## Schmidt Newtonian and Maksutov Newtonian Collimation

Amateur astronomers who have collimated a "straight" Newtonian already know now to collimate SNTs and MNTs. You haven't? Don't worry; it's harder than adjusting an SCT, but is easy enough to accomplish once the process is understood. A lot has been written about Newtonian collimation, but it's really a simple procedure that can be broken into three steps: center the reflection of the primary mirror in the secondary mirror, center the reflection of the secondary in the primary, and do final tweaking on the diffraction rings of a slightly out of focus star.

Step one, centering the primary in the secondary, is done by observing the reflection of the primary in the secondary mirror while looking into the focuser with no eyepiece installed. Take a look. The reflection should resemble what's seen in Figure 8. If the reflection of the primary mirror is not centered, adjust the secondary mirror via three or four screws on the exposed surface of the secondary holder. Collimate as with an SCT, tightening screws to move the reflection of the primary mirror until it's centered. Note that some MNTs and SNTs have small covers on their secondary holders that must be removed for access to the adjustment screws.

In rare cases it may be impossible to center the primary reflection in the secondary because the secondary and holder have rotated with respect to the focuser. It's easy to adjust the rotation of the secondary assembly in a standard Newtonian, but not so easy with an SNT—the corrector complicates matters. If the mirror only needs to be rotated a small amount, maybe 1/4-inch or so, it's permissible to loosen the

Figure 8. (SNT Collimation) Schmidt Newtonian collimation is harder than SCT collimation, but not much harder, and very necessary.

Figure 8. (SNT Collimation) Schmidt Newtonian collimation is harder than SCT collimation, but not much harder, and very necessary.

corrector retaining ring and turn the whole thing—corrector and secondary. If more rotation is required, forget that. Rotating the corrector may cause optical problems. The corrector will have to be removed for the secondary's rotation to be adjusted (usually via a central screw on the secondary holder). For many if not most SNT/ MNT owners, that is a good time to call the scope maker.

In step two—still with no eyepiece in the focuser—check that the secondary reflection is centered in the primary. Center it by adjusting the scope's primary mirror collimation screws. On the Meade SNTs and many MNTs, there are six bolts on the main mirror cell. Three are locking bolts that must be loosened before adjustments can be made. Adjust the primary slowly and carefully, doing plenty of checking until the secondary reflection is centered. Once it is, lock the primary mirror by tightening the lock screws. Don't tighten each locking screw all the way at once. Instead, turn each screw small amounts until all three are tight, checking the secondary's reflection as you go. Tighten or loosen one or more lock screws slightly if collimation has been thrown off.

A Newtonian collimation tool, a "Cheshire" eyepiece , can help a lot when adjusting the primary mirrors of these scopes. It's essentially a peep sight that fits into the focuser in place of an eyepiece, and, if the center of the primary mirror is marked (as many are with a small sticker), the Cheshire will make primary adjustment very easy. Line up the sticker with the "dot" that is the reflection of the Cheshire's peephole, and mirror adjustment is done.

The final act in the Schmidt/Maksutov Newtonian collimation drama is a star check. If the diffraction ring bulls-eye is skewed, fix it as described in the SCT fine collimation section. The only difference is that the primary mirror on an SNT or MNT is adjusted at this stage rather than the secondary, as on an SCT.

Optical Cleaning SCT users get off easy when it comes to collimation. There's only one element, the secondary mirror. We (and other CAT owners) also luck out when it comes to optical cleaning . For most users only one surface ever needs attention: the exterior surface of the corrector plate. Cleaning correctors is pretty much a no-fault operation. A Newtonian user always runs the risk of scratching a primary or secondary mirror's delicate coating during cleaning. The SCT's corrector plate, in contrast, is like a camera lens; it's reasonably tough and will survive some wrong-headed cleaning.

Still, the last thing a CAT user should do is clean optics that don't need it. The usual advice to novices regarding cleaning is don't! Though correctors are tough compared to mirrors, there is still the chance of doing more harm than good. Don't set out to clean the corrector because of a speck of dust or two. There will always be a little dust on a telescope that's actually used, and a few dust motes on a corrector will not hurt a thing. A speck of dust will most assuredly do less harm than a big scratch left on the lens as a result of one cleaning too many. This really can't be overemphasized: leave the corrector alone unless it's dirty. It would do novices good to see how dirty the optics of professional telescopes are allowed to get before they are cleaned.

The time will come when an SCT or MCT will need a gentle corrector cleaning, of course. It may have accumulated a fingerprint or two at the last star party. In the spring many areas are plagued by airborne pollen that can potentially damage coatings and should be removed. It's not unheard of for birds to undertake "bombing missions" against CATs, too. What's the secret to safely cleaning the corrector plate? Treat it with respect like, a fine camera lens. It is a fine lens that is beautifully polished and multicoated for optimum performance.