14
The key factors for good CCD imaging are; exposure time, field-of-view, image size, and pixel resolution. As
the F-number goes down (or gets faster), the exposure times needed decreases, the field-of-view-increases, but
the image scale of the object gets smaller. What is the difference between f/2 and f/10? F/2 has 1/5 the focal
length of f/10. That makes the exposure time needed about 25 times shorter than at f/10, the field of view 5
times larger and the object size 1/5 compared to that of f/10. (see Table below)
Telescope
Model
Standard f/10
Configuration
f/2
Configuration
Focal
Length &
Speed
8"
80" (2032mm) 16" (406.4mm)
9.25"
93" (2350mm) 18.5" (470mm)
11"
110" (2800mm) 23.1" (587mm)
14"
154" (3910mm) 28" (711mm)
Figure 3-5
Table 3-3
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