Sky & Telescope - April 2019 - 70

Telescope Care

distance to the lower half of the secondary from the focuser.
can see the entire primary mirror when you look through
(Objects farther away from your eye look smaller, so most
the peephole. If you see something like ﬁgure 1, where
ATMs and some commercial manufacturers put more mirror
the primary mirror looks off-center (don't worry about the
on the far side to compensate.) So just pay
reﬂection of the secondary and spider for
attention to the reﬂection of the spider and
now), then you need to adjust the secondary
The faster your
focuser drawtube.
until the primary appears centered. Use the
Once you've aligned the primary, that's
adjustment screws on the secondary mount
scope, the more
it. You're collimated. Lock down the primary
for motion toward or away from the focuser,
critical the
with the lock screws if you have them. Just
and rotate the secondary rather than tilt it
snug them up. Don't tighten hard.
for left-right motion. (Tilting it sideways
collimation. The
will de-center it in the focuser's drawtube.)
"sweet spot" in
Once the primary looks centered within
Technological Assist
the secondary, look at the reﬂection of your
But what about lasers?! Doesn't everybody
the ﬁeld of view
secondary, spider, and focuser drawtube.
need a laser? Or at least a combination
gets smaller
If it looks like ﬁgure 2, you need to adjust
Cheshire/sight tube?
Nope. Because you can star test. Once
your primary mirror until the reﬂection
and smaller with
you've gotten the collimation as close as poslooks like ﬁgure 3. Centering can be estifaster primaries.
sible by sighting through the empty focuser,
mated much more easily if you mark the
take the telescope outside at night and aim
exact center of your primary mirror (if it's
it at Polaris or, in the Southern Hemisphere,
not already marked by the manufacturer,
Sigma Octans (because those stars won't move much while
as some are). A small dot with a Sharpie works well, though
you're performing the test). Put your highest-power eyepiece
a self-adhesive hole reinforcing ring used in 3-ring binders
in the focuser and center your test star in the ﬁeld of view.
is even better for this. In either case, the center marker will
Now rack the image out of focus just a little and look at the
not affect your image when observing at all, since it's in the
fuzzy donut of light. If it's round with the shadow of the
shadow of the secondary mirror.
secondary mirror in the middle, you're done. If it's oval or
Don't be confused by the perimeter of the secondary mircomet-shaped and the secondary shadow is squished toward
ror. That will probably not look centered when everything
one side, move the telescope so the image shifts around in
else is, because your secondary has likely been offset slightly
away and downward from the focuser to account for the extra the ﬁeld of view and note in which direction the image looks
q SECONDARY ADJUSTMENTS Left: The secondary mirror can be tilted and rotated. Some adjusters have three screws, some four (not counting
the center bolt). Right: The primary mirror adjustment will generally have three screws that pull and three that push. Some may just have three springloaded screws.

Primary
mirror cell

A PR I L 2 019 * SK Y & TELESCOPE

Sky & Telescope - April 2019

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