Dedicated equipment
Collimation cap: A collimation cap, or sight tube, is a plug that fits in your reflector’s focuser. It has a small central hole. Although it’s mainly used to ensure that your secondary mirror is aligned with respect to your focuser, it can also be used to ensure correct alignment of the secondary and primary mirrors.
Cheshire: This collimator adds a set of crosshairs to a sight tube. Many amateurs I know who own reflectors think a Cheshire is the best way to go in terms of proper collimation.
Laser collimator: Even though a laser collimator has advantages and disadvantages (the optics can be slightly out of alignment and yet look perfect), it’s always been my preferred “quick” method. It sits in the focuser and shines a beam that ideally reflects onto the hole it came from.
But if the telescope is so far out of collimation that the return beam doesn’t even hit the secondary mirror, it’s useful to point it at a nearby wall to help make the initial adjustments. In a pinch at night, I’ve aimed it at a friend. (If you do this, be certain their eyes are closed because laser light can harm vision.)
Before using any laser collimator, make sure it’s trued. Turn it on and roll it along a flat surface, projecting the beam on a wall. If it scribes a straight line, you’re good to go. If not, there’s usually a way to adjust it.
Star test: A star test is free, easy, and works with any type of telescope. The only disadvantage is that not all nights have good enough seeing (atmospheric steadiness) to ensure that the diffraction rings are centered around the star.
Collimating an SCT
Make sure your mount is on and tracking. Insert an eyepiece that will give you moderate to high magnification, and center a fairly bright star, but not the brightest you can see. If you typically observe with a star diagonal, make sure it’s in place. If you’re not using a motorized mount and you’re in the Northern Hemisphere, you can collimate on Polaris.
Defocus slightly until you see a doughnut of light. If your scope is out of collimation, the doughnut’s hole won’t be centered. On the corrector of most modern SCTs, you’ll find a plastic circle, which is the back of the secondary mirror’s mount. There you’ll find three adjustment screws. If you don’t see them, there’s probably a cover to remove.
You’ll need a Phillips screwdriver or an Allen wrench. The screws tend to be small, so be certain to use a tool that fits properly. Because using metal tools so close to optics makes me nervous, I recommend replacing the screws with a set of knobs for safer and easier adjusting. Make small corrections, one screw at a time, look through the eyepiece, and observe the change in the alignment. Continue to adjust these screws until the hole is centered in the doughnut. Once it is, focus a bit more until you can see the diffraction rings, and use them to fine-tune the collimation. Ideally, you should need to adjust only two of the screws.
Some SCTs, particularly older ones, suffer from mirror shift, where the primary flops slightly to one side of the cell as the telescope crosses the meridian. For the best results, collimate on the side of the sky where you’ll be observing. If you pass the meridian, you should check collimation again. The process might seem slow at first, but you’ll gain speed with practice.
There are tools that let you collimate an SCT during daylight, but they are fairly expensive. They can be a good purchase if your time is valuable, or if you are part of an observing group that can split the cost.