Figure 2 shows how the limiting magnitude changes with focal ratio
for a given focal length.
You can compensate completely for the lower magnification inherent to shorter focal ratios
by using eyepieces with shorter focal lengths.
Having a fast focal ratio
meant that the Schmidt camera configuration had a very small depth of focus--i.
There is no all-purpose telescope for deep-sky photography, but if I had to pick one that can do a lot, I'd choose something with about 6 to 8 inches of aperture and a focal ratio
of f/4 to f/8.
The difference is that a fast scope (with a focal ratio
of f/6 or lower) provides a wide field of view that's best for panoramic views of the Milky Way and observing large deep-sky objects (targets beyond the solar system).
Meade LightBridge Mini Specifications Mini 82 Mini 114 Mini 130 Aperture 82 mm 114 mm 130 mm Focal length 300 mm 450 mm 650 mm Focal ratio
The StarBlast has an unusually short focal ratio
(f/4), making it ideal for deep-sky browsing with its standard 17-millimeter eyepiece, while the 6-mm eyepiece yields a respectable 75x for planetary viewing.
The mirror is comparable in size to Hubble's primary but, thanks to its faster focal ratio
Until recently, that required a long focal ratio
, but modern apochromats can achieve superb color correction in short, convenient tubes.
Ah, but try to find one with an f/3 focal ratio
, or with a collapsible mount, or made of inlaid oak.
The focal ratio
or f/number (such as f/4 or f/8) is simply the focal length divided by the aperture.
Orion Telescopes Model WIDESTAR 80 SpaceProbe 3 EQ Price (US$) $119 $129 Optics type refractor reflector (achromat) (Newtonian) Aperture 80 mm 76 mm Focal length 400 mm 700 mm Focal ratio
f/5 f/9 Finder reflex reflex Focuser size 1.