Collimation is critical to obtaining the best performance from your
telescope. There is a bit of an art to collimating a
you have done it a few times, it will start to become second nature.
Unless you have a truss-tube telescope which must be assembled each time it is
used, collimation should be an infrequent necessity. You should be able to
go for weeks or months without collimating a telescope under normal use. If you
find collimation to be necessary every couple weeks, you may not be properly
locking down the mirrors after collimation, or a component may be loose
somewhere in the optical train.
There are two basic tools necessary for accurate alignment of Newtonian
Above: A laser collimator and collimation eyepiece
A collimation eyepiece is a metal tube with crosshairs in it. It fits
into the focuser just like an eyepiece. Looking through it you see the
crosshairs overlaying the telescope optics, allowing you to determine the
alignment of the mirrors. A laser collimator also fits into the focuser
like an eyepiece. It projects a red laser beam, placing a dot onto the
primary mirror which is reflected onto the
secondary mirror then back onto the
collimator (when everything is properly aligned).
In addition to these items, you will need the appropriate wrenches or
screwdrivers to adjust your telescope's mirrors. A common set of tools
would be a Phillips screwdriver and hex-head wrenches for the secondary mirror,
and a flat-head screwdriver for the primary mirror, but each telescope is
Outline of Procedure
Before beginning, know your plan of attack. There is a correct
procedure to use when collimating a Newtonian. The basic steps are as
Align the secondary mirror using the collimation eyepiece
Align the secondary mirror using the laser collimator
Align the primary mirror using the laser collimator
Double check everything with the collimation eyepiece
One of the common misconceptions about collimation is that most of the
adjustment must be done on the primary mirror. In fact, the opposite is
true. Most of the collimation adjustment is with the secondary mirror,
especially in a truss-tube telescope. The secondary mirror is more likely
to be bumped out of alignment, and it has more degrees of freedom than the
Visual Alignment of Secondary Mirror
The first step is aligning the secondary mirror using the collimation
eyepiece. As mentioned above, do not underestimate the importance of
proper secondary mirror alignment. Begin by inserting the collimation
eyepiece into the focuser drawtube and take a look through the eyepiece.
An out-of-collimation instrument will appear like the diagram below. A
properly collimated telescope is shown as well.
Above: Uncollimated Newtonian as viewed through a collimation
eyepiece. Note the points of interest: collimation eyepiece crosshairs, primary mirror
clips, primary mirror center dot, secondary mirror reflection, focuser drawtube.
Above: A perfectly collimated Newtonian
viewed through a collimation eyepiece.
How Secondary Mirror Adjustment Works
Keep in mind that the secondary mirror has three degrees of freedom:
tilt, rotation, and longitudinal adjustment. Tilt is controlled by three
screws behind the secondary mirror. These screws push against the flat
back of the mirror holder. They also lock the mirror in place when
collimation is complete. Rotation and longitudinal adjustment (movement up
and down the tube along the optical axis) are controlled by a single larger
screw in the center of the mirror holder. This screw actually attaches to
the mirror assembly, holding in onto the spider vanes.
Above: Three outer screws control tilt of secondary mirror
Above: Main central screw controls rotation and longitudinal
Adjusting the Secondary Mirror
Below are diagrams depicting the appearance through the collimation eyepiece
of errors in tilt, rotation, and longitudinal adjustment. Keep in mind
that these errors will normally occur in combination. The first thing to
try is adjusting the tilt of the mirror because it is the easiest error to see.
Once this is eliminated, you can check for errors in the other degrees of
Above: Tilt error (see description below)
Above: Rotation error (see description below)
Above: Longitudinal error (see description
Tilt error shows up as a displacement of the reflection of the secondary
mirror (the dark circle near the center of the view). Correct this by
adjusting the three screws pushing the mirror assembly until the reflection of
the secondary is centered. At this point, the primary mirror clips should
be visible evenly around the perimeter of the view. (There will be three
or four depending on the model. These diagrams show four.)
secondary reflection and clips are displaced up or down as shown in the second
rotational adjustment is necessary. After rotating the mirror, readjust
the tilt as well.
Finally, if the focuser drawtube is not centered in the
secondary mirror reflection, while everything else appears centered, a
longitudinal adjustment is required. In the third diagram above, the secondary
mirror sits too far down the tube (too close to the primary) and needs to be
moved up. This is done by loosening the tilt screws and tightening the
main screw. After this it will be necessary to readjust the tilt once
Laser Alignment of Secondary Mirror
The next step is to refine the tilt alignment of the secondary mirror using
the laser collimator. Insert the laser into the focuser and turn it on.
Look down the front of the tube at the primary mirror. You should see the
laser produce a red dot on the primary. This dot should coincide with the
dot marking the center of the primary mirror. If there is no center dot on
your mirror, use the distance of the laser dot from the mirror clips to estimate
the center. Adjust the tilt of the secondary mirror, if necessary, to
place the red dot directly on the center of the primary.
Above: Misaligned secondary produces a red dot offset from
the primary mirror center. Adjust the mirror until the dot hits the center
mark of the primary.
Before proceeding to the primary mirror, be sure to double check the
alignment once more using the collimation eyepiece. This will verify that
tilting the secondary to align the laser dot did not introduce any rotational
error. If needed, repeat the rotation and tilt adjustments until
everything is perfect.
Laser Alignment of Primary Mirror
Finally, the primary mirror can be adjusted. You can take a quick look
through the collimation eyepiece to see what the appearance of the optics are
when the secondary is aligned but the primary is not. The general
appearance is that the primary mirror center dot and mirror clips are centered
on the crosshairs of the eyepiece, but the reflection of the secondary mirror
itself is offset.
Above: Properly aligned secondary with unaligned primary
Insert the laser into the focuser drawtube and turn it on. This time,
look for the reflection of the secondary mirror. You should see two red
dots. One comes from the bottom of the laser itself, and the other is a
reflection of the laser beam back onto the bottom of the collimator (or onto the
focuser if the alignment is way off). The trick now is to get the two dots
to coincide, sending the laser beam right back on itself.
Above: Laser dots produced by a misaligned primary
Most primary mirrors have six screws on the mirror cell: three push
screws and three pull screws. The push screws are normally smaller and
lock the mirror in place. The push screws are usually bigger and have
springs on them. These control the main adjustment of the mirror.
Above: Collimation screws on a typical Newtonian
Loosen the lock (push) screws to allow the mirror to be adjusted.
Unless you have a helper or a very short telescope, you will have to make an
adjustment to the mirror then walk up to the front of the scope and see whether
things got better or worse. Continue this until the two red dots perfectly
coincide. Keep the laser on while you tighten the lock screws.
Tighten each screw about a quarter turn at a time and continue working around
until all three are snug. This will tend to keep the mirror in alignment
while you lock it down. Double check the laser again after tightening the
Once again, double check the alignment with the collimation eyepiece to be
sure everything looks okay.
Above: Appearance of a perfectly collimated Newtonian through
a collimation eyepiece
Collimating a Telescope