# Collimating a dobsonian telescope

**URL:** <https://boards.straightdope.com/t/collimating-a-dobsonian-telescope/677279>\
**Category:** Factual Questions\
**Created:** [December 27, 2013, 9:54am UTC](https://boards.straightdope.com/t/collimating-a-dobsonian-telescope/677279 "2013-12-27T09:54:59Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Willcross](https://avatars.discourse-cdn.com/v4/letter/w/b2d939/32.png) [@Willcross](https://boards.straightdope.com/u/Willcross)\
**Post date:** [December 27, 2013, 9:54am UTC](https://boards.straightdope.com/t/collimating-a-dobsonian-telescope/677279/1 "2013-12-27T09:54:59Z")

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I’ve had my dobsonian telescope for about a month now and have had it out several nights. When looking through the collimating cap, everything seems to be fine.

My question is about performing a star test. I have a pretty bright star centered in the view and I defocus it. Instead of a series of concentric circles (like all the online examples) The star just turns into a single halo. Not smooth at all, but very jagged. Imagine a high frequency sine wave that was cut and wrapped into a circle.

I get clear views of Jupiter and Saturn at 200x, the moon looks clear and stars are usually pinpoints of light and not distorted. Could my star test results be bad seeing, or user error? Any experience with this?

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**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\
**Post date:** [December 27, 2013, 10:16am UTC](https://boards.straightdope.com/t/collimating-a-dobsonian-telescope/677279/2 "2013-12-27T10:16:28Z")

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> **[Astro Babys Guide to Collimation](https://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/)**
>
> The toughest job a beginner has to learn is Collimation of a Newtonian telescope. This Simple to use guide is used by beginner/experienced alike.

has examples of various types of errors…

The one which closest match is the one that says “there is nothing wrong” - well the problem is the atmosphere. You might also have air currents/density issues - you would be in very cold air outside ? you need to let the telescope become just as cold as air it is in !