72 AUSTRALIAN SKY & TELESCOPE February | March 2019
but he wanted the capability to use
a 3-inch corrector for a wider, fully
illuminated field. That would require a
3-inch focuser, which is both heavy and
expensive.
Mel and Pierre Lemay were in
constant contact during the building
process, and when Pierre heard about
this problem, he said, “Why don’t you
use the barrel of the coma corrector
as the drawtube of a helical Crayford
focuser?” In fact, Pierre offered to make
just such a focuser for him.
A helical Crayford foregoes the
threaded barrel that standard helical
focusers use in favour of angled
bearings that rest against the drawtube.
Rotating the tube makes it crawl inwardWPierre has designed an easy and
inexpensive two-inch plywood version
of his two-speed helical Crayford
focuser.The secondary mirror is
supported using solid
wire and only requires
minor adjustment after
each setup.or outward along the bearings’
angle of attack, while a friction
pad holds the tube against those
bearings and provides the right
amount of resistance.
Somewhere along the line as
he was designing the focuser,
Pierre had a brainstorm: If he put
the friction block on a screw that
pulled it from side to side, he could
provide a fine motion control. And
thus the two-speed helical Crayford
focuser was born.
The focuser coarse-focuses with a
twisting motion like any helical design,
and it fine-focuses with a threaded
shaft that pulls and pushes the friction
block tangentially across the drawtube.
The pitch of the screw determines
how incremental the fine motion is.
Experimentation quickly showed that
Pierre would need a coarse thread to
keep from making multiple turns for
even the finest adjustment. He settled
on a multi-lead screw with a draw of 3
turns per 2.5 cm, which gave a coarse-
to-fine focus ratio of about 32:1, which is
perfect for fast telescopes where the tight
depth of field leaves no room for error.Pierre’s first attempt, using a
cylindrical Delrin friction block,
proved too slippery, but he re-did the
friction block with a rectangular cross
section and that worked like a charm.
Mel reports that the focuser works
beautifully on his new scope.
An interesting detail: The coma
corrector intrudes into the light path
by a couple of centimetres or so. That
enabled Mel to use a smaller secondary
mirror, which provides less overall
obstruction than if he had moved the
coma corrector farther out.
One of Mel’s big observing interests
is tracing out the delicate web of galactic
cirrus, the gas and dust within our
galaxy that’s illuminated not by any
single star but by the combined glow
of the entire Milky Way Galaxy. He
continues to search out new patches of
galactic cirrus whenever he can. This
new scope is great for that. With its wide
field and large aperture, faint nebulosity
really stands out nicely. Mel reports
that the Ring Nebula (M57) also sports
colour when viewed directly on.
I had the pleasure of viewing through
this innovative optical masterpiece,
and I can confirm that it is a joy to use.
With the Veil Nebula nearly straight
overhead, I was guiding the scope with
the ease of a ball-mounted scope and
enjoying the sharp-focused view from a
62.5-cm aperture mirror, seeing details
I’d never seen before — all with both my
feet flat on the ground.
Now that’s what I call a multiple
breakthrough.
For more information about
this telescope and many of Mel’s
other designs, visit his website at
bbastrodesigns.com.
Visit Pierre Lemay’s website at
telescopelemay.com if you’d like to
learn more about the focuser, including
plans for a plywood 2-inch version
that can be built for about $20 in parts
with little more than hand tools and a
drill press.■ Contributing Editor JERRY OLTION is
easily impressed... with innovations as
impressive as these! PLYWOOD FOCUSER: PIERRE LEMAY; SECONDARY MIRROR: MEL BARTELSASTRONOMER’S WORKBENCH