Newtonian Reflector Collimation

You’ll need a good carpenter’s or framer’s square. You also need to temporarily remove the diagonal mirror and its spider. First use the carpenter’s square to check that the tube’s front opening is square with the tube axis. Then insert the laser collimator in the focuser and turn it on.

Put one leg of the carpenter’s square inside the tube, flush the square up against the end of the tube, and mark where the beam from the collimator hits the square inside the tube. The beam should hit the same point on the carpenter’s square no matter whether the square is held on the side of the tube with the focuser or on the side opposite the focuser. Tip the focuser forward and backward as needed to make the beam hit the square at the same point on both sides of the optical tube, shimming the focuser as needed tightening it down. This will assure that the focuser is at right angles fore and aft to the optical path.

Next, cut a strip of cardboard the exact length needed to span the tube’s inside diameter and mark the exact center of the strip. Place it in the tube at right angles to the focuser, and adjust the focuser from side to side until the laser beam falls precisely on the center mark. Shim the focuser as needed and tighten it down. This will assure that the focuser is aligned properly from side to side on the optical path. Reinstall the diagonal mirror and spider.

The holographic collimator allows precise positioning of the diagonal mirror on the eyepiece axis without resorting to marking the center of the diagonal as some recommend. The diagonal should not be marked because, unlike the primary, its center is in use. The diagonal is positioned by moving it to center the projected grid pattern on it, or by centering the shadow of the diagonal within the grid pattern that’s projected on the tube wall behind the diagonal. This produces the proper “offset” towards the primary.

Next, the angular alignment of the diagonal mirror is adjusted so that the beam strikes the center of the primary mirror. With the single beam laser collimator, you will need a dot or “donut” placed on the center of your primary as an aid to precise beam centering. A donut is preferable, as a completely opaque dot will not allow reflection of the beam, preventing primary mirror alignment. However, a dot made with a permanent felt-tip marker will transmit enough light to allow collimation. Don’t worry about putting a dot on your primary mirror. The center of the mirror is not in use because it’s in the shadow of the diagonal. With the holographic collimator, the center dot is not necessary, as you can center the grid pattern on the edges of the mirror.

Next, the primary mirror is adjusted so that the beam retraces its path, and returns to the laser opening in the front surface of the collimator. The returned beam can be seen striking the face of the collimator, and the primary is adjusted to move this spot to the center of the collimator. The best way to view the returned beam impact on the collimator face is from the front of the telescope, looking down the tube, by double reflection in the primary and diagonal mirrors. The primary adjustment can also be done by making the up- and down-going laser beam’s impact points on the diagonal coincide.

There is an additional method of primary adjustment with the holographic collimator that takes advantage of the fact that a telescope will work in reverse to turn the diverging rays from the hologram into a parallel beam. The holographic pattern can be projected a great distance from the scope, and will remain constant in size. The primary can be adjusted by observing the pattern projected by the telescope on a screen or wall at right angles to the telescope tube, and centering the projected grid pattern that’s seen in the projected aperture. This procedure makes solo collimation easy. For it to work accurately, you must first align the focuser with the main tube, and center the primary mirror within the tube.