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from Astronomy Technologies, 680 24th
© 2009 by Astronomy Technologies
Specifications, features, and descriptions are effective 12/1/2009, but are subject to correction and/or modification without notice and/or obligation.
Scope in collimation
Scope out of collimation
Diffraction pattern
Diffraction pattern
Rough focus
Rough focus
Sharp focus
Sharp focus
APPEARANCE OF A STAR DURING COLLIMATION
Patterns have been exaggerated for clarity
............
A.-
C.-
B.-
➞
➞
➞
➞
.......
A.-
Screw 3 is loosened;
Screw 2 is tightened; Screw
1 is tightened slightly; the
resulting star motion is shown
by the center arrow
B.-
Screws 1 and 3 are
loosened; Screw 2 is
tightened; the resulting
star motion is shown by
the arrow
1
1
1
2
3
2
2
3
3
C.-
Screw 3 is loosened;
Screws 1 and 2 are tightened
evenly; the resulting star motion
is shown by the center arrow
➞
FORCE VECTOR DIAGRAMS FOR VARIOUS
SECONDARY MIRROR SCREW ADJUSTMENTS
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in the illustration on the previous page. Each pair consists of
a smaller black screw and a larger chrome screw. These must
be adjusted in tandem. As you loosen one, tighten the other
in each pair to adjust the tilt of the optical axis in relation to
the secondary mirror. This procedure will require only micro-
adjustments, if any. When properly aligned you will see a
concentric outer white circle around the perimeter of your
view through the Cheshire eyepiece and all circular light and
dark elements will be concentric.
Once the optical axis has been collimated, recheck the
secondary mirror collimation and tweak as necessary, then
confirm the optical axis collimation one last time.
Star Testing:
For optimum imaging performance, perform
a star test to confirm the accuracy of your collimation. The
star test relies on your eye and an out of focus star for
collimation, rather than a Cheshire eyepiece. Seeing conditions
will affect the end result, so it is somewhat more difficult than
collimating indoors.
Install all three extension rings between the scope’s rear
cell and the focuser. Using the 1.25” compression ring adapter,
insert an eyepiece directly into the focuser drawtube and
visually center and focus on a bright star at a reasonably high
magnification. Do not use a star diagonal in the system and
be certain that the focuser tension and drawtube lock knobs
are tightened firmly after focusing. Choose a star close to the
zenith rather than at the horizon to minimize atmospheric
distortions.
The diagram at the top of the next column illustrates the
appearance of collimated (top) and out of collimation (bottom)
images of the star being examined. The top left image is the
diffraction pattern in a collimated scope. The center and right-
hand images show what the star looks like when roughly
focused and sharply focused. The bottom row of images show
the same sequence through an out-of-collimation scope.
If collimation is needed, begin by placing a bright star in
the center of a low to medium power eyepiece field (again
without using a star diagonal). Defocus the image until it is
about the apparent size of a dime or nickel held at arm’s
length. This will show the diffraction pattern, which should
look like a bull’s-eye with the circular shadow of the secondary
mirror holder in the center, as shown in the illustration in the
next column. If the shadow of the secondary is not precisely
in the center of the diffraction rings, adjust the collimating
screws to tilt the secondary mirror until the shadow of the
secondary is centered in the diffraction pattern and the
diffraction rings are concentric.
Always make adjustments to the collimating screws in tiny
increments, only a fraction of a turn at a time. The image of
the secondary shadow will move in the direction of the
collimating screw that is being tightened. If the secondary
shadow needs to be shifted in a direction between two screws,
those two must be tightened to make the image shift in that
direction, while the single screw on the opposite side should
be loosened. As each adjustment is made, the secondary
shadow will move off center. Recenter the star’s image in the
field before making the next adjustment. You need to keep
the star precisely centered in your field of view while
collimating, which is critical to avoid false negatives.
Refer to the diagrams below, which show the direction the
star image will move when different combinations of collimating
screws are loosened and tightened. In all cases, the star image
needs to be shifted in the 3 o’clock direction. The screws that
must be adjusted depend on the orientation of the three
collimating screws in relation to the desired star movement
direction.
Repeat the collimation procedure several times, using
successively higher power eyepieces, until you are sure the
collimation is exact. Finally, after the final adjustments have
been made, make sure that all of the collimating screws are
snugged down tightly and evenly to ensure that the collimation
will hold for many trips out into the field.
Figure 3: Optical axis
out of collimation
(not to scale)
Viewing aperture is centered in
Cheshire’s reflective surface
End of optical tube
and secondary mirror
are not centered in end
of focuser drawtube
