Not my words but good ones none the less concerning this thread 8)
DETAIL IN EXTENDED OBJECTS. When you look at any extended object, the telescope image is made up of an arrangement many diffraction disks of a size and spacing determined by the diameter of the objective. The smaller the disks, the more detail you can see, just as more detail is visible in a fine-screen magazine halftone than in a course newspaper halftone. When the telescope image is magnified 13x per inch of objective diameter, the diffraction pattern becomes equal to the resolving power of the eye. This is the equivalent to a 150-line halftone screen. Such a pattern or screen readily allows about 2.5x magnification in order to produce a larger picture and yet not, make the screen pattern too prominent. The pattern or screen is then 60-line, the same as used in newspaper halftones. This is the approximate effect you get at 32x per inch magnification. This is the ideal high power. Definition remains good to about 50x per inch and deteriorates sharply: at 60x per inch, the diffraction pattern has a structure of about 25 lines per inch. Being a picture painted with light disks, you can't view a telescope image and actually count off the lines-per-inch structure. However, the effect of a too-course screen is readily apparent, the picture becoming soft and woolly like a photograph with too much enlargement. REMEMBER: You can use all the power you like when looking at single stars or double stars or open clusters, because what you are looking at is a single diffraction disk, or a pair of disks, or an open cluster of disks. However, when you look at an extended object, you want to see the picture as a whole without making the disks of light which comprise it too prominent.
Keith
PS: Aperture rules but the seeing seldom allows it
