Small f/number, “fast” system, little depth of focus, tight tolerances on placement of components

4. F/number = focal length ÷ aperture Also, reciprocal of “relative aperture” • 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

5. Same primary mirror, but very different f/numbers

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

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

8. 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

9. Chromatic aberration

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

15. 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

17. 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

18. Field curvature

19. Schmidt camera

20. Distortion

21. y = distance of image from optical axis h = distance of object from optical axis • 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 1.125 • Curvature reduced to 0.25 of previous amount • Blurs due to astigmatism or curvature will of 0.375 of original size