Limit of Resolution
Diffraction by a circular aperture (such as a telescope objective) results in point source being imaged as a central maximum surrounded by circular rings. Rayleigh's criterion says that the limit of resolution of a telescope when looking at a double star is given by (1.22 x wavelength/D) radians (about 4.56/D arc-seconds in the visible if D is in inches). At this limit the maximum of the diffraction pattern of one star will fall on the first minimum of the other star.

Magnification with eyepiece
The magnification of a telescope is only useful for extended objects (e.g. the moon, planets, nebulae, galaxies), not unresolved objects such as stars (though magnification does apply to the angular separation of two stars). It is most easily calculated from the ratio of the effective focal length of the telescope to the focal length of the eyepiece (m = Fscope /feyepiece ). Typical eyepieces are 25 mm, 12.5 mm, and 6 mm. With a telescope of 1 meter focal length these eyepieces would give magnifications of about x40, x80, and x170 power. This means that objects such as the moon would appear 40, 80, or 170 larger in diameter (and hence they would appear to be 40, 80, or 170 times closer).
Minimum and Maximum Useful Magnification
The pupil of a typical human eye has an opening about 5mm in diameter in subdued daylight. The pupil may contract to as little as 2.5 mm in bright light, and it may open to 8 mm when the eye is dark adapted. Magnifications smaller than values equal to the telescope's diameter in cm. will result in a bundle of light larger than the pupil, thus loosing image brightness. Magnifications larger than about 10 times the aperture in cm. will result in a bundle so small that the image quality will suffer. Example: The RHO 46 cm. scope can use magnifications between about 50x and 500x to good effect. Since the focal length is about 480 cm., thus means eyepieces of about 10 mm (480x) and 100 mm (48 x). Note that eyepieces are generally not available with focal lengths longer than about 50 mm (100x with the 46 cmm. telescope).
Field of View
In addition to magnification the field of view of an eyepiece is important. The field of view is a function of the optics of the eyepiece itself and its magnification which is a function of the telescope focal length. Typical eyepieces have field of view ranging from 40� to 65� or more. The intrinsic eyepiece FOV must then be divided by the magnification to get the effective field of view at the telescope. A 25 mm eyepiece on a 1 meter focal length telescope has a magnification of x40. This will yield a 1� field of view if the eyepiece has a 40� intrinsic field of view