Secondary Mirror in and out ajustments

[Catanonia]

Can someone tell me what moving the secondary mirror in and out with the centre screw on a newtonian does to colimation please.

I know it will move the focuser point of aim towards and away from the centre of the secondary mirror, but what effect does that have on collimation ?

[Merlin66]

If the secondary mirror is large enough....not much effect on collimation...the return image (light cone) just moves up/ down the surface of the mirror.

HOWEVER, I did find that it can cause issues with vignetting and shadowing.

Let me explain something first:

We all call these secondary mirror "ellipticals" as all the diagrams show them as being a 45 degree cut section of a cone (of light) and the surface shape of such a section is indeed an ellipse. (By definition!)

Have you ever thought how an actual secondary mirror is cut to shape?? Certainly NOT on a CNC router running in a conical/ elliptical path; it's done by using a cylinder cutting through (or a variation) a piece of glass inclined at 45 degrees....the resulting shape is a 45 degree cut though a cylinder NOT a cone....So what? I hear you say...

Well, when you superimpose the cylinder section on an ellipse section what to you see??

It's wider at the middle and narrower at the ends... in a scope this means that it will "shadow" the primary in the middle and not catch all the light at the ends ( somewhere about a 12% nett loss!!!)

So, if you move the secondary up/ down this issue becomes much worse and lots of light is lost....

Not a primary collimation problem but a vignetting/ shadowing problem.....

[barkis]

And in short focal length scopes in particular, it is why the secondary needs offset away from the focuser,

as the mirror end of the elipse, is catching the reflected light at a wider point in the beams upward path to the secondary.

If it was central on the optical axis, light would bypass it.

Ron.

[Catanonia]

If the secondary mirror is large enough....not much effect on collimation...the return image (light cone) just moves up/ down the surface of the mirror.

HOWEVER, I did find that it can cause issues with vignetting and shadowing.

Let me explain something first:

We all call these secondary mirror "ellipticals" as all the diagrams show them as being a 45 degree cut section of a cone (of light) and the surface shape of such a section is indeed an ellipse. (By definition!)

Have you ever thought how an actual secondary mirror is cut to shape?? Certainly NOT on a CNC router running in a conical/ elliptical path; it's done by using a cylinder cutting through (or a variation) a piece of glass inclined at 45 degrees....the resulting shape is a 45 degree cut though a cylinder NOT a cone....So what? I hear you say...

Well, when you superimpose the cylinder section on an ellipse section what to you see??

It's wider at the middle and narrower at the ends... in a scope this means that it will "shadow" the primary in the middle and not catch all the light at the ends ( somewhere about a 12% nett loss!!!)

So, if you move the secondary up/ down this issue becomes much worse and lots of light is lost....

Not a primary collimation problem but a vignetting/ shadowing problem.....

ah that would explain the quite bad vignetting on my 1st test image in the DSO section.