Celestron Compustar Manuál s instrukcemi stáhnout pdf (Strana 7)

crosses the meridian, but this will not occur in the guide scope. Thus, even if the guide star is

tracked perfectly in the guide scope, differential flexure between it and the imaging scope can

result in a trailed image. Ironically, efforts to thwart this problem can be counterproductive.

Attaching the guide scope to the imaging scope using rigid hardware may reduce the flexion, but

it adds a significant weight load to the drive, making drive rate errors more likely. The other

disadvantage of the guide scope is cost; one has to purchase a separate telescope for guiding.

Usually, this is a refractor.

An off-axis guider is a T-shaped connector (Figure 2A) placed between the telescope and the

camera (Figure 3). The cross bar of the T is the main imaging axis through which light passes

from the telescope to the camera. Inside the stem of the T is a small 90-degree reflecting prism,

called a pick-off prism (Figure 2B), which is positioned (generally) outside the field of view of

the imaging camera, but still within the field of view of the imaging telescope. The light from the

prism is directed to an eyepiece holder, which comprises the stem of the T. The guiding reticle

eyepiece is placed in that holder (Figure 4). The telescope is focused so that the image in the

camera is sharp. Accordingly, the guiding eyepiece cannot be focused using the telescope‟s

focusing knob, but must be focused by sliding the eyepiece in and out of the eyepiece holder, so

as to not disturb the focus of the imaging telescope. The chief advantage of the off-axis guider is

that the guiding optics and the imaging optics are one and the same. This eliminates the problem

of differential flexure. Even guiding errors due to mirror flop of Schmidt-Cassegrain telescopes

will be properly corrected. Other advantages to off-axis guiders are low cost and low weight.

Off-axis guiders are inexpensive. They are also light weight and do not require additional

mounting hardware and therefore do not add a significant weight load to the drive motors.

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Celestron Compustar Manuál s instrukcemi Strana 7