Celestron OMNI XLT 102 manual appear, pattern, diffraction

When satisfied with the collimation, tighten the small locking screws.

Figure 7-7

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.

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		Figure 7-8
exactly which way the movement occurs. It may be helpful to have two people	A collimated		telescope
working together: one viewing and instructing which screws to turn and by how	should	appear		as a
much, and the other performing the adjustments.	symmetrical ring			pattern
	similar	to the	diffraction
	disk seen here.

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 of them to achieve the necessary mirror movement.

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