Refractor and Cheshire collimation

[AlistairW]

Hello,

I read that it is possible to use a Cheshire eyepiece to check collimation of a Refractor. But, I am not sure how this really works, or indeed what Cheshire would be appropriate. I think you do the following :

> Insert Cheshire in to the drawtube

> Put lens cap on OTA

> Shine very bright LED on to side of Cheshire (or sunlight)

> Look down the Cheshire to see donut rings (the idea being they should be inside on anther, and look a bit like a donut).

If that is correct, what sort of Cheshire could I consider getting, as I have also read that people have to remove the "crosshairs" from certain Newtonian based Cheshires in order to get this technique to work.

Thanks

Alistair

 

 

[Merlin66]

Yes, that's the technique. Any Cheshire will work.

If you add a collimated laser to the Cheshire and cover the aperture with a paper mask which has a center spot the laser will hit the middle when the focuser is square to the optical axis.

 

[John]

Your process is good for the objective lens tilt.

I also use a well collimated laser collimator to check that the axis of the focuser is true with the axis of the objective (sometimes they are not !). This is done before testing the tilt of the objective with the cheshire.

I use a low cost plastic cheshire with no cross hairs for the objective tilt testing. It also serves pretty well for collimating my 12" dob as well !