7
Fastar Lens Assembly Option – Using your CGE telescope at f/2 with optional Fastar
Lens Assembly
Some C8 and C14 optical tubes are equipped with a removable secondary mirror (see Table 1-1) that allows you to convert your
f/10 telescope into an f/2 imaging system capable of exposure times 25 times shorter than those needed with a f/10 system! With
the optional Fastar lens assembly you can easily convert your Fastar compatible telescope to f/2 prime focus use in a matter of
seconds.
The Fastar compatible CGE telescope's versatility allows it to be used in many different f-number configurations for CCD
imaging. It can be used at f/2 (with optional Fastar Lens Assembly), f/6.3 (with the optional Reducer/Corrector), f/10, and f/20
(with the optional 2x Barlow) making it the most versatile imaging system available today. This makes the system ideal for
imaging deep-sky objects as well as planetary detail. Described below is the configuration of each F-number and the type of
object best suited to that kind of imaging.
The above figure shows how the secondary mirror is removed when using the optional CCD camera at f/2 and the Fastar Lens
Assembly.
Warning:
The secondary mirror should never be removed unless installing the optional Fastar Lens Assembly. Adjustments to
collimation can easily be made by turning the screws on the top of the secondary mirror mount without ever having to remove the
secondary mirror (see Telescope Maintenance section of this manual).
The F/# stands for the ratio between the focal length and the diameter of the light gathering element. A C8 optical tube 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).
However, the C14 optical tube has a 154 inch focal length with a F-ratio of f/11. When the secondary is removed and the CCD
camera is placed at the Fastar position, the system becomes f/2, this is a unique feature to some Celestron telescopes (see figures
below).
Figure 1-7 - The Fastar Compatible Optical System
Secondary
Mirror
Secondary
Mirror
Retaining Ring
Corrector Plate
Secondary
Mirror Mount
Handle
