Section 3 - At the Telescope with a CCD Camera
This preliminary step will save you much time in initially finding focus. The approximate distance behind the eyepiece tube for each of our CCD cameras is listed in Table 3.1 below:
Camera | Distance |
0.660 inch | |
0.680 inch | |
0.560 inch | |
0.920 inch |
Table 3.1 - Camera Back Focus
Diffuser
Back Focus Distance from Table 3.1
To achieve fine focus, insert the CCD head into the eyepiece tube, taking care to seat it, and then enter the CCDOPS FOCUS mode. The Focus command automatically displays successive images on the screen as well as the peak brightness value of the brightest object in the field of view. Point the telescope at a bright star. Center the star image in the CCD, and adjust the focus until the star image is a small as can be discerned. Next, move the telescope to a field of fainter stars that are dimmer so the CCD is not saturated. Further adjust the focus to maximize the displayed star brightness in counts and minimize the star diameter. This can be tedious. It helps considerably if a pointer or marker is affixed to the focus knob so you can rapidly return to the best focus once you've gone through it.
An exposure of 1 to 3 seconds is recommended to smooth out some of the atmospheric effects. While you can use the Full frame mode to focus, the frame rate or screen update rate can be increased significantly by using Planet mode. In Planet mode the Focus command takes a full image and then lets you position a variable sized rectangle around the star. On subsequent images the Planet mode only digitizes, downloads, and displays the small area you selected. The increase in frame rate is roughly proportional to the decrease in frame size, assuming you are using a short exposure.
The telescope focus is best achieved by maximizing the peak value of the star image. You should be careful to move to a dimmer star if the peak brightness causes saturation. The saturation levels of the various resolution modes are shown in Table 3.2 below. Another point you should also be aware of is that as you approach a good focus, the peak reading can vary by 30% or so. This is due to the fact that as the star image gets small, where an appreciable percentage of the light is confined to a single pixel, shifting the image a half a pixel reduces the peak brightness as the star's image is split between the two pixels. The Kodak CCD pixels are so small that this is not likely to be a problem.
Resolution | Saturation Counts |
High Res | ~20,000 for |
| ~40,000 for |
| ~65,000 for |
Med/Low Res | 65,000 |
| Table 3.2 - Saturation Values |
Once the best focus is found, the focusing operation can be greatly shortened the second time by removing the CCD head, being careful not to touch the focus knob. Insert a high power eyepiece and slide it back and forth to find the best visual focus, and then scribe the outside of
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