|
Short focal length means smaller tubes provided there are no multiple internal reflections as in a SCT. eg an 8" SCT has a f.l. of 2000mm but is about 1/4 of that in legth.
Short focal ratio may still mean a long tube if the mirror diameter is large.
Fully illuminated field is influenced by many factors namely secondary size, tube size, focuser height (distance from secondary to focal plane). f ratio does not really come into it but a short f ratio usually means a larger secondary to get a better fully illuminated field. The cone of light from a short f ratio mirror is wider than that for a long f ratio mirror so the light fall off from centre to off axis is faster.
Long f length does give greater mag. than short f length for a given eyepiece. Note long f ratio doesn't necessarily do the same unless the aperture is the same.
Some non-corrected simple lens eyepieces do perform better at long f ratios, say around f15 perhaps lower. Note f ratio is the determining factor not f length.
Long f length mirrors are easier to figure but a little harder to test.
Apparent field of view is a function of the eyepiece.
A 16" f8 mirror would have a focal length of 3.25m and the focuser height would simply be too high off the ground for comfortable viewing when at zenith. However, such a telescope would indeed produce some fine images since it could have a much smaller secondary for better contrast. It would be a visual only instrument as the mount would have to be very large and sturdy to cope with imaging.
|