Sky and Telescope - February 2018 - 30

Scope Maintenance

donut half the size as it was previously, and then see if the dark hole is
still centered. If it is, you can move on
to ﬁne collimation. If not, follow the
same steps until it's as well centered as
you can get it. Some people stop at this
"donut stage," but that is rarely good
enough for demanding tasks like planetary observing.

Fine Collimation

You can begin collimation with any
of the screws. Just pick one and, using
the proper tool, tighten it experimentally. In this stage, you may need to
turn screws a fair amount; but even
so, make only 1/8 turn at a time. If the
reﬂections moved in the proper direction, continue turning the screw in
small amounts until the reﬂections
in the telescope's primary mirror are
concentric. If not, pick another screw,
tighten it, and check to see if you are
going in the correct direction. When all
the reﬂections line up, it's time to move
to rough collimation.
In all stages of adjustment, try to
avoid loosening the screws; that will
ensure the secondary is snug and the
telescope will remain in collimation for
months or even years. If you still need
to move the secondary in a particular
direction but the corresponding screw
is alredy hand-tight, you can still make
the adjustment in the same direction by
loosening the other two screws slightly.

Rough Collimation
Start with Polaris centered in the ﬁeld
30

FE B RUA RY 2 018 * SK Y & TELESCOPE

and defocus until the star looks like a
donut with a dark center. Is that donut
hole (which is actually the shadow of
the secondary mirror) in the middle, or
is it off to the side? If it's not centered,
adjust the screws as described above
until it is.
Note that turning a screw will move
Polaris to the edge of the ﬁeld, so be
sure to re-center it following each
adjustment. When the donut looks
good, move the focus in to make the

In-focus Collimation
If you have excellent seeing conditions,
you can attempt the ultimate in SCT
collimation - in-focus collimation.
This is made possible by the fact that,

p FINE COLLIMATION Left: While aimed at Polaris, rack the focus out slightly so the star's diffraction rings are visible. Imperfect collimation will make the rings appear skewed in one direction.
Right: Adjust the collimation screws until the rings are perfectly concentric, so that the star looks
like a perfect bull's-eye target.

TOP: ROD M OLLISE; BOT TO M: SK Y & TELESCOPE

p SMALL TURNS When performing the fine collimation steps, limit the amount you turn your tool
to no more than 1/8 of a turn to avoid pushing Polaris out of the eyepiece field.

Re-focus Polaris until it is almost sharp,
such that diffraction rings appear
around the star. If the bull's eye of
these rings isn't concentric, then adjust
the secondary screws until it is. There
are various methods that can be used
to determine which screw needs to
be adjusted, but I like to keep things
simple, so I use a variation of my lowtech "pick one" strategy.
Look at the bull's eye. Which side is
squished? Move the telescope until that
side is against the edge of the eyepiece
ﬁeld. Now, move the screws by small
amounts until you ﬁnd the ones that
move the star toward the center. Use
that screw or screws to put the star back
in the center of the ﬁeld. Keep repeating
this, moving the star to the ﬁeld edge
closest to the compressed side of the
bull's eye and re-centering it with the
screws, until the rings are concentric.

Sky and Telescope - February 2018

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