8
Fas tar L ens As sem bl
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 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
Cassegrain f/10
With Reducer/Corrector
f/6.3
With Fastar Lens
Accessory f/2
CGE 800
80" (2032mm)
50.4" (1280mm)
16" (406.4mm)
FOCAL
LENGTH
& SPEED
CGE 1400
154" (3910mm)
88.2" (2239mm)
29.4" (746mm)
CGE 800
8 x 6.1 (arc min)
12.6 x 9.7 (arc min)
40 x 30 (arc min)
ST 237
F.O.V.*
CGE 1400
4 x 3 (arc min)
7 x 5.5 (arc min)
22 x 17 (arc min)
* Field of view calculated using SBIG ST 237 CCD camera with 4.7mm x 3.6mm chip.
Care and Cleaning of the Optics
Occasionally, dust and/or moisture may build up on the corrector plate of your telescope. Special care should be taken when
cleaning any instrument so as not to damage the optics.
If dust has built up on the corrector plate, remove it with a brush (made of camel’s hair) or a can of pressurized air. Spray at an
angle to the lens for approximately two to four seconds. Then, use an optical cleaning solution and white tissue paper to remove
any remaining debris. Apply the solution to the tissue and then apply the tissue paper to the lens. Low pressure strokes should
go from the center of the corrector to the outer portion.
Do NOT rub in circles!
You can use a commercially made lens cleaner or mix your own. A good cleaning solution is isopropyl alcohol mixed with
distilled water. The solution should be 60% isopropyl alcohol and 40% distilled water. Or, liquid dish soap diluted with water (a
couple of drops per one quart of water) can be used.
Occasionally, you may experience dew build-up on the corrector plate of your telescope during an observing session. If you want
to continue observing, the dew must be removed, either with a hair dryer (on low setting) or by pointing the telescope at the
ground until the dew has evaporated.
If moisture condenses on the inside of the corrector, remove the accessories from the rear cell of the telescope. Place the
telescope in a dust-free environment and point it down. This will remove the moisture from the telescope tube.
To minimize the need to clean your telescope, replace all lens covers once you have finished using it. Since the rear cell is NOT
sealed, the cover should be placed over the opening when not in use. This will prevent contaminants from entering the optical
tube.
CCD Camera
Figure 1-8
Figure 1-9
Table 1-2
