Collimating a Schmidt Cassegrain

The only adjustment on a Schmidt-Cassegrain telescope is the alignment of the secondary mirror, located in the middle of the corrector plate. To adjust it, tighten or loosen any of the three adjusting screws (Figure 5.8); the secondary mirror is balanced on a pivot between them.

To do the adjustment, aim the telescope at a star high in the sky, throw it out of focus, and then adjust until the image is a concentric doughnut (Figure 5.9). Rather than try to figure out in advance which screw to turn, just try one of them and see what it does; if it's the wrong one, undo what you did and try another. The image will move when you turn a screw; re-center it before judging the collimation, because the center of the field is where you want the collimation to be best.

In a pinch, if no star is available, you can use a spotlight reflected off a ball bearing on the other side of a large room. Don't try to perform a complete star

Figure 5.8. Collimating a Schmidt-Cassegrain telescope with an Allen wrench.

Figure 5.8. Collimating a Schmidt-Cassegrain telescope with an Allen wrench.

Figure 5.9. Collimating a Schmidt-Cassegrain. View a star out of focus and adjust until the secondary shadow is centered. Then switch to a higher power eyepiece and try again. Finally, go to Figure 5.13(d) and eliminate any remaining error.

test this way, though, since the telescope is not focused at the distance for which it was designed.

After making a noticeable improvement, bring the image closer to focus or switch to a higher-power eyepiece and try again. When you think the collimation is perfect, compare the in-focus image at high power to Figure 5.13(d). Chances are, you can see and correct a small remaining error. The ultimate accuracy you can achieve will be limited by the steadiness of the air.

Work with very small increments. A tenth of a turn usually makes a noticeable difference. When making larger changes, the rule is to tighten when possible, but if a screw is already tight, loosen the others instead. Never tighten a screw with the full force of the wrench; the screws should be finger tight or slightly

Schmidt Cassegrain Collimation
Figure 5.10. Using a ruler to check that the secondary holder is centered. Note thumbscrews installed in place of original collimation screws.

tighter. Never loosen all three screws at the same time, or the secondary mirror may come off its support.

The adjustment normally requires an Allen wrench, but you can install thumbscrews (visible in Figure 5.10) so that you can collimate by hand. One supplier of suitable thumbscrews ("Bob's Knobs") is Morrow Technical Services, 6976 Kempton Rd., Centerville, IN 47330, U.S.A., When installing them, never remove more than one screw at a time, or the secondary may fall off its support.

All this is not as complicated as it sounds. I can collimate a Schmidt-Cassegrain in two or three minutes, and even a beginner should finish in ten minutes. Much time is saved by not trying to predict which screw to turn; just trying them is a lot faster.

Is the secondary centered?

If collimation seems unduly difficult or if some astigmatism (Figure 5.14(b)) remains after your best efforts, take a ruler and see if the secondary holder is actually in the center of the corrector plate (Figure 5.10). On the NexStar 5 and possibly other telescopes, the plastic mount of the secondary mirror is held in place by tension in an oversized hole in the corrector plate. The corrector plate itself may also be loose in its mount. Either element can shift sideways, producing a telescope that is seemingly impossible to collimate. This is a relatively rare problem, but I've seen it in two telescopes.

The cure is simple - just push the secondary holder back into position. You can move it laterally by applying gentle pressure with your thumb and fingers. Centering it to within 0.5 mm should be good enough. It doesn't much matter whether you are moving the secondary holder or the whole corrector plate; whatever is slightly loose will move, and slight decentering of the corrector plate is not serious as long as the secondary mirror is centered.

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