Celestron Compustar Instrukcja Obsługi Pobierz pdf (Strona 8)

Figure 2A (left). An off-axis guider is a T-shaped connector that connects to the threads of

the rear cell of the Compustar with the threaded ring at left. The threads to the right

accept a T-ring for a camera or other adapter. The vertical “stem” of the T accepts an

illuminated cross hair reticle eyepice with which the precise position of the guide star and

guide errors can be judged.

Figure 2B (right) A view through the main optical axis of the off-axis guider shows the

small pick-off prism that directs light from the guide star into the eyepiece barrel that

accepts the cross hair guiding reticle eyepiece.

The chief disadvantage of off-axis guiders is their limited choice of guide stars. The field of view

of the prism in the off-axis guider is quite small. Finding a suitable guide star that appears in

prism at the same time that the subject is anywhere in the field of view of the camera is one of

the most exasperating problems in astrophotography. Once the subject is centered in the field of

view of the camera, no guide star may be visible in the prism. Then, the entire camera/off-axis

guider complex can be rotated around the optical axis of the telescope to see if a star appears at

some other angle. If that fails, the subject can be shifted slightly in the frame of the camera and

the process repeated. This can take a considerable amount of time. The problem is particularly

acute when imaging galaxies, which tend to be in the portions of the sky less populated by

foreground stars in our own Milky Way. Thus, the farther the subject is from the plane of the

Milky Way, the more the choice in guide stars diminishes.

Obviously, once a suitable guide star is located, the subject may not be ideally framed in the field

of view of the camera. It may be somewhat off-center, or even near a corner of the frame. This is

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Celestron Compustar Instrukcja Obsługi Strona 8