6
to another object and fixing the crosshairs on the exact point
you want to look at. Then look through the telescope’s eye-
piece to see if that point is centered in the field of view. If it is,
the job is done. If not, make the necessary adjustments until
the two images match up.
5. Collimation of the Optics
(Aligning the Mirrors)
Collimation is the process of adjusting the mirrors so that they
are perfectly aligned with each other. Accurate alignment is
important to insuring the peak performance of your telescope
and should be done on a regular basis. Collimation is easy to
do and you should become familiar with the procedure, as
you will perform it often. Your telescope’s optics were aligned
at the factory, but may have jiggled out of alignment during
shipment or with rough handling. A simple star test will tell you
whether the optics are properly collimated.
Star-Testing Your Telescope
Point the telescope at a bright star and slowly rack the image
out of focus with the focusing knob. If the telescope is cor-
rectly collimated, the expanding disk should be a perfect
circle. If it is unsymmetrical, the scope is out of collimation. In
reflectors and Schmidt-Cassegrains, the dark shadow cast by
the secondary mirror should appear in the very center of the
out-of-focus circle, like the hole in a doughnut. If the “hole”
appears off-center, the telescope is out of collimation.
Use a Collimation Tool
To aid in centering your line of sight down the focuser draw-
tube, and in centering the mirror reflections during collimation,
it is very helpful to use a precision collimating tool containing
crosshairs, such as the Orion Collimating Eyepiece #3640.
We highly recommend that you purchase one, as you will use
it for as long as you own this, or any, reflecting telescope.
Mechanically Centering the Mirrors in the
Optical Tube
With the telescope pointed away from the Sun, stand a couple
feet away from the front of the telescope and look down the
optical tube. Check that the secondary (or diagonal) mirror is
positioned in the center of the tube. Use a ruler if necessary
to measure the distance from the center of the secondary
mirror holder to the inside of the tube along each of the four
“spider” vanes; the distance should be the same along each
vane. The spider adjustment is set at the factory, so it will
probably be fine right out of the box.
Should you have to adjust the secondary mirror position, do
so by turning the Allen-head spider vane adjustment bolts on
the outside of the optical tube with the small Allen wrench
provided. Unthread one bolt a small increment while thread-
ing in the opposite bolt the same increment. Work with two
bolts at a time until the mirror is centered.
Now check that the primary mirror is centered in the optical
tube. There should be an equal amount of space between the
edge of the primary mirror and the inside of the optical tube
all the way around. A quick visual inspection usually suffices.
If the primary is obviously not centered, it will need to be
repositioned in its mirror cell.
Centering the Secondary Mirror Under the Focuser
If there is an eyepiece in the telescope, remove it. Look down
into the open focuser drawtube at the secondary (diagonal) mir-
ror. It should be centered in the field of view. You will see the
secondary mirror and mirror holder as well as reflections of the
secondary mirror and holder and the primary mirror, and your
eye. It’s pretty confusing, so refer to Figure 2 to sort them all out.
(This figure shows what you would see if both the primary and
secondary mirrors were out of alignment and the secondary
mirror were not centered below the focuser tube.)
If the secondary mirror is not in the center of the focuser draw-
tube, it must be adjusted. It really helps to put a piece of white
paper on the inside of the optical tube opposite the focuser.
The white paper forms a bright background behind the sec-
ondary mirror as you look down the focuser drawtube, making
it easier to distinguish the mirror holder from the background.
The secondary mirror is moved forward and back on a thread-
ed rod that extends from the secondary mirror into the
four-vane spider center support. The threaded rod has nuts
on both sides of the spider to hold it in place. By threading the
two nuts in one direction or the other, you can move the rod
forward or back to correctly center the secondary mirror under
the focuser. When you’ve achieved the correct position
(Figure 3), make sure the two nuts are threaded against the
spider center support to hold the rod securely in place.
Adjusting the Tilt of the Secondary Mirror
Look in the focuser drawtube and see if the entire reflection
of the primary mirror is visible and precisely centered in the
secondary mirror. It’s centered if there is an even ring of
space between the reflection and the edge of the secondary
mirror (Figure 4). If there is not, you will need to adjust the tilt
of the secondary mirror. (It helps to adjust the secondary mir-
ror in a brightly lit room with the telescope pointed toward a
bright white surface, such as white paper or a wall.)
First, loosen the four Allen-head alignment screws in the sec-
ondary mirror holder enough to be able to rotate the holder
with your hand. Now look into the focuser and rotate the sec-
ondary mirror holder slightly one way, then the other, with
your hand, until the reflection of the primary mirror is as cen-
tered in the secondary mirror as it will get. It still may not be
perfectly centered yet, but that is OK. Now secure the holder
in that rotational position by threading the four alignment
screws back in.
Now, if the entire primary mirror reflection still is not visible
and centered in the secondary mirror, adjust the secondary
mirror tilt further by alternately loosening one of the four align-
ment screws and tightening the opposite one a turn or so.
Remember, the goal is to center the primary mirror reflection
in the secondary mirror, as depicted in Figure 4. Don’t worry
that the reflection of the secondary mirror (the smallest circle
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