Aligning the secondary to be "straight"

[OneOfOne (Trevor)]

I have been trying a few things with the scope lately to get some confidence in adjusting screws and moving things in the optical train. At the weekend I checked the centering of the secondary under the focus tube and decided it was too close to the primary, so I loosened the centre screw and adjusted it to bring the secondary holder closer to the end of the tube. Of course, the mirror "went limp". As I was adjusting it to a more central position, a question struck me. How critical is it that the plane of the secondary be at right angles to the optical axis of the focusser/eyepiece? If it is perfectly square to it, the reflected light from the primary will be reflected along the optical axis of the eyepiece and you should get a good image.

If the mirror is not square, the light will be reflected at an angle to the optical axis. I expect this would cause some sort of result in the image. I guess, if you think about it, one side of the secondary will be closer to the eyepiece than the other "skewing" the light path. Perhaps, because the distance is much shorter than primary to secondary it is much less important?

It seems that there are all these things to check the secondary is in the middle of the focusser, the primary is square to the optical axis. Adjustments of the primary may require moving it half a bees d... in one direction. But any instructions that mention the secondary seem to discount it as being particularly important.

Most seem to indicate that measuring the centering of the secondary in the tube is good enough with a ruler. As for the squareness of the surface to the optical axis, most guides seem to indicate that "the secondary mirror should appear to be round and in the centre of the field". But how well can the eye determine if something is "round". Is there a way of checking this alignment. I also find it difficult to hold the secondary in position while tightening the centre screw and watching it doesn't move in the focusser, is there a trick to it?

[Starkler (Geoff)]

Originally Posted by OneOfOne

most guides seem to indicate that "the secondary mirror should appear to be round and in the centre of the field". But how well can the eye determine if something is "round". Is there a way of checking this alignment.

In practical terms, You want to make the secondary appear round and centred under the focuser while blocking the view of the primary with a sheet of paper. Then check again after removing the paper whilst taking note of centering the reflected view of the primary mirror.

The most important thing is to adjust the tilt of the secondary so that the optical axis is aligned with the focuser tube axis. If it is not, then the focal plane at the eyepiece will also not be at right angles, but rather tilted.

This may cause the focus point for different points in the field to differ.

You can check that the optical axis are lined up with a laser, or by checking with a combo cheshire tool that the cross hairs line up with their reflections whilst centred on the primarys centre spot.

If your optical axis is aligned as above, any physical offset of the secondary from optimum will merely result in the illuminated field being offset from centre.

Before i discovered that my secondary wasnt centred properly ( it was offset in the 90 degree plane to the focuser), I could never get the crosshair reflections to stack. This is more important than centering the secondary under the focuser. Ultimately, you are aiming for both.

[OneOfOne (Trevor)]

That sounds like a good way to determine "squareness". I haven't actually tried checking that the reflections of the cross hairs line up...it is probably the easiest way. I was wondering if there was some way of making the alignment visible. I will give it a try tomorrow...I am looking forward to spending some time tomorrow afternoon to take the focusser off and move it back a tad as I think it is a couple of mm too far down the tube. BTW, it will give me a chance to avoid seeing the footy...avoided it at school and still do!

[Don Pensack]

The secondary can be too far up the tube, too far down the tube, and slightly uncentered under the focuser, not to mention slightly off-round, and still be collimated. There are many optical configurations where these other positions are PURPOSELY chosen.
The centering of the secondary under the focuser (essentially offsetting the secondary toward the primary, but not away from the focuser) is to result in even edge illumination for the light cone from the primary.
If you draw a cross-section of a reflector on graph paper, you will see that the primary mirror's beam does not strike the exact center of the centered secondary mirror--it strikes a little lower than center (toward the primary) on the secondary mirror. This is because the light cone gets narrower the farther it is from the primary, and it is narrower when it strikes the part of the secondary farther from the primary.
That means the edge of the secondary farthest from the primary fields none of the axial light beam from the primary, and more of the off-axis light than the part of the secondary closest to the primary. So we drop the secondary closer to the primary to evenly illuminate the edge of the secondary (which is what happens when you center the secondary in the focuser). The center of the beam from the primary now strikes the secondary slightly away from the exact center of the secondary (offset, essentially, up the secondary away from the primary), but we achieve better edge-of-field illumination.
If the secondary is slightly off-center, does it matter? No. It just means (and it probably won't be visible) that the edge-of-field illumination won't be exactly the same all the way around.
Can the secondary be rotated slightly around its axis and still be collimated? No. The return reflection of the primary mirror's center mark will not line up with the crosshairs in the sight tube. So the sight tube will correct the misalignment even if the mirror is slightly out of perfect position.
If you're really fussy and want to have everything perfect, a telescoping sight tube, a good cheshire, and a good autocollimator will allow collimation to the small thousandths of an inch.
I recommend Catseye tools. There are also some excellent tutorials on collimation on Jim Fly's site. see: www.catseyecollimation.com
Don

[Roger Davis]

And have you checked that the axis of the tube of the focuser is perpendicular to the optical axis? That both the mechanical axis and the optical axis coincide?