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Suppose the aberrations are *just* below detection at a moderate exit pupil, but are made visible at a smaller exit pupil. Think of doing a star test test with an artificial, point source.While the image will look 'softer' at the higher power, the detail revealed *must* be no less than at the 'cleaner' lower power. This kind of visual test using other than a point source is dependent on target contrast (and brightness), and when optical quality is better than mediocre does not necessarily well differentiate between diffraction and aberrations (spherical, primarily.) A point source is rather superior because one is not subject to the blending of image units of scale of order the Airy disk. Unless the aperture is very small, the star test is misleading.(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.127 divided by the aperture of the telescope; that is, by 4, is nearly 32X per inch of aperture.) Now look for some very small, almost invisible detail on the bright surface; say, a thin crack with two almost invisible specks below the crack and a tiny notch above the crack.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.

Into your scope, place a good quality eyepiece which will magnify around 30X per inch of aperture.I don't know how the math works out but I do know that for some scopes, the Newtonian for example, that correction is perfect only at infinity...Personally, I am not so worried about seeing everything there is to see in an individual telescope because I know that no matter how perfect the scope might be, a decent quality larger scope will show more... Jon Note that focusing relatively nearby is to use a target at a conjugate much nearer than the nominal optical infinity telescopes tend to be corrected for.Differential refraction due to mixing of air 'air parcels' having differing temperature is what makes a nearby target desirable--even necessary.But depending on aperture and focal length, one must observe the minimum target distance to avoid imparted spherical aberration.

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