optical telescopes

basic principles

refraction & reflection

Snell's Law

collecting light

the amount of light gathered per unit time

amount of time it takes to collect a given amount of light

focal length, image scale and focal ratio

image scale

how many arcseconds of sky fall on each pixel

focal ratio

focal / length

surface brightness and image brightness

surface brightness

brightness per unit area

how bright an extended object appears in the telescope depends on

its surface brightness

the surface brightness of the background

the focal ratio of the telescope

image brightness

light gathering power divided by the image area

types of telescope

refractors

compound

only lenses

only mirrors

catoptric

dioptric

the both

catadioptric

aberrations

the deviations from the ideal deffraction pattern at the focal plane

off-axis aberrations

vignetting

field of view

the angular size of the area of sky seen in the eyepiece