Refractor Collimation Question.

[Franklin]

I have an Omni XLT120 which I recently upgraded with a Skywatcher dual-speed crayford focuser (Wow, what an improvement!). So I thought I would check the focuser alignment and the collimation of the objective lens. Using a laser collimator without the diagonal in the focuser and an aperture mask with a centre mark I was happy and relieved to see the laser beam pass directly through the centre of the objective, even when the new focuser is rotated. All good there. I then replaced the lens cap and put a small cheshire eyepiece in, again without the diagonal. Pointing the cheshire window at a light source the primary and secondary reflections revealed a figure of eight pattern through the peephole. Out of collimation. After a little experimentation tweaking the three sets of collimation screws on the lens cell I managed to get the cheshire reflections to be perfectly concentric through the peephole. Great, my focuser is square on and my objective is collimated. All is good in the world. Then I put in the diagonal. Now the laser is about an inch higher than the centre. My 2" mirror dagonal has no collimation screws so I placed a 0.2mm thick teflon shim along the lower edge of the diagonal mirror to tilt the mirror very slightly. Success, the laser is now central. But when I put the cheshire in the diagonal the figure of eight reflection is back. Now, my question is this. Do I re-collimate the objective with the newly tilted mirror in the diagonal or do I leave the collimated objective as it is? When using the cheshire without the diagonal the light source just bounces off the back of the lens but when in the diagonal the cheshire light source is reflected off the diagonal mirror then the back of the lens and then the diagonal mirror for a second time. As I have tilted the mirror slightly with a shim to centralize it, is it this misalignment that is causing the cheshire reflection to appear wrong, even though the objective is collimated? I'm starting to confuse myself now. Help!

[jonathan]

My advice: just use the scope on a clear night and see how it looks to your eyes.

[AstroMuni]

My suggestion is get it all aligned with the Cheshire and then do a star test to check the airy disk pattern. Laser collimators can have their own alignment errors which need to be corrected before using them.

Just out of interest, did you have the same problem before upgrading your focusser?