http://www.skyandtelescope.com.au 71There was no way Mel was going to
polish and parabolise a mirror that size
and that rough by hand. So he spent a
year making a grinding and polishing
machine. Because he eventually plans
to make a 105-cm ultra-fast scope, Mel
designed it to handle up to that size.
He put the 62.5-cm on the machine,
and 74 hours of polishing later, using
pads on the grinding tool, there were
no pits left. But polishing pads leave
an irregular figure, so Mel made a
hydrostone-and-pitch tool and polished
for another 69 hours, eventually
bringing it to spherical.
Then came the hard part:
parabolising. I’ll spare you the gory
details, other than to point out that
Mel did the entire process with the
matching Ronchi test and star testing,
eventually (after 181 hours) coming
up with a parabola that provides (with
a coma corrector) pinpoint stars from
edge to edge of the field. The temptation
to tweak it further was great, but as Mel
says, “Somewhere in the intersection
between personal skill, Ronchi
matching test and star test results,
and the risk of making the mirror’s
profile worse and having to start over,
the ‘observer’ seizes the mirror from
the ‘maker’ and sends it off to be
aluminised”.
With the mirror done, Mel turned to
the scope that would hold it. At f/2.6,
the eyepiece would be below eye level
at zenith, which meant he had some
room to play around with a concept he
had been considering while he polished:
how to eliminate the dreaded Dob’s hole
with an alt-az mount.
The solution was simple, elegant, and
apparently brand-new: add a second
altitude axis! When the scope is raised
toward the zenith, instead of twisting it
in azimuth for sideways motion the way
a regular Dobsonian mount requires,
just push it sideways on another axis
90° from the standard one. Mel built
a second flex rocker on top of the first,
placed the OTA on it, and shoved. It
worked beautifully! And thus the alt-
alt-az mount was born.
When aimed near the horizon,
the scope behaves just like a standard
Dobsonian: You raise and lower it
for altitude, and you push and pull itaround in a circle for azimuth. As you
raise it up toward the zenith, however,
the second altitude axis comes into
play. By 60° elevation or so, the scope
gently starts to rock sideways as well as
rotate when you push, and the higher
you’re pointed the more rocking and
less rotation you get. At the zenith it’s
all rocking motion, no matter which
direction you push.
Mel designed the second altitude
axis to have about 15° — one hour — of
motion. He also gave himself an extra
7½° beyond vertical in the regular
altitude direction. That lets him observe
an object right on through the zenith
without twisting the scope in azimuth.
With a telescope this fast, a coma
corrector is essential. At f/2.6 Mel can
just squeak by with a 2-inch Paracorr,TLeft: Pierre Lemay’s two-speed helical Crayford focuser incorporates the 3-inch paracorr as a
drawtube. Middle: The fine-motion mechanism moves a nylon block sideways to provide minute
adjustment to the helical twist. Right: A helical Crayford focuser uses angled bearings to direct
the drawtube inward and outward as it’s twisted.
Dob’s Hole
“Dob’s hole” is the gymbal lock
experienced when observing near
the zenith with a Dobsonian mount,
although Mel points out that it’s
“rather unfair to call this Dobson’s
hole since John Dobson hardly
wanted the telescope he invented
to be called the Dobsonian in the
first place.”
X An extra flex rocker provides a second
altitude axis, completely eliminating
“Dob’s hole”.