Small f/number, “fast” system, little depth of focus, tight tolerances on placement of components Large f/number, “slow” system, easier tolerances, nearly parallel rays through filter F/number = focal length ÷ aperture Also, reciprocal of “relative aperture”

Same primary mirror, but very different f/numbers

Primary is f/2.63, but system is f/31.2 !!

Spherical aberration—focal length depends on radius where ray strikes mirror/lens.

Note circle of least confusion, the “best” focus Image of a point source is usually a bright dot surrounded by a halo of light Effect on extended image is to soften the contrast and blur the details

Chromatic aberration

The primary “monochromatic” aberrations are: Spherical aberration Comatic aberration (“coma”) Astigmatism Petzval field curvature Distortion

Coma Affected by where the light rays hit the lens/mirror Same focal plane, but different magnification Coma varies with the shape of the lens, and the position of any apertures/stops. Size of coma patch varies linearly with its distance from the axis

Astigmatism Image of a point source is not a point, but takes the form of two separate lines Between the astigmatic foci, the image is an elliptical or circular blur Circle of least confusion increases in diameter as the object moves further off-axis Image loses definition around its edges

Field curvature

Schmidt camera

Distortion

Example: If we increase aperture diameter by 50%, and reduce field of view by 50%, then ‘y’ is 1.5x the original, ‘h’ is 0.5x the original Coma increases by Curvature reduced to 0.25 of previous amount Blurs due to astigmatism or curvature will of of original size y = distance of image from optical axis h = distance of object from optical axis