Aberrator 3.X
Generates images that show the effects of several types of aberrations (distortions) that influence the quality of a telescope's performance. You basically input the values of a given amount of parameters and the program simulates eight tests, which produce images depicting the effect: startest; intra/extra focal; planet; animation; chromatic; PSF/MTF; turbulence; and aperture.
See a screenshot.

TelOptiCulator 3.08
Input a certain amount of data (focal length, aperture, eyepiece...) and the program calculates the magnification; focal ratio; true field of view; maximum amplification power and theoretical resolution of a telescope. Also has a tool to compare the size of the field of view with the one of the moon.
A real must have for all those owning a telescope, or in the process of choosing which one to buy. User friendly display. See a screenshot.

Telescope Optical Parameters Calculator [NEW ADDITION]
A simple utility to calculate various telescope characteristics both related with the theoretical performance (given the aperture and f/ratio values) and eyepieces: « eyes = surface area of objective divided by area of 5mm human eye pupil; Magnitude = limiting magnitude of dark adapted eye under very dark, clear skies, using an unobstructed objective of the given size. Assumes unaided eye can perceive Mag 6.2 stars; Res (arcseconds) = Dawes Limit theoretical resolving power of diffraction limited, unobstructed objective; Low Useful X = lowest useful magnification. Determined by power at which the exiting light cone will just fit in a fully dialated 7mm pupil; Low Pract. X = lowest practical magnification. Determined by power at which the exiting light cone will just fit in a normally dialated 5mm pupil; Hi Pract. X = magnification at which unavoidable diffraction effects begin to be apparent, limiting further resolution. Roughly 35X per inch of objective aperture; Hi Useful X = magnification at which Airy disc is obvious. Further magnification produces no additional information or benefit. Useful only for close double stars. 60X per inch of objective aperture; F.L.(mm) = focal length of eyepiece, in millimetres. A.F.(deg) = Apparent Field of view, measured in degrees. The angle of view the eye sees when presented with the magnified True Field; Mag (X) = Magnification power. Calculated by: Objective Focal Length divided by Eyepiece Focal Length. Usually a very good approximation of angular magnification; T.F.(deg) = True Field. The actual angular space included in the view. Much smaller than Apparent Field; Brightness = Relative surface brightness of magnified view. The image is made brighter by the increased light gathering power of the objective, yet it is made dimmer by the spreading effect of magnification. A 5mm eye pupil is used as the basis for this calculation. Any value greater than 1.00 (one) is meaningless; Exit Pupil = the diameter of the light-cone exiting the eyepiece. Measured in millimetres.»

Important notice: the program seems to suffer from the known decimal separator bug (in some countries Windows uses a coma instead of the dot). If you get a «floating point» error, just replace 7.5 by 7,5 in the eyepiece list.

See a screenshot.

Newt 2.5
Assists the design of Newtonian telescopes. Allows to quickly test several parameters and displays the effect graphically on a telescope's model. Ideal also for those willing to understand the functioning of a reflector scope of the Newtonian type. Simple, attractive and small. See a screenshot.

OSLO LT 6.X
Professional optical design program. Splendid for specialists, useless for most of us. See a screenshot.

Other freeware programs in this category (links to the homepages)

OmegaLab

Eyepiece

Modas - Modern Optical Design and Analysis Software