Celestron 11055, 91525, 11065 instruction manual Description of F-numbers, Fastar Configuration

Description of F-numbers

The F/# stands for the ratio between the focal length and the diameter of the light gathering element. A C14 optical tube has a focal length of 154 inches and a diameter of 14 inches. This makes the system an f/11, (focal length divided by diameter). When the secondary is removed and the CCD is placed at the Fastar position, the system becomes f/2.1, this is unique to Celestron telescopes (see figures below).

Fastar Lens Assembly

PixCel 237 CCD Camera

Figure 7-2 -- Light path at f/11 focus Figure 7-3 -- Light path at Fastar f/2.1 focus

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/2.1 and f/11? F/2.1 has 1/5 the focal length of f/11. That makes the exposure time needed about 25 times shorter than at f/11, the field of view 5 times larger and the object size 1/5 compared to that of f/11. (see Table below)

	Standard Cassegrain			With Reducer/Corrector	With Fastar Lens
				Accessory	Accessory
Focal Length & Speed	154"(3910mm) @ f/11			98" (2488mm) @ f/7	29.4" (747mm) @ f/2.1
PixCel 237 F.O.V.	4.1 x 3.2 (arc min)			6.5 x 5 (arc min)	22 x 17 (arc min)
			Table 7-3
		The following is a brief description of the advantages of imaging at each f-
Fastar Configuration
		number configuration and the proper equipment needed to use the telescope in

any of its many settings. Refer to Figure 7-6for a more detailed description of the accessories offered for each configuration.

60 • Celestial Photography