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Collimate - Noob question

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I'll leave the experts to tell you, but my Celestron may as well have come as a kit! The secondary mirror was barely hanging on the central adjuster! It needed full collimation. Quite disgusting really as some youngster could've received this and had no idea how to collimate, not that I did either as I had to learn, but I'm very good with technical bits and got it done.  

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Hi imballinger 

yes, SCT scopes do require collimation!

but it is very INFREQUENTLY required as they hold collimation very well.

you can tell when it needs doing by doing the following.

- select a bright star

- put your HIGH power eyepiece in, x300 is a good magnification but put in the lowest mm eyepiece you have.

- centre the star in the view

- defocus by a QUARTER turn of the focus knob

= you should see diffraction RINGS appear around the star. If the rings are even and  nicely circular then collimation is fine. STOP

if the rings are UNEVEN and tighter on one side then collimation is required...


there are THREE SMALL SCREWS on the secondary mirror in the middle of the FRONT of your scope. These are collimation screws. Tightening them by VERY SMALL amounts 1/10 turn changes the mirror angle and alters the diffraction rings you see.


if your scope needs collimating there are plenty of good guides on this forum, search for "collimating SCT"



Edited by alanjgreen
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  • 1 month later...
On 03/01/2017 at 17:11, imballinger said:

This may be a stupid question, I have a Celestron Nexstar evolution 8, does it need collimating?

Hi - as a new owner of your telescope I can recommend thread I started about Jupiter being blown out. Not for its author sadly but for all great advice I got on this site re. Collimation for the scope.

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