3
and then moving the ring down the telescope optical tube
until properly positioned. Retighten the tube ring lock knob
when done. Attach the dovetail mounting plate so that the
smooth, flat side of it faces the telescope optical tube.
Now that the dovetail mounting plate is attached to the tube
rings, the optical tube fits into the dovetail slot on the head of
the GP-DX mount. Make sure both included counterweights
are placed on the counterweight shaft; both weights are nec-
essary to properly balance the load. Secure the optical tube
in place with the dovetail tube plate lock screw. Also tighten
the knurled metal safety screw. Refer to the manual that
comes with the GP-DX mount for instructions on how to bal-
ance and properly use the mount.
3. The Optics
The Maksutov-Newtonian optical design yields exquisite,
refractor-like images yet has a much more compact optical
tube than an equivalent 6" refractor. It employs a spherical
primary mirror. A spherical mirror of this aperture would, by
itself, yield an unacceptable degree of spherical aberration,
so the design incorporates a deeply concave “corrector” lens,
often called a “meniscus,” at the front of the scope. This lens,
at the center of which the secondary mirror holder is mount-
ed, provides sufficient correction to render a sharp,
undistorted image. The diameter of the central obstruction
has also been minimized to further increase image contrast.
The purplish tint of the meniscus is caused by multi-layer
coatings that have been applied to the surfaces of the glass
to enhance light transmission and image contrast.
Collimation
Collimation is the process of adjusting the optical compo-
nents of a telescope so they are perfectly aligned with each
other. Your telescope’s optics were aligned at the
factory, and should not need much adjustment unless the
telescope was roughly handled during shipment. You will
never need to adjust the positioning of the front corrector
lens; only the primary and secondary mirrors. Accurate align-
ment is important to insure the peak performance of your
telescope, so it should be checked occasionally. Collimation
of the mirrors is easy to do and should be done in daylight.
To check the collimation, remove the eyepiece and look
straight down the focuser drawtube. Turn the focus knob until
the drawtube is in its most recessed position, and hold your
eye just above the end of the drawtube. Make sure you are
looking straight down the focuser, or your adjustments will not
be correct. You should see the secondary mirror centered in
the drawtube, the reflection of the primary mirror centered in
the secondary mirror, and the reflection of the secondary mir-
ror centered in the reflection of the primary mirror (behind the
reflection of the primary mirror’s central black circle), as in
Figure 1D, page 7. If anything is off-center, follow the
collimation procedure below.
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 you purchase one.
Aligning the Secondary Mirror
With the eyepiece removed, look straight down the open
focuser drawtube at the secondary (diagonal) mirror. It helps
to adjust the secondary mirror in a brightly lit room with the
telescope pointed toward a bright surface, such as white
paper or a wall. Ignoring the reflections, the secondary mirror
itself should be centered in the field of view. If it isn’t, it must
be returned to Orion Telescopes to be adjusted, as this can-
not be corrected by the user. This has been checked during
assembly at the factory, so there should not be a problem.
If the entire primary mirror reflection is not visible in the sec-
ondary mirror (as it is not in Figure 1B), you will need to adjust
the tilt of the secondary mirror. This is done by alternately
loosening one of the three alignment screws visible in the
center of the corrector lens a turn or two and tightening anoth-
er one. You will need a flat-head screwdriver to do this. (Do
not adjust the central Phillips-head screw!) The goal is to cen-
ter the primary mirror reflection in the secondary mirror, as
depicted in Figure 1C. Don’t worry that the reflection of the
secondary mirror (the smallest circle, with your eye reflected
in it) is off-center behind the reflection of the primary’s central
black circle (as also is the case in Figure 1C); you will fix that
in the next step.
Adjusting the Primary Mirror
The final adjustment is made to the primary mirror. It will need
adjustment if, as in Figure 1C, the secondary mirror is cen-
tered under the focuser and the reflection of the primary
mirror is centered in the secondary mirror, but the small
reflection of the secondary mirror (with your eye inside) is off-
center behind the reflection of the black circle in the center of
the primary. The tilt of the primary mirror is adjusted with the
three sets of collimation screws on the back end of the opti-
cal tube (bottom of the mirror cell). The collimation screws
can be turned with Allen-head wrenches.
Each set of collimation screws works together to adjust the
tilt. The smaller screw pushes the mirror cell forward, while
the larger screw pulls the mirror cell back. One must be loos-
ened and the other tightened by the same amount to adjust
the tilt. Try tightening and loosening one set of collimation
screws one turn. Look into the focuser and see if the sec-
ondary mirror reflection has moved closer to the center of the
primary mirror reflection. Repeat this process on the other
two sets of collimation screws, if necessary. It will take a little
trial and error to get a feel for how to tilt the mirror in this way
to center the reflection. (It helps to have two people for pri-
mary mirror collimation, one to look in the focuser while the
other adjusts the collimation screws.)
The view through the Collimating Eyepiece should now
resemble Figure 1D. The secondary mirror is centered in the
focuser; the reflection of the primary mirror is centered in the
