Fine Collimation

Rough collimation will improve a scope's performance somewhat, but not enough to support high power observing. Fine tuning is needed for that. To do a fine collimation, center Polaris and defocus just slightly until a series of diffraction rings

Figure 6. (Fine Collimation) If the 'bull's-eye' diffraction rings are not concentric, there's collimating to do!

similar to what's shown in Figure 6 appears. It doesn't matter which "side" of focus the telescope is on; if one side looks clearer than the other, use that. In order to make the rings large enough to show collimation errors clearly, pump up the power. Use a minimum of 250 x for fine collimation. If Polaris can't be used for some reason, choose a similar star that's at least 30 degrees above the horizon. If diffraction rings are not obvious no matter how high the magnification, it's possible atmospheric seeing is too poor to allow fine collimation or that the scope is not yet adequately cooled.

Center the star as precisely as possible (a crosshair eyepiece may help), and when the SCT is properly defocused, examine the bulls-eye formed by the star's airy disk and diffraction rings carefully. Are the rings concentric or does the bull's-eye look squished on one side? If the rings are skewed, there's adjustin' to do.

To make adjustments, follow exactly the same procedure as during rough collimation: tighten screws by small amounts until the rings are perfectly concentric. If a screw is snug, loosen the opposite screw(s). One trick you can use is placing the squished side of the bull's-eye at the edge of the field. To collimate, then turn the screw or screws that move the bulls-eye to the center of the field. If the rings are still not perfect, move the bulls-eye back to the field edge and center it with the collimation screws again, repeating this procedure until the diffraction rings are as concentric as you can make them.

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