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I own an orion laser collimator which I had not used in a very long time. I decided to collimate an old Celestron NextStar 114 and I realized that the laser did not produce a circle but an oval. Apparently the telescope is roughly collimated using a collimation cap. There seems to be a corrector less in the focuser which I don't dare to disassemble. Could this oval be produced by this corrector or perhaps the laser collimator needs collimation too or maybe there is a problem in the secondary mirror?

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  • $\begingroup$ What happens if you shine the laser on a flat surface? You should get a small round circle. $\endgroup$ – Dr Chuck Aug 25 '17 at 9:23
  • $\begingroup$ @DrChuck that is correct. It's only when I put it on the focuser and it's reflected on the primary mirror. $\endgroup$ – PbxMan Aug 25 '17 at 9:26
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This is a slightly tricky one. For a conventional Newtonian reflector, shining a laser collimator back over the secondary to the primary should present a small dot on the primary - the task then is to adjust the secondary to center the dot on the primary - there is hopefully a mark on the primary.

This is a reasonable general primer: https://starizona.com/acb/basics/using_collimating_newt.aspx

HOWEVER, the 114 is special, having a Barlow lens embedded in the bottom of the focuser. This would have the effect of spreading the laser light back into the telescope, leading to a projected shape (a disk or oval) on the primary. It's not really possible to conclude whether the collimator is collimated from this.

Reading the manual ( http://www.nexstarsite.com/Manuals.htm ), the manufacturer recommends a simple sight test: remove the eyepiece, and look down the focuser, adjusting the primary to center your eye (hence not worrying about the secondary alignment).

Alternatively, you should be able to remove the Barlow lens by racking the focus tube all the way out and unscrewing it (do remember which way around it is mounted ...), which would allow the use of the laser.

Simplest solution would be to use a low tech Cheshire collimator, I think. These forum discussions might give useful background:

http://www.astronomyforum.net/celestron-nexstar-telescope-forum/94220-celestron-114eq-collimation-woes.html

https://www.cloudynights.com/topic/117354-nexstar-slt-114-viewing-collimation/

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First of all, what do stars look like through the scope, particular just inside and outside focus? If they are oval, then read on.

I'm not familiar with this telescope, but I suspect the most likely sources of the problem are 1. The corrector lens you mention (which is a relatively unusual thing to have) is distorted. If you can't remove this, then you are stuck with it. 2. The adjustment screws on the secondary mirror may be too tight, thereby distorting the shape of the . Try loosening each screw by a small amount, say 1/8th of a turn, and see if this improves things. Make sure the telescope tube is horizontal when you do this in case the secondary slips off its mount.

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  • $\begingroup$ Definitely agree that you should orientate the tube horizontal when doing this, in case the secondary assembly drops away - this is easy to put back but scary when it happens ! $\endgroup$ – MartinV Aug 25 '17 at 13:10

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