Rough Collimation

Set up the SCT as usual, insert an eyepiece that yields a magnification of 100 to 150x, and aim it at Polaris or an artificial star. When the star is in the center of the eyepiece field, defocus (either way) until it becomes a donut that covers about 1/4 to 1/2 of the field, and recenter if necessary. Is this donut's hole, the shadow of the secondary mirror , more or less centered? If so, move on to fine collimation. If not, secondary adjustment is required.

Older Celestrons have an orange plastic cover on the secondary mounting . It must be removed to expose the three adjustment screws. This cover is held in place by two plastic tabs inserted into the secondary assembly and is removed by snapping it off. If this cover has never been removed, it may be necessary to pry gently with a small screwdriver until it comes free. Just remember the cardinal rule of telescope maintenance: never force anything. A few newer Celestron models feature a rotating cover that must be turned to reveal the screws. If in doubt, check the manual. Meade secondary holders usually don't have covers, so the screws should be immediately visible. Meade currently uses Allen-head screws that require a small wrench for adjustment. A small Allen "key" may have been included with the telescope, but if not, these tools are very inexpensive and can be purchased at almost any hardware store. Later Celestron SCTs replace these Allen screws with standard Phillips ("cross-point") screws, which are turned with a screwdriver.

A few older telescopes, both Meade and Celestron, have a fourth screw in the center of the secondary mount. Don't touch it. In these CATs, the secondary mirror is attached to the mount via this central screw. Remove it, and the secondary drops onto the primary. Modern CATs use secondary backing plates with three threaded holes for the collimation screws and a central pivot or leaf spring the mirror rides on. The secondary will remain attached unless all three screws are removed. Actually, the Meade LX400 and some other recent Meade models use six screws rather than three or four. These are arranged in three pairs of outer and inner screws. The inner screws provide collimation adjustment; the outer ones hold the secondary assembly to a mounting assembly on the corrector. The advantage is that the secondary can be taken out without removing the corrector plate, a difficult operation on the LX400. Unlike the adjustments on other SCTs, the LX400 collimation screws (and those on a few other newer Meade models) are spring loaded, so there's no need to worry about leaving them too loose at the end of collimation; the spring applies even tension across the whole range of adjustment.

How does turning the collimation screws adjust the aim of the secondary mirror? Since the mirror rides on a central pivot, tightening or loosening the screws causes them to push or pull the secondary mirror backing plate and tilt it and the secondary in or out, causing collimation to change and the target star to move in the field of the eyepiece.

Now to the task at hand. Looking in the eyepiece, it should be fairly obvious which direction the donut hole needs to be "moved" in order to center things up. What may not be obvious is which screw needs to be turned to move the dark spot the correct way. Don't waste time trying to figure out which screw will move the spot which way. Instead, just pick a screw and tighten it a little. Wrong way? Try another screw. In the rough stage of collimation, turning a screw by small amounts won't have a dramatic effect on the donut hole. If the secondary shadow doesn't seem to move, turn the screw a little more, but resist the temptation to turn by large amounts. The secret to successful collimation, even in the rough stage, is working slowly and methodically.

When it's clear which screw (or combination of screws) needs to be tightened to center the secondary shadow, slowly turn that screw, stopping frequently to peep through the eyepiece and moving the scope to re-center the donut in the ocular (with the telescope's slow motion controls or hand control) after each adjustment until the secondary shadow is centered. Always adjust the secondary by tightening the screws. Only if a screw is snug—hand tight—and can't be turned easily should an "opposite" screw or screws be loosened to continue moving the dark spot in the proper direction by tightening the original screw some more.

Occasionally a telescope is so far out of collimation that it's difficult to get a clear image of the donut. That is usually the result of the owner having turned one or more adjustment screws by large amounts in the wrong direction. To get a scope like that back in the ballpark, stand about 6 feet from the corrector and look down the front of the tube. Do the mirrors' reflections look concentric? Or is the reflection of the secondary off to one side? If it is, adjust the collimation screws until it is roughly centered. That will get the secondary back to the point where a rough collimation can be performed.

Once the donut hole is centered, stop. Don't tighten any screws. Nothing needs to be locked down. As long as the rule "Never loosen a screw unless its opposite number can't be tightened easily" is followed, the secondary will be perfectly secure.

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