Secondary mirror axis dimensions

[Dave_D]

Ok this has me really puzzled. I just measured the major and minor axis of my secondary (with a digital vernier) and the major axis length is longer, by almost 3mm to what an ellipse major axis should be to present a circle at 45 degrees.

Now obviously, using a cheshire or webcam to align the secondary to present a circle to the focuser will put the secondary at an angle other than 45 degrees.

Is there some mystic bit of knowledge about secondary size that i haven't found on google or has this secondary been manufactured at the wrong angle?

[Peter Drew]

Sounds like it has been edged to the wrong angle. Optically this doesn't matter as long as the mirror is large enough. And flat!. 

[Moonshane]

the OOUK secondary on my 6" f11 is not elliptical either and I cannot get a circle. As Peter says, it matters not a lot really.

[Dave_D]

the OOUK secondary on my 6" f11 is not elliptical either and I cannot get a circle. As Peter says, it matters not a lot really.

funnily enough, it's also an OOUK scope (GX250)

i added the outlines of true 45 degree ellipses to my secondary template and it collimated easily. you can see the physical outline by measurement of the secondary (white) and the true 45 degree ellipses (red). Doesn't look much but it's enough to alter the angle by a few degrees if the secondary is aligned using a sight tube or webcam by using the outer edge of the mirror.

[Moonshane]

as I understand it secondary alignment and shape affects the contrast and sharpness very little if at all, it just ensures all the light is collected from the primary and sent up the drawtube and that any diffraction is minimised. it is the alignment of the primary to the optical axis that makes the most difference to the fine details.