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Celestron Omni XLT 150 Manual

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46
Night Time Star Collimating
After successfully completing daytime collimation, night time star collimation can be done by closely adjusting the
primary mirror while the telescope tube is on its mount and pointing at a bright star. The telescope should be set up
at night and a star's image should be studied at medium to high power (30-60 power per inch of aperture). If a non-
symmetrical focus pattern is present, then it may be possible to correct this by re-collimating only the primary
mirror.
Procedure -- Please read this section completely before beginning.
To star collimate in the Northern Hemisphere, point at a stationary star like the North Star (Polaris). It can be found
in the north sky, at a distance above the horizon equal to your latitude. It’s also the end star in the handle of the
Little Dipper. Polaris is not the brightest star in the sky and may even appear dim, depending upon your sky
conditions. If you are in the Southern Hemisphere, point at Sigma Octantis.
Prior to re-collimating the primary mirror, locate the collimation screws on the rear of the telescope tube. The rear
cell (shown in Figure 7-6) has three large thumbscrews which are used for collimation and three small thumbscrews
which are used to lock the mirror in place. The collimation screws tilt the primary mirror. You will start by
loosening the small locking screws a few turns each. Normally, motions on the order of an
1
/
8
turn will make a
difference, with approximately a
1
/
2
to
3
/
4
turn being the maximum required for the large collimation screws. Turn
one collimation screw at a time and with a collimation tool or eyepiece see how the collimation is affected (see the
paragraph below). It will take some experimenting but you will eventually get the centering you desire.
It is best to use the optional collimation tool or collimating eyepiece. Look into the focuser and notice if the
secondary reflection has moved closer to the center of the primary mirror.
With Polaris or a bright star centered within the field of view, focus with either the standard ocular or your highest
power ocular, i.e. the shortest focal length in mm, such as a 6mm or 4mm. Another option is to use a longer focal
length ocular with a Barlow lens. When a star is in focus it should look like a sharp pinpoint of light. If, when
focusing on the star, it is irregular in shape or appears to have a flare of light at its edge, this means your mirrors
aren’t in alignment. If you notice the appearance of a flare of light from the star that remains stable in location, just
as you go in and out of exact focus, then re-collimation will help sharpen the image.
Figure 7-6
Primary mirror. Large thumbscrews
are for collimation and small
thumbscrews are for locking the
mirror in
p
lace.
… 46 …


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