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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 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. For 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 7-1) has three large screws
which are used for collimation and three small screws 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 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.
When satisfied with the collimation, tighten the small locking screws.
Take note of the direction the light appears to flare. For example, if it appears to flare toward the three o'clock position in
the field of view, then you must move whichever screw or combination of collimation screws necessary to move the star’s
image toward the direction of the flaring. In this example, you would want to move the image of the star in your eyepiece,
by adjusting the collimation screws, toward the three o'clock position in the field of view. It may only be necessary to
adjust a screw enough to move the star’s image from the center of the field of view to about halfway, or less, toward the
field's edge (when using a high power ocular).
Collimation adjustments are best made while viewing the star's position in the field of view and turning the adjustment
screws simultaneously. This way, you can see exactly which way the movement occurs. It may be helpful to have two
people working together: one viewing and instructing which screws to turn and by how much, and the other performing the
adjustments.
IMPORTANT:
After making the first, or each adjustment, it is necessary to re-aim the telescope tube to re-center the
star again in the center of the field of view. The star image 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 screws are present, it may be necessary to move at least two
to achieve the necessary mirror movement.
Figure 7-2
Even though the star pattern appears the same on both sides of focus, they are asymmetric. The dark obstruction is
skewed off to the left side of the diffraction pattern indicating poor collimation.
Figure 7-3
A collimated telescope should
appear as a symmetrical ring
pattern similar to the diffraction
disk seen here.
