## F4 OpticsPosted By Vladimir Sacek |

Going faster with parabolic mirror results in:

- coma limited linear field diameter decreased in proportion to the cube of f#

- coma limited angular field decreased in proportion to the square of f#

- eyepiece astigmatism increased in proportion to the

f-ratio

-eyepiece spherical aberration increased in proportion to the cube of f-ratio

An f/4 parabola has 1/4 wave of coma at 0.55mm off-axis

(from F^3/116). However, an 18" mirror will have field of best definition roughly twice larger, as it seldom will operate at better than ~1/2 wave, due to accumulated errors

(seeing, miscolimation, c.obstruction, etc.). Good definition in the center of the field takes an eyepiece

well corrected for spherical aberration; a Kellner would likely have 5 to 6 arc minutes central blur, and even best corrected eyepieces wouldn't satisfy a ~1' or smaller central spot criterion. How much it really matters depends on one's individual eye acuity - most people won't notice difference bellow 2-3' (besides, an average "good seeing blur" with an 18" is likely to be larger than 1', even at lowest magnifications) .

- coma limited linear field diameter decreased in proportion to the cube of f#

- coma limited angular field decreased in proportion to the square of f#

- eyepiece astigmatism increased in proportion to the

f-ratio

-eyepiece spherical aberration increased in proportion to the cube of f-ratio

An f/4 parabola has 1/4 wave of coma at 0.55mm off-axis

(from F^3/116). However, an 18" mirror will have field of best definition roughly twice larger, as it seldom will operate at better than ~1/2 wave, due to accumulated errors

(seeing, miscolimation, c.obstruction, etc.). Good definition in the center of the field takes an eyepiece

well corrected for spherical aberration; a Kellner would likely have 5 to 6 arc minutes central blur, and even best corrected eyepieces wouldn't satisfy a ~1' or smaller central spot criterion. How much it really matters depends on one's individual eye acuity - most people won't notice difference bellow 2-3' (besides, an average "good seeing blur" with an 18" is likely to be larger than 1', even at lowest magnifications) .