ETX TECHNICAL TIPS Last updated: 16 November 1998

Collimation Test

From: Larry Janowicz (ljanowicz@metrohealth.org)

On a calm steady night (no twinkling stars!) slip a higher power eyepiece into the ETX and point to a bright star (at least 3rd or 2nd magnitude). Make certain your scope has adapted to the outside temperature and that you are not looking over a warm roof, driveway, or car hood with a still warm engine. With a 10mm or shorter focal length eyepiece you should be able to see at least one diffraction ring surrounding the star at a close distance. If you move the focus in either direction from "dead on" you should observe a dark center (the secondary mirror) surrounded by alternating light and dark rings that expand in size as you move further away from focus. If these rings are perfectly round and concentric around the center dark spot your instrument is collimated. These intrafocal and extrafocal rings also tell other tales about the quality/alignment of the optics but that is another and lengthier topic. D. Suiter has a great book about star testing scopes. New users should refrain from attempting to collimate their own ETX. You are much safer sending it back to Meade. I have seen one "factory fresh" ETX that was not collimated correctly when it was received. Once collimated this instrument should not require frequent further adjustments since the push-pull screw method used to align the optics is fairly robust.

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