Fastar L ens Assem bl |
CCD Camera |
Figure 7-4 |
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 | Standard | With | With Fastar |
| | Model | Cassegrain f/10 | Reducer/Corrector | Lens Accessory |
| | | | f/6.3 | f/2 |
| Focal | CGE 800 | 80" (2032mm) | 50.4" (1280mm) | 16" (406.4mm) |
| | | | |
| CGE 1100 | 110" (2800mm) | 69.5" (1764mm) | 23.1 (587mm) |
| Length & |
| Speed | | | | |
| CGE 1400 | 154" (3910mm) | 88.2" (2239mm) | 29.4" (746mm) |
| |
| | | | | |
| | CGE 800 | 8 x 6.1 (arc | 12.6 x 9.7 (arc min) | 40 x 30 (arc |
| | | min) | min) |
| ST 237 | | |
| CGE 1100 | 5.8 x 4.4 (arc | 9.2 x 7.0 (arc min) | 28 x 21 (arc |
| F.O.V.* | | min) | min) |
| | |
| | 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.
Table 7-3
The following is a brief description of the advantages of imaging at each f-number configuration and the proper equipment needed to use the telescope in any of its many settings
Fastar F/2 Imaging
As stated above, the exposure times are much shorter at f/2 than at f/6.3 or f/10. The field-of-view is wider, so it is easier to find and center objects. Also with a wider field-of-view you can fit larger objects (such as M51, The Whirlpool Galaxy) in the frame. Typical exposure times can be 20-30 seconds for many objects. Under dark skies you can get an excellent image of the Dumbbell Nebula (M27) with only a few 30 second exposures (see figure 8-5 below). The spiral arms of the Whirlpool galaxy (Figure 8-6) can be captured
49
