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