Telescope Optics Explorer
Three modes: single telescope analysis with an optical ray diagram, side-by-side comparison of two telescopes, and an eyepiece finder that shows results for seven standard eyepiece focal lengths. Six presets from 10×50 binoculars to the Hubble Space Telescope.
Mode
Presets
Optical Diagram
Optical Properties
200 mm | f/6.0 | 25 mm EPReference Guide
Resolution and Dawes Limit
The Rayleigh criterion gives the smallest angle a telescope can resolve, set by diffraction at the aperture.
In arcseconds, multiply by 206 265. For visible light (550 nm) through a 200 mm aperture, resolution is about 0.69″.
The Dawes limit is an empirical formula for resolving equal-brightness double stars.
Magnification and Field of View
Magnification equals the objective focal length divided by the eyepiece focal length.
The true field of view (TFOV) seen through the eyepiece equals the apparent field (AFOV, typically 50°) divided by magnification. Higher magnification gives a narrower view.
Maximum useful magnification is roughly 2× the aperture in mm. Above that, you magnify atmospheric blur more than detail. Minimum useful is D/7, where the exit pupil equals the eye's dark-adapted pupil (7 mm).
Light Gathering and Limiting Magnitude
A telescope collects light in proportion to the area of its aperture compared to the dark-adapted eye (7 mm pupil).
A 200 mm telescope gathers about 816 times more light than the naked eye, allowing you to see objects roughly 7.3 magnitudes fainter.
The limiting magnitude formula estimates the faintest star visible through the scope under ideal conditions.
Choosing Eyepieces
The exit pupil is the diameter of the light cone leaving the eyepiece and entering your eye.
If the exit pupil exceeds 7 mm (dark-adapted pupil), light is wasted. If it falls below about 1 mm, the image becomes uncomfortably dim and eye floaters become visible.
