Ultimate Newtonian Telescope Seconadr Mirror

The Ultimate Newtonian
The Secondary Mirror Holder Described and Explained
and
A simple and practical collimation procedure described

The secondary mirror holder in this telescope is different in that instead of using three screws to adjust the elliptical flat mirror it makes use of the fact that the face of the secondary mirror of a Newtonian is not set at right angles to the incoming light path as is the primary mirror but at an angle of 45 degrees. This very fact, ignored by everyone else, makes the adjustment of the Newtonian secondary essentially different than the primary. The secondary mirror optical axis is really at 45 degrees to the central optical axis (90 degrees off the face of the mirror) so attempting the adjust the mirror with three screws aligned along the central axis is illogical and leads to many of the common problems with collimation. Rather than try and tip-tilt the mirror around a skewed axis a better plan is to design a holder that has three adjustment functions: rotation about the central axis, longitudinal motion, and tilt at right angles to that axis, in other words, adjustment of the 45 degree angle only. By rotating the mirror about its axis and tilting the mirror, all required collimation adjustments are easily made. If a threaded rod and central threaded hub is used the required longitudinal motion (accurate to the nearest thread spacing - plenty good enough) is achieved by rotation.

How many times have you struggled with a conventional three-screw holder, and, after much frustration and bad language, found yourself looking at a secondary mirror skewed off to one side under the focuser? That's because the basic adjustment design process is incorrect and illogical and does not follow right angle relationships. All the adjustments with this holder are both at right angles to the central optical axis and focuser axes, so that can't happen.

The basic concept can be seen here. The left hand picture shows the hinged mirror holder plate and its adjustment pull screw. Between the faces of the holder, where the screw is, there is compressible rubber so that only a simple turn on the screw with an Allen key is required to hold everything in place. At night, in the dark, you don't want to be fooling with multiple screws and things becoming loose. At the right can be seen how the mirror is rotated by simply turning the mirror with your hand.

The central threaded rod is kept from freely rotating but has enough friction to allow easy adjustment by using a Belleville washer and a friction adjustment knob on the outer end of the rod.

This arrangement is easily manageable in the dark with a long Allen wrench through the end of the tube.

Collimation Procedure

The procedure for adjusting the mirror is different than with a conventional holder.

Begin the daytime. If you are assembling a new holder and spider begin here, otherwise, if you are just adjusting under normal conditions, start at 2. Before installing the holder in the spider hub, using the tip-tilt adjustment screw, adjust the angle of the secondary to the central threaded rod until it is as close to 45 degrees as you can reasonably get. Finer adjustments can be made later. Screw the holder into the central hub from the back to front and then put on washers and spring washer and friction nut.
Extend the focuser with the 1.25" adapter in place out to its fullest extent. This gives you a good long sighting tool to insure accurate centering of the diagonal mirror.
Tighten the friction nut against the spring washer until the secondary mirror shaft with stay put but still turn relatively easily. Rotate the mirror about its axis until it sits exactly under the focusing tube when looking from front to back. You should be able to see the primary mirror in the secondary
Rotate the mirror a tiny bit as needed until the primary mirror is centered from left to right. The secondary mirror should now be sitting exactly under the end of the focusing tube, if it is not, adjust or shim the tilt of the focuser relative to side of the tube until it looks exactly at the secondary mirror when the secondary is at the best possible adjustment position. This step of centering the mirror exactly under the tube and shimming where necessary is critical and should not be ignored.
Adjust the secondary mirror tip-tilt screw until the primary is centered within the secondary mirror.
Go back and review the previously described adjustments again until everything is as perfect as you can get it.
Adjust the primary mirror until the reflection of the secondary mirror is exactly in the center of the secondary mirror or offset off to the rear of the telescope very slightly.

Images above show what the picture through the focuser looks like. Note the very small offset. The diagonal shadow is slightly high. Also note that the black center hole of the sight tube is centered with respect to the primary mirror, not the diagonal.

At f/6, that's about it folks. You may be done. Now, with faster mirrors a bit more is required but with a slower telescope like this we're about done until dark. After the telescope is set up at night put a bright star in the center of the field and defocus. The black spot of the secondary mirror should be exactly in the center of the expanded star disk. If not, adjust the primary until it centers - and that's about as good as it can get. While there can be other early errors that can lurk undetected, if you make the proper and accurate adjustments outlined above they are extremely tiny and their impact is unnoticeable. If you want to use a tool use a film can with a 1/4" hole drilled through (if you can find them nowadays) just to center your eye in the focuser tube. I use a piece of plastic 1.25" drain tubing about 3" long with a 1.25" fender washer with a 1/4" hole in the center glued to one end. If you think you can't center the secondary mirror in the middle of the primary mirror it's not important because any errors there are the ones you will correct at night with a defocused star.