Collimating a Newtonian Reflector

Begin by squaring the focuser to the light path. Lay the optical tube on the floor and prop with pillows until a carpenter's level indicates the tube is level. Then, place the level alongside the focuser perpendicular to the length of the optical tube to check alignment. The focuser should be at a 90 degree angle to the tube. If the focuser is seriously out of square, you may see the secondary mirror offset to one side of the optical tube when looking through the focus tube as illustrated at left. Another visual indication of a misaligned focuser is a reflected image of the focus tube in the secondary mirror showing the outside of the tube. The reflected image of the focus tube should only show the circular bottom edge of the tube.

The secondary mirror should appear centered within the focus tube as illustrated at left. Looking through the focus tube at the secondary mirror's outline, rack the focuser in-and-out until the primary mirror appears to fill the secondary. The secondary mirror should appear centered within the focus tube. If the secondary appears offset, either toward or away from the primary mirror, move the secondary until it appears centered. Does the secondary look circular in shape. If it has an oval outline, the secondary needs to be rotated until its shape is perfectly circular.

A film canister lid is an inexpensive but useful tool in this process. Simply drill a hole through the center of the lid and place the lid over the focus tube opening. Looking through the hole aligns your eye to the light path, which should be centered within the focus tube.

I'm not a big fan of expensive collimation tools. Collimation has been done successfully for hundreds of years without laser collimators and the like. I did plunk down about $20 for a collimating eyepiece available from Orion Telescopes and Binoculars. The current version costs a little more but I think you'll find it very useful.

The primary mirror should appear centered within the secondary mirror. The illustration at left shows this. There should be three or four screws on the backside of the secondary mirror mount. Adjust these screws to change the primary mirror's appearance within the secondary. It's a trial-and-error process. Make small adjustments to one screw and check the effect before moving on to the next screw. Use the retaining clips holding your primary mirror in its cell to define the outside edge of the mirror. There edge of the reflected image of the primary should meet the outside edge of the secondary mirror.

The final step is to center the reflected image of the collimation tool. This is done by adjusting collimation screws on the primary mirror cell. As with the screws you adjusted to fine tune the secondary mirror, this is a trial-and-error process. Make a small adjustment to one screw and check the effect. The illustration at left shows the view you get when looking through the focus tube at the secondary mirror of properly collimated Newtonian.

Fast Newtonians with focal ratios of f/6 or less will look a bit different than the illustration. The reflected image of the collimation tool within the reflected image of the secondary will appear offset toward the front of the optical tube. Optically slower Newtonians have an offset, too. It's just not as dramatic. You can learn more about this process by visiting Mel Bartels' excellent Newtonian Collimation page.

You can check the collimation by doing the star test.