Sky & Telescope - March 2024 - 63

To avoid seeing the confusing reflection from the primary
mirror, it's helpful to insert a black plastic bag into the tube
between the secondary support and primary to mask it off.
Once this step is completed, remove the plastic bag.
Insert a peep-sight collimator cap into the focuser and
rack the drawtube all the way in. Peer into the cap's hole and
center the secondary mirror's round reflective surface under
the focuser by adjusting the center-post bolt to move the mirror
towards or away from the primary mirror as needed. You
should see the same spacing all the way around the secondary
in the peep sight. Taping a white piece of paper inside the tube
opposite the focuser makes it easier to make this adjustment.
The goal here is twofold: First, you need to exactly center the
converging light beam in the focuser drawtube to minimize
any vignetting. Second, this also aligns the drawtube to be
perpendicular to the converging beam in order to minimize
tilt errors, which we'll discuss later. This alignment procedure
works for both normal and offset secondary mirrors.
Now, use the three secondary-mirror adjustment screws
to set the tilt of the secondary mirror to center the primary
mirror. You should see an even gap all the way around the
primary, and its retaining clips should be plainly visible. The
secondary mirror is now roughly positioned and ready for
laser collimation refinement.
Remove the sighting cap (as well as the plastic bag) and
insert your super collimator into the focuser. Fine-tune the
Laser
source
Returning
beam
Center of
primary
LASER COLLIMATING Use the reflection from the primary mirror to
see the returning laser beam on your painted mask. Here the beam is off
to the lower right, revealing focuser tilt that must be corrected.
tilt of the secondary mirror using its three screws to center
the tiny laser dot in the exact center of the primary mirror.
If your primary mirror lacks a central mark, you'll have to
remove the mirror cell from the back of the telescope tube
and, using the machinist's scale, locate and mark its very
center. This is essential for accurate collimation. Even if your
scope came from the factory with its center marked, there's
no guarantee it was done correctly. You should verify that it is
before moving on to the next step.
CENTERING THE SECONDARY STALK Use the machinist's scale
to measure the centering of the secondary-mirror holder. Be sure to
measure from the inside of the tube to the center of the secondary mirror
mounting post on each side, then determine the precise length that both
vanes need to be. After adjusting the left and right spider vanes, do the
same for the top and bottom pair.
CENTERING THE SECONDARY MIRROR Use a collimation cap
to see the exact centering of the diagonal mirror. The spacing indicated
by the arrows here should appear equal. The reflection of the
primary mirror has been blocked for clarity. The goal is to capture all
of the reflected cone of light from the primary to minimize vignetting in
your images.
sk yand tele scope .o r g * MARCH 2024 63
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Sky & Telescope - March 2024

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