The Newtonian Reflector

Collimation may be the dirtiest word in amateur astronomy. Some amateurs fear collimation more than an IRS audit. It shouldn't be that way. Collimation is the key to getting the best images from your telescope and it's not that difficult. This section takes you step-by-step through the collimation process for Newtonian reflector and Schmidt-Cassegrain telescopes using plain language and easy-to-understand illustrations. Get the most from your telescope. Collimate it!

Collimation is a process. You'll get the best results if you remain consistent in how you approach the task. Also, the right tools are essential. One of the most important tools is a center mark on the primary mirror. This is especially important for telescopes with f/8 and faster focal ratios. The thought of marking the primary mirror usually produces feelings of revulsion in a new scope owner. However, this step is essential to accurate collimation and will not degrade the telescope's performance one iota. The center of the primary mirror falls within the shadow of the secondary mirror. Therefore, this section of the primary is not used to collect light and form the image you see in the eyepiece. Marking the center of the primary mirror will not reduce it's optical performance.

I recommend you use one of those loose-leaf paper hole reinforcers to mark the center. I use the following procedure:

• Carefully draw a square on thin packing tissue. Each side of the square should be equal in length to the diameter of the primary mirror.
• Cut out the square and fold into quarters. The intersection of the folds marks the center of the square.
• Place a hole reinforcer on the tissue at the center.
• Cut around the hole reinforcer.
• Lay the square over the primary mirror and tape in four places to the side of the mirror. Do not affix tape to the aluminized surface of the primary.
• The hole in the center of the tissue square will be over the center of the mirror. Use a black Sharpie pen (felt-tipped marker) to mark the center of the primary mirror with a small dot.
• Carefully place a hole reinforcer on the mirror around the center mark.
• Remove the tissue square.

There, you're done. Install the primary in its cell and replace in the optical tube. You'll find instructions on how to collimate your Newtonian on the next page.

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Revised: February 11, 2002 [WDF]