(So, for example, if you have four inch refractor with a focal length of forty inches (F10), you will want an eight to nine millimeter eyepiece.
An F10 four inch telescope has a focal length of forty inches or 1,016 millimeters. Thus, an 8 millimeter eyepiece in a 1,016 millimeter focal length provides a magnification of 127X.
And a point source has the additional distinct advantage of providing a target which is inherently azimuthally symmetrical, which well reveals small amounts of astigmatism. SA is evident due to the fact that you are focusing in the very near field, not at close to infinity.
The point is, you want to select some details which you can only be sure are present if you have to look and peer carefully at the tree or barn or fence or lock. Now, choose a selection of eyepiece or eyepiece and barlow which provides a magnification of 100X per inch of aperture.Increasing the magnification makes the object larger which generally makes details more easily seen but it also dims them substantially, this would seem to be something of an eye test.If one wants to know that the telescope is showing everything that a telescope of that aperture can see, a comparison with some standard would seem appropriate... That requires nearly 3 inches of outward focus travel for a scope with a 1000mm focal length.(I would appreciate feedback of those who know that the aforementioned test does not provide the knowledge of one's telescope it claims to provide.If your knowledge of optics, telescopes, vision, stargazing is such that you know this "simple test" is just wrong or needs to be amended in some way; by all means, please add your comments here.) Edited by Otto Piechowski, 13 April 2015 - PM.
We want to be assured that they optical elements are properly collimated.