http://www.skyandtelescope.com.au 63TOP: ROD MOLLISE; BOTTOM: SEAN WALKER /
S&T
It’s fortunate that collimating a Schmidt-Cassegrain is
this simple, because good collimation is critical for peak
performance. The standard f/10 SCT uses a 5× magnifying
secondary mirror to increase the focal ratio of the scope’s
‘fast’ f/2 primary mirror to f/10. That gives the SCT a long
focal length packed into a short tube, but this magnifying
secondary mirror also exaggerates collimation errors.
So, how do you collimate an SCT? Which accessories do
you need? Forget basic Newtonian collimation tools like
lasers and Cheshire sights. Variations in the mechanical
alignment of the commercial SCT render most of these
devices useless. Only one ‘tool’ is needed, and it’s free: a
suitable bright star. While you can collimate a Schmidt-
Cassegrain using an artificial star device, it’s better to use
a bright star. Those in the Northern Hemisphere have the
perfect star in Polaris, which is available every clear night.
Here in the Southern Hemisphere we can use any comparably
bright star, though you’ll benefit from having a tracking
mount. Also, having a friend to adjust the collimation screws
while you watch the movement through the eyepiece can
greatly speed up the process.
Before beginning, it’s important to know how the
SCT’s secondary-mirror adjustments work. On almost
all modern SCTs, you’ll find three Allen- or Phillips-
head screws on the secondary mount in the centre of
the corrector lens. They are arranged in a triangle and
thread into a metal backing plate onto which the convex
secondary mirror is glued. This backing plate rides on a
central hump on the secondary mount; tightening and
loosening the screws tilts the mirror on this hump. Never
remove or completely loosen all three screws at the same
time. Unless your telescope has a fourth, central screw,
these three are all that holds the mirror onto the mount —
removing all of them will drop the secondary mirror onto
the primary mirror.
Your first step in collimation is determining whether
the telescope actually needs to be adjusted. On a night of
good seeing — that is, steady atmospheric conditions when
stars are not twinkling madly — remove the star diagonal
and insert a medium-power eyepiece so that you’re looking
straight through the instrument. The eyepiece should yield a
magnification of about 150×. Now centre your chosen star in
the eyepiece field, bring it to sharp focus, and then defocus
just slightly. If the star looks like a big donut, you’ve gone too
far. Defocus only until the star looks like a small bullseye
target, when you see a series of diffraction rings surrounding
a small bright point, the star’s disk.
Take a critical look at the bullseye. Are the rings
concentric, or are they compressed on one side? Does the
whole thing look squished? If it is, you have collimating to
do. Before beginning, however, double-check that the star is
centred in the field. Due to the curved field of the standard
SCT, an off-centre star might appear distorted, implying that
the scope needs collimation even if it doesn’t.Ballpark collimation
The only reason to do ‘ballpark’ collimation is if stars look
like comets and it’s impossible to see diffraction rings. Stand
about 2 metres in front of the telescope and look straight down
the tube. Observe the series of your reflections that are visible
in the primary mirror. Are they concentric? Does everything
line up, or do these reflections look tilted to the side? If they’re
skewed, adjust collimation with the three screws on the
secondary mount until the reflections are centred.S TRIPLE PLAY The secondary-mirror collimation screws are spaced
120° apart, making adjustments easy and intuitive.S ROUGH ALIGNMENT If your telescope is significantly out of
collimation, like this one is, stand about 2 metres in front of the scope
and look straight down the tube. If the reflections aren’t perfectly
concentric, you’ll need to adjust the collimation screws a fair amount
before moving on to the next stage.