When these requirements are met, and we can consider one common optical axis, the following supplementary requirements should also be met:Ģ - The optical axis should strike the optical center of the secondary mirror. 1B - The optical axes should be parallel. To simplify the error analysis, this can be broken down into two parts, giving separate kinds of error if violated:ġA - The optical axes should intersect at the common point of focus. The two optical axes should be coincident, forming one common axis. 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) 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. So if you like, skip to the " End of heavy theory" for some more practical stuff. 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. Here comes some heavy theory - do I really have to read it? This done, the two optical axes are brought together. You first adjust the position and tilt of the secondary mirror to center the (reflected) eyepiece axis on the primary mirror, and then adjust the tilt of the primary mirror to center its (reflected) optical axis in the focuser. In most instruments, the focuser is fixed (or at least not readily adjustable), so it is practical to use the focuser axis as a reference. The main purpose of collimating is to align the two axes to form one common axis. The secondary will also "reflect", or rather deflect, the optical axes - it has an optical center, but no optical axis to concern us. 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 axis of the eyepiece is usually taken as the center of the focuser drawtube. The distance along the optical axis, from the mirror center to the focus, is the focal length. Other stars will form images around the focus, in the focal plane (actually, the focal "plane" is part of a sphere, with its radius equal to the focal length). The light from a star in the exact direction of the primary mirror axis will be reflected and "focused" to a sharp image at the focal point or focus on this axis. For convenience, this is often marked with a spot of paint or tape. The axis of the primary mirror is perpendicular to the mirror at its optical center - for practical purposes assumed to be the center of the circular glass mirror. There are two optical axes in a Newtonian telescope: the optical axis of the primary mirror, and the optical axis of the eyepiece. How are they supposed to be aligned when the scope is well collimated? The tube, in turn, is supported by some mounting that lets you aim it at your chosen celestial object, and perhaps track its apparent motion as the Earth rotates. These optical parts are held in mechanical alignment by a tube of sorts. 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. 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. This is a more or less fancy magnifying glass, used to see the image of the star or whatever else you look at. Commonly, the mirror holder has a center bolt and three screws for adjustment. It can be (more or less easily) moved sideways and along the tube, and it can be tilted (or rotated) slightly. The secondary mirror holder, and often the spider itself, is adjustable. It is used to deflect the light from the primary mirror sideways, so that you can see the image without having your head in the way of the incoming starlight. 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. This is a smaller mirror with an elliptic face (its size is given as the length of its minor axis, i.e.
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