The optical axis (between the main and secondary mirrors) should be centered in the tube.

A Newtonian telescope can be collimated to meet each of these requirements more or less closely, but as with mechanical adjustments in general, they cannot and need not be met exactly.

If we understand the effects of the separate errors, we can decide on the maximum error tolerances. We can then be sure that the telescope will perform as well as it should, if the collimation is done to within these tolerances. Here is a brief discussion of the effects of the errors:

Error type 1A - The optical axes are separated at focus by a distance D. The eyepiece focus is in the main mirror's focal plane, but at a distance D from its focus.

This is the crucial error for visual use. Images by the paraboloid mirrors of Newtonians can be close to perfect near the focal point, but suffer from increasingly severe coma at increasing distances from it. Coma is an optical aberration that causes loss of contrast and detail resolution. It is approximately proportional to the distance from focus, and inversely proportional to the third power of the focal ratio (f, this is the focal length of the main mirror divided by its diameter).

Any good eyepiece gives a very sharp view near its focus - that is in the center of the field of view. Towards the edge, however, all eyepieces cause more or less unsharpness of star images. This is mainly due to astigmatism (I won't explain that here!) of the eyepiece - it is not the mirror's fault - but shows up worse with a short focus mirror (with a low focal ratio). For most eyepieces, the coma of the main mirror gives much less contribution to the unsharpness.

But if the main mirror focus is in the focal plane away from the eyepiece focus, there will be some coma at the center of the field, where the image should be sharpest, and the image is not quite as clear and crisp as it could have been - particularly when you use high magnification to catch the subtle details of planet surfaces.

Under ideal conditions, the maximum distance from focus where coma barely affects the diffraction image from a star, has been given (Sidgwick) as 0.0036*f³ mm (0.000143*f³ in.). At a distance of 0.0088*f³ mm (0.00035*f³ in.), coma introduces a wavefront error of ¼ wave in addition to any other error present (Sinnott). For high resolution images, the tolerance should be set somewhere between these values (see table below). The tolerances may be relaxed for wide-field photography and for low-power viewing only, and perhaps also for instruments of very large apertures where seeing will limit the possible resolution. Surprisingly, the size of the telescope mirror doesn't come into the calculations, a larger telescope doesn't have a larger "sweet spot" than a smaller. You will notice that short focus telescopes (low f/ numbers) are very much more critical to collimate - that is one price you must pay for the convenience of a short, wide field telescope.

If you have a center spot that happens to be at a distance D from the true optical center, and do a perfect collimation against it, the collimation error 1A is half the distance, or D/2. Thus, the allowable miscentering of the center spot is much less than twice your tolerance!

Date: 2015-12-11; view: 718