Collimation is critical to obtaining the best performance from your
telescope. Aligning the optics of a Schmidt-Cassegrain telescope (SCT) is
much easier than collimating a Newtonian telescope and can easily be learned by
any user. However, there are some tricks to doing it right, and some
things to avoid. If done right, collimation should only be necessary every
few months. If you find it necessary to collimate your telescope every few
weeks, the mirror is probably not being locked down properly after adjustment.
Note: In our experience, the original screws on an SCT secondary mirror
are much better to use for collimation purposes than the aftermarket thumbscrews
that can be added. Thumbscrews cannot be turned as precisely, making
accurate alignment difficult. Also, thumbscrews do not hold the mirror as
tightly, increasing the need to collimate more often. Thumbscrews also
tend to make people "collimation happy," tending to collimate a scope far more often
than necessary. Under normal use, you should be able to go months without
collimating a telescope.
Unlike collimating a Newtonian, there are no special tools required for an
SCT. However, you will need to test the collimation on a star, so it must
be clear and dark. Otherwise, all you will need is a screwdriver to adjust
the screws on the secondary mirror.
Outline of Procedure
Collimating an SCT is simply a matter of adjusting the three screws on the
secondary mirror. This changes the tilt of the mirror and aligns it with
the (fixed) primary mirror. The tilt of the mirror is tested by viewing an
out-of-focus star image through the telescope.
It is very important to let the telescope thermally stabilize before
collimation. A scope that is still cooling down to ambient temperature
will produce a heat spike as warm air radiates off the optics. This can
distort the star image and make the telescope appear out of collimation when it
For the star test, use a relatively high power eyepiece. A 10 or 12mm
is a good choice for most SCTs, providing a magnification of 200-300x.
Choose a fairly bright (1st magnitude) star for the test. It is
important that the star be centered in the field of view when testing
collimation. A star at the edge of the field may be distorted, especially
at lower powers, and could make the telescope appear out of collimation when it
Defocus the star to produce a donut shape. The hole in
the donut is the shadow of the secondary mirror. If the hole is offset
from the center of the star, the collimation must be adjusted.
Above: Appearance of a defocused star in an
Adjusting the Collimation
Now the trick is figuring out which screw to turn. The
low-tech trick is to reach up in front of the telescope and stick a finger in
front of the aperture. You will see the shadow of your hand in the star
image. Move your hand around until it reaches the narrowest (or widest)
part of the donut. Take a look at the secondary mirror and see what screw
your finger is nearest to (or opposite from). It doesn't matter whether
you use the narrow or fat part of the donut, or whether your finger ends up next
to a screw or across from one. The only difference will be whether you
tighten or loosen the screw.
Above: Finding the correct screw to adjust by
pointing to the narrow part of the donut
Whether you tighten or loosen the screw you have
found, depends on whether the image is inside or outside of focus. The
usual method is to try tightening first and see if the star image improves.
Also, note that turning a screw the correct direction will cause the entire star
image to move toward the fat part of the donut (upper left in the diagram
above). Begin by turning the screw about 1/8th of a turn.
If tightening one screw makes the collimation
worse, return that screw to its starting position and try tightening the other
two screws. The most important thing is that the screws end up snug in
the end. Try not to loosen a screw without tightening the others to
compensate. Leaving the screws loose can cause the collimation to be lost
when moving the telescope.
After adjusting the screws, be sure to return the
star to the center of the field of view by adjusting the position of the
telescope. Once the adjustments have been completed, the screws should be
snug and the image of the defocused star should appear perfectly concentric, as
Above: Perfectly collimated
defocused star image
The star should focus down to nice point, with
no asymmetric flaring. The overall sharpness of the star image will depend
on the magnification and seeing conditions, but it should be symmetrical and
Collimating a Telescope