A Center Donut for your Telescope Mirror

DISCUSSION: When collimating your reflecting telescope, I recommend a 2" barlowed laser collimator. These are available from a couple of different sources, and are an excellent way to bring the optics into perfect alignment. After an interesting discussion with Bob Fies of Alcoat Aluminium (Coatings), I've learned that using the paper "donut" that is commonly found for loose-leaf notebook paper is "NOT" the best idea. It is the glue, says Bob, that is the problem. Over time, as one cleans the mirror, the glue tends to liquefy and can contaminate the aluminum surface of the coating. Bob says it's better to use a "magic marker". The magic marker is easy to clean away and reapply, when necessary. Plus, there are no problems with contamination.

With that in mind, I started using this process. The pictures that follow should be helpful to those of you who wish to adopt this method of center marking your telescope mirror..

If anyone has other ideas, and if you're willing to have them posted here, please email me. If you have one, a picture would be help.


Whatever means you employ to put a center "donut" on your telescope mirror, your laser collimator with barlow will return a shadow of the donut onto the center of the collimator. Centering the donut ensures an accurately collimated optical system


Photo, astrozap.com, with permission.

Here are the few items you'll need to place a center donut onto your telescope mirror:
  • Staedtler professional SKETCH MASTER template, model # 977-135.
  • Sharpie "twin tip" marker, # 32162.
  • Paper template with center hole.

I found these items at my local Staples store.

I use a brown wrapping paper, but a paper shopping bag from you local super market will work. Cut a circle the same size of your telescope mirror and fold into quarters. Cut a hole in the center with a pair of scissors. When the paper is unfolded, the hole will be in the exact center.

Place the paper template on the front surface of your telescope, making sure it is centered.

Carefully place the Staedtler SKETCH MASTER template over the center of the mirror and paper template. The Sketch Master is transparent enough so you can center the circle you wish to use. I used the 5/16" size circle.

The final step is to scribe a "donut" or circle onto the center of the telescope mirror. Hold the marker vertical. You don't need to press very hard. It's a good idea to practice another surface first. I use the back of the telescope mirror if I can. That way, I get a good sense of the technique needed.

Let the ink dry for a couple of minutes and you're ready to go.

Get a 2" laser with barlow:

Howie Glatter with "blug"
Kendrick Astro Systems

Of course, these collimators can be purchased from the many telescope "dealers" out there. Scope City, OPT, Astronomics and Anacortes are just a few.

As a final note, one can use a 1.25" or 2" laser collimator and a barlow (the one used with eyepieces). These work just as well. I like the 2" barlow because it tends to fit the focuser better.



For more information about your requirements:
--Dennis Steele
--Tel: 650-315-6578 (anytime)
--Email: densteele@dobstuff.com



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