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Celestron Omni XLT 150 Manual

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42
Pick a bright star and center it in the field of the telescope. Study the image of the star while racking it in and out of focus using
an eyepiece that yields 30 to 60 power for every inch of aperture. If an unsymmetrical focus pattern is present, then collimation is
necessary. (If the telescope is properly collimated, the out of focus star image will appear as a concentric ring pattern similar to
that shown in Figure 7-2).
To collimate, the telescope should be on either a motor driven (i.e., tracking) equatorial mount that is approximately polar aligned
or pointed at a stationary star without the motor drive running. Polaris, the North Star, is the perfect collimation star for northern
hemisphere observers since it appears motionless against the background sky long enough to perform the collimation procedure.
Polaris is the last star in the handle of the Little Dipper (Ursa Minor) and its distance above the northern horizon is always equal
to your latitude angle.
Prior to collimating, locate the three (3) mounting screws on the objective lens housing on the front of the tube. (These screws
attach the objective lens housing to the main tube and should not be removed). It may be necessary to remove the lens shade from
the front of the tube to allow easy access to the collimation screws. Next to each mounting screw is a shorter Allen screw
(collimation screw) that pushes against the optical tube to pivot the objective lens housing (see Figure 7-1). In order to make an
adjustment, the mounting screw is loosened while the Allen screw is turned in or out. Then, the mounting screw is tightened.
Only one of the three (3) sets is adjusted at a time. Normally motions on the order of 1/8 turn will make a difference, with only
about 1/2 to 3/4 turn being the maximum required. DO NOT remove or back out the mounting screws more than one (1) to
two (2) turns!
With Polaris or another bright star centered in the field of view, focus with your highest power
eyepiece (i.e., one with the shortest focal length). This includes eyepieces in the 4mm to 6mm range.
The star should be well centered in the field of view of the eyepiece. It may be helpful for two people
working together, while one views and instructs the other which screws are correctly turned and by
how much. Start by loosening one Phillips head (mounting) screw about 1 turn and advancing the
Allen screw to see if the motion is correct. If not, undo what you did and try another set of screws.
After making the first adjustment, it is necessary to re-aim the telescope tube to center the star again in
the field of view. It can then be judged for symmetry by going just inside and outside of exact focus
and noting the star’s pattern. Improvement should be seen if the proper adjustments are made. Since
three (3) sets of screws are present, it may be necessary to move at least two (2) sets of screws to
achieve the necessary lens movement. DO NOT over tighten the outer mounting screws!
Once in collimation, your telescope should not need additional collimation unless the telescope has
been bumped or jarred severely.
Refractor Collimating Eyepiece- Your refractor includes a collimating eyepiece that can help you to
roughly check the alignment of your telescope in the daytime. The collimating eyepiece has a pinhole site that helps you
determine if the optics are properly aligned with the tube. With the focuser racked in all the way and the diagonal removed, place
the collimating eyepiece inside the focuser tube. If the telescope is properly collimated, you should be able to see the entire edge
of the objective lens when looking through the pin hole. If the objective lens appears oval, then it may be necessary to collimate
the telescope as described above.
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The optical performance of your telescope is directly related to its collimation, the
alignment of its optical system. Your telescope was collimated at the factory after it
was completely assembled. However, if the telescope is dropped or jarred severely
during transport, it may have to be collimated. The only optical element that may need
to be adjusted, or is possible, is the tilt of the secondary mirror.
To check the collimation of your telescope you will need a light source. A bright star
near the zenith is ideal since there is a minimal amount of atmospheric distortion.
Make sure that tracking (with an optional motor drive) is on so that you won’t have to
manually track the star. Or, if you do not want to power up your telescope, you can
use Polaris. Its position relative to the celestial pole means that it moves very little
thus eliminating the need to manually track it.
Figure 7-2
A collimated telescope
should appear as a
symmetrical ring pattern
similar to the diffraction
disk seen here.
Figure 7-3
The three collimation screws are
located on the front of the
secondary mirror housing
.
… 42 …


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