My first Magazine Sky & Telescope - 04.2019 | Seite 72

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 fi gure 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
fi
eld
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?
your primary mirror until the reflection
Nope. Because you can star test. Once
and
smaller
with
looks like figure 3. Centering can be esti-
you’ve gotten the collimation as close as pos-
faster primaries.
mated much more easily if you mark the
sible by sighting through the empty focuser,
exact center of your primary mirror (if it’s
take the telescope outside at night and aim
not already marked by the manufacturer,
it at Polaris or, in the Southern Hemisphere,
as some are). A small dot with a Sharpie works well, though
Sigma Octans (because those stars won’t move much while
a self-adhesive hole reinforcing ring used in 3-ring binders
you’re performing the test). Put your highest-power eyepiece
is even better for this. In either case, the center marker will
in the focuser and center your test star in the field of view.
not affect your image when observing at all, since it’s in the
Now rack the image out of focus just a little and look at the
shadow of the secondary mirror.
fuzzy donut of light. If it’s round with the shadow of the
Don’t be confused by the perimeter of the secondary mir-
secondary mirror in the middle, you’re done. If it’s oval or
ror. That will probably not look centered when everything
comet-shaped and the secondary shadow is squished toward
else is, because your secondary has likely been offset slightly
one side, move the telescope so the image shifts around in
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 spring-
loaded screws.
Primary
mirror cell
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A PR I L 2 019 • SK Y & TELESCOPE