Celestron Ultima 8 - Description of F-Numbers

85
Fastar Lens Assembl
y
PixCel 237 CCD Camera
Description of F-numbers
The F/# stands for the ratio between the focal length and the diameter of the light gathering element. A
standard C8 has a focal length of 80 inches and a diameter of 8 inches. This makes the system an f/10,
(focal length divided by diameter). When the secondary is removed and the CCD is placed at the Fastar
position, the system becomes f/1.95, this is unique to Celestron telescopes (see figures below).
The key factors for good CCD imaging are; exposure time, field-of-view, image size, and pixel resolution.
As the F/# 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/1.95 and f/10? F/1.95 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 9-3)
Standard Cassegrain With Reducer/corrector
Accessory
With Fastar lens
Accessory
Focal Length & Speed 80" (2000mm) @f/10 50" (1250mm) @f/6.3
16"(400mm) @
f/1.95
PixCel 237 F.O.V. 8.1 x 6.3 (arc min) 13 x 10 (arc min)
.69 x .53 (degrees)
41 x 31 (arc min)
PixCel Sampling (arc
sec/pixel)
.75 (arc sec) 1.2 (arc sec)
3.8 (arc sec)
Table 9-3
•
Fig 9-4 -- Light path at Fastar f/1.95 focus
•
Fig 9-3 -- Light path at f/10 focus