At The Telescope - Santa Barbara Instrument Group ST-i Operating Manual

3. At the Telescope

Connect the camera to the computer and start CCDOps. Establish a link to
the camera and run the Camera Setup command. At this point we're ready
to hook up to the telescope.
Focusing the ST-i camera is much easier than most CCD cameras due
to its faster frame rates. Nevertheless, it can be difficult the first time when
the star images are likely to be well out of focus and difficult to detect.
Use 3 second exposures the first time in order to detect even stars that are
out of focus. As you get closer to focus you can reduce the time to increase
the update rate and use just the brightest stars for final focusing. The
backfocus distance (the distaqnce from the front of the camera tube to the
CCD focal plane) is 0.53 inches.
To achieve fine focus, first center a bright star then insert the CCD
head into the eyepiece tube, taking care to seat it, and then enter the
CCDOps Focus Mode. Do not initially try to focus on the moon or planet –
they are harder. As we learned in the tutorial, the Focus Mode
automatically displays successive images on the screen as well as the peak
brightness value of the brightest object in the field of view. Using the
telescope controls, center the star image in the CCD, and adjust the focus
until the star image is a small as can be discerned. Next, shift the telescope
to fainter stars 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.
With the fast update rate of the ST-i camera, use the Full frame mode
to focus, 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.
SBIG ST-i Manual
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