Cheshire disagrees with Laser

Am using a recently acquired Cheshire e.p. and a Laser collimator to align an 8" Discovery PDHQ. They appear to disagree on the secondary tilt setting. When the Cheshire says tilt is correct (full reflection of primary in secondary), the laser hits primary about an inch above the center spot.

When secondary tilt is adjusted so laser hits in middle of center spot, the primary reflection's 10:30 quadrant is truncated in the Cheshire image. In fact, you notice the truncation just looking into the focuser without the Cheshire.

Laser collimation appears to be on; it's a little sloppy in the focuser but very consistent once set screw is tightened. Tried shimming with scotch tape and got the same result. Also tried different 1.25" adapter and got the same results. Spent some time trying to improve centering of secondary under focuser...it didn't seem to help any.

It's cloudy here and will be for awhile; otherwise I'd just trot the thing outside and see which alignment is better. I'm new to Newts. Is my procedure right? Any ideas?
Bob

It sounds like your optical axis is not perfectly coincident with the mechanical axis of your optical elements. One likely source for this problem is a focuser that is not square with the diagonal. I had this problem with my dob until I leveled the focuser and now the cheshire and laser agree. Until you get the two axes aligned, you will chase collimation between the cheshire (optical axis) and the laser (mechanical axis).

BTW--Others may disagree with this, but I found the cheshire will give me good optical collimation, while the laser will give good mechanical collimation. What this means is that my images were fine when I collimated with the cheshire but soft when I collimated with the laser. Only when I got the mechanical axis aligned with the optical axis did the laser give an accurate indication of the optical collimation.

Randy

S&T had an article about a year ago explaining how to modify a Barlow such that when combined with a laser, it delivered the same accuracy as a Cheshire.

Let us know which delivers the best images. My money would obviously be on the Cheshire.
Renato