Once fine tuning is complete an SCT is ready to take on most observing assignments, but there is a final step for observers engaged in demanding pursuits such as high-resolution planetary imaging. In-focus collimation is done by observing
Poor Collimation Good Collimation
Figure 7. (In-focus Collimation) If the seeing is good enough, an in-focus collimation can be performed to get the scope dialed-in precisely.
the precisely focused image of a star and its first diffraction ring. The requirements for performing in-focus collimation are high power (300 x and more) and steady seeing. The slightest turbulence will make a star's in-focus diffraction ring and Airy disk smear together into a blob.
If the seeing is right, center Polaris in the field and focus up at high power. A tiny disk, the Airy disk, surrounded by one bright and (maybe) one or more dim rings (Figure 7) should be visible. Take a close look at the first diffraction ring. Is it complete and unbroken? If not, adjust the collimation screws by small amounts until it is complete around the star as in Figure 7b. This is very critical work. You might sometimes find it necessary to wait for the heat waves left in front of the corrector by your hand after adjusting the secondary to dissipate before you can see the first diffraction ring clearly again!
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