Question-Help!

Roland, Barry, Someone,
When I check collimation on my refractor(TMB APO triplet) I get the doughnut with more or less concentric rings inside and at the center the hole or dark spot. Everything looks like the books say it should look except for one thing. On the outer edge of the dough nut there is a small flat spot in the circle. It is on top when in focus and moves 180 degrees when in out focus. Imagine a perfect circle except for a tiny section that does not curve but is straight like a flat spot on a tire. I would sure appreciate some input on this. BTW I am using a Takahashi Cheshire type collimator.
Respectfully Submitted,
Dan

>>When I check collimation on my refractor(TMB APO triplet) I get the doughnut with more or less concentric rings inside and at the center the hole or dark spot. >>

You're checking collimation with what? On a star? With an eyepiece? With a Cheshire? I'm confused.

RC

So this device relies on you shining a light into
the side of it to illuminate the 45 degree mirror?
Is it possible you are not uniformly illuminating the
mirror? Take the collimation device off the scope
and look over the device -- is there anything cutting
off the edge of the aperture?