Any ideas? how practical is an F4 reflector assuming that the optics are of the best quality? In other words can a system this short provide nice visual images from a moderately dark sky site? What are the advantages or disadvantages? Thanks in advance for the input. Tony Ennis in Charleston SC
F4 Optics
Started by Tony Ennis, 07/17/2002 03:20AM
Posted 07/17/2002 03:20AM
Opening Post
Posted 07/17/2002 12:36PM
#1
The practical advantages I can think of are: wider maximum field of view, lighter weight, and smaller size.
The wider maximum field of view becomes less useful depending on how much light pollution you have.The disadvantages would be less contrast (due to larger secondary), more coma at the edge of the field of view, and harder to collimate. If you are considering a schmidt newtonian, that design at F4 is supposed to limit coma to the equivalent of an F8, but the high power performance still won't be as good as an F8 newtonian.
If you are considering newtonians 8 inches in aperture or less, I'd aim for at least F6 or F7. With apertures 10 inches and up, F ratios usually get smaller in order to allow use of the scope without a ladder.
Of course, portability will be more important to you if you need to fit the scope in a small car.
Joplin
The wider maximum field of view becomes less useful depending on how much light pollution you have.The disadvantages would be less contrast (due to larger secondary), more coma at the edge of the field of view, and harder to collimate. If you are considering a schmidt newtonian, that design at F4 is supposed to limit coma to the equivalent of an F8, but the high power performance still won't be as good as an F8 newtonian.
If you are considering newtonians 8 inches in aperture or less, I'd aim for at least F6 or F7. With apertures 10 inches and up, F ratios usually get smaller in order to allow use of the scope without a ladder.
Of course, portability will be more important to you if you need to fit the scope in a small car.
Joplin
Posted 07/18/2002 12:17PM
#2
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) .
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