The main, or primary, mirror.

This is the large mirror in the bottom of the tube, with a concave, aluminized face figured to an extremely accurate paraboloid surface. It concentrates the light from a star into a sharp image - not really a point, but a diffraction pattern with a small circle of light surrounded by small, faint rings.

It is held in some kind of mirror cell, fancy or simple, that rests on 3 set screws. By adjusting these screws, you can finely adjust the tilt of the main mirror, this is an important part of collimation (you only need to adjust 2 of them - it might be wise to leave the third in a middle position). Often there are 3 extra screws for locking the mirror cell in place, once it is adjusted. It may look something like this:

The secondary, or diagonal, mirror.

This is a smaller mirror with an elliptical face (its size is given as the length of its minor axis, i.e. its "width"). It is suspended by a spider with one or several vanes inside the tube near its opening, and the face is at 45 degrees to the tube. It is used to deflect the light from the main mirror sideways, so that you can see the image without having your head in the way of the incoming starlight.

The secondary mirror holder, and often the spider itself, is adjustable. It can be (more or less easily) moved sideways and along the tube, and it can be tilted (or rotated) slightly. Commonly, the mirror holder has a center bolt and three screws for adjustment.

The eyepiece

This is a more or less fancy magnifying glass, used to see the image of the star or whatever else you look at. It has a certain focal length, and with several eyepieces of different focal lengths, you can select the magnification (often called "power") that you want. The focuser is where you put the eyepiece, it has a drawtube that holds the eyepiece and can be moved a little bit in and out, as needed to "focus" to get the sharpest view.

These optical parts are held in mechanical alignment by a tube of sorts. The tube, in turn, is supported by some mounting that lets you aim it wherever you want in the sky, and perhaps track its apparent motion as the Earth rotates.

How are they supposed to be aligned when the scope is well collimated?

There are two optical axes in a Newtonian telescope: the optical axis of the main mirror, and the optical axis of the eyepiece. The axis of the main mirror is perpendicular to the mirror at the optical center - for practical purposes taken as the center of the circular glass mirror. For convenience, the is often marked with a spot of paint or tape (more about that later). The light from a star in the exact direction of the main mirror axis will be reflected and "focused" to a sharp image at the focus on this axis. The distance along the optical axis, from the mirror center to the focus, is the focal length. Stars will be focused to sharp images in the focal plane, at or near the focus. (Actually, the focal "plane" is part of a sphere, with its radius equal to the focal length).

The axis of the eyepiece is usually taken as the center of the focuser drawtube. The secondary mirror reflects the incoming light to the side of the tube, and here the focused image forms, and is seen with the eyepiece. The secondary will also "reflect", or rather deflect, the optical axes - it has an optical center, but no optical axis to concern us.

The main purpose of collimating is to align the two axes to form one common axis. Normally, you do this by adjusting the position and tilt of the secondary mirror, and the tilt of the main mirror.

Here comes some heavy theory - do I really have to read it?

Glad you asked - if you read this for the first time, you will probably find it a bit difficult to chew and swallow in one bite. So if you like, skip to the "End of heavy theory" for some more practical stuff. But I am sure the theory will make you understand the practical things better, and you may go back to read it any time later.

A systematic background:

I propose the following system of requirements for collimating Newtonians, and the corresponding errors, to facilitate understanding of the process (for illustrations, see the section on the corresponding errors)

The first and foremost requirement is:

Date: 2015-12-11; view: 808