I was on the BBC last week recording an interview which will go out this week. One of the topics discussed (in addiition to how to take a pee in the dark) was how to collimate a telescope.
Two types of telescopes – Refracting and Reflecting
There are two basic types of telescopes, refracting telescopes (which use lenses) and reflecting telescopes (which use mirrors). Either type of telescope can become mis-aligned, usually through the telescope being knocked. Putting it back into alignment (technically called collimation) is not a difficult process, and should be the kind of thing anyone can do with a little patience.
There are many expensive gadgets available to help you align a telescope, but none of them is really necessary. The easiest way to do it is to simply point the telescope at a star and then de-focus the image. You will end up with a blury image of the star. When the telescope is not collimated this blurry image will look distinctly asymmetrical.
In a reflecting telescope you would see something like this:
In a refracting telescope it will look something like this:
In either case, the mis-alignment of the telescope is shown by the asymmetry of the out-of-focus star’s image. You then need to adjust the optical components’ alignment to collimate the telescope. Usually it is only the primary mirror (in the case of a reflecting telescope) or the objective lens (the front lens) in the case of a refracting telescope, which will need adjustment. There should be screws which you can turn to change the tilt of the mirror or lens, allowing the necessary adjustment to be made.
What you are trying to do is get the out-of-focus image to look symmetrical, so it looks like the following:
or, for a refracting telescope
A very good article on aligning a reflecting telescope can be found here.
Even if you do not think you have knocked your telescope, it is worth checking your alignment on a regular basis, and certainly if you have not used your telescope in a while. Using the technique I have outlined above of looking at an out-of-focus star, checking the alignment can be done in a matter of minutes, so there is no excuse not to do it regularly!