My first Magazine Sky & Telescope - 02.2019 | Page 74

With a telescope this fast, a coma
corrector is essential. At f/2.6 Mel can
just squeak by with a 2-inch Paracorr,
but he wanted the capability to use
a 3-inch corrector for a wider fully
illuminated fi eld. That would require
a 3-inch focuser, which is both heavy
and expensive.
Mel and Pierre Lemay were in con-
stant contact during the building pro-
cess, 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 favor of angled bearings
that rest against the drawtube. Rotat-
ing the tube makes it crawl inward or
outward along the bearings’ angle of
attack, while a friction pad holds the
tube against those bearings and pro-
vides 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.
p Left: Using the 25-inch, Mel traced out tendrils of galactic cirrus all around M51. Right: The Ring
Nebula shows color in a scope this large.
The focuser coarse-focuses with a
twisting motion like any helical design,
and it fi ne-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 fi ne motion is. Experi-
mentation quickly showed that Pierre
would need a coarse thread to keep
from making multiple turns for even
the fi nest adjustment. He settled on a
multi-lead screw with a draw of 3 turns
per inch, which gave a coarse-to-fi ne
focus ratio of about 32:1, which is per-
fect for fast telescopes where the tight
depth of fi eld leaves no room for error.
Pierre’s fi rst attempt, using a cylin-
drical Delrin friction block, proved
too slippery, but he re-did the friction
block with a rectangular cross section
q Left: Pierre Lemay’s two-speed helical Crayford focuser incorporates the 3-inch paracorr as a drawtube. Middle: The fi ne-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.
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