Celestron AstroMaster LT Instruction Manual page 22

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):
Figure 7-1
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.
Prior to re-collimating the primary mirror, locate the collimation screws on the rear of the telescope tube.
The rear cell (shown in Figure 7-1) 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 mir-
ror. 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 following 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.
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