In the example scope, however it would
make the tube about two feet longer.  That pretty much dictates a
complicated structure: Mirror box, truss, tertiary cage, truss, secodary
cage.  Collimation then becomes a problem.
Is it worth it???  I guess it depends on your observing style.  I tend to
jump all over the place, so I really like the fact that the eyepiece of my
8" SCT doesn't move much.  I love the views of planets through my 8.5"
f/12.5 refractor, but what a pain following the eyepiece!  And, in between
is my 16" f/5.4 Dob.  Also good points and bad points about eyepiece
movement.
I guess it's like when my extremely computer-savvy wife and I discussed
gettng our first home computer 15 years ago.  "Gee, what kind of processer
do we need?" I asked.  "Wrong question," she replied.  By way of snappy
comebacks, I came up with "Huh?"
"The First Question," she replied, "is 'What do we want it to DO?' "

 *  Best regards, Bob
*
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