Celestron Compustar Bedienerhandbuch PDF herunterladen (Seite 6)

Figure 1. An illuminated red cross hair reticle. This reticle has a double cross hair. The

guide star is centered in the cross hairs so that guiding errors can be detected and

corrected by returning the guide star to its intended position in the cross hair using the

motion keys on the drive corrector.

Notably, the Compustar has a manual drive corrector incorporated directly into the computer

module. The four motion control keys in the upper right hand corner that are used for slewing

and setting the telescope can also be used for making guiding corrections. However, the “SLEW”

and “SET” speeds are far too fast and jerky for guiding corrections. Before manually guiding,

one must switch the speed on the drive motors to “GUIDE” speed by pushing the “SPEED”

button followed by the “GUIDE” button. The Computer module will beep three times indicating

that it has changed the drive rates to guiding speeds. “GUIDE” speed is usually some fraction of

the sidereal rate, which is an appropriate speed for guiding corrections.

Two Main Techniques Used for Guiding

How does one acquire the guide star? There are two main approaches: one is to use a separate

guide scope and the other is to use an off-axis guider. Each has advantages and disadvantages.

A guide scope is another telescope, often a refractor, mounted on the same mounting as the

imaging telescope. The guiding reticle eyepiece is inserted into the viewing end of the guide

scope. Although the guide scope is mounted roughly parallel with the main imaging telescope

using mounting rings or a dovetail plate, there are provisions by which the aim of the guide

scope can be adjusted a fair amount in any direction. This accounts for the chief advantage of the

separate guide scope. Namely, it provides an ample selection of suitably bright guide stars. The

guide star can be quite some distance from the subject being imaged and this increases the odds

that one can select a fairly bright star guide star that is easier to track.

The chief disadvantage of a separate guide scope is differential flexure. The guide scope and the

main imaging telescope are two independent optical systems on the same mount and, as the

mount moves, they may not remain stationary with respect to one another. Rather, the telescope

tubes and their mounting hardware may flex in different directions and at different rates. With

Schmidt-Cassegrain telescopes, the primary mirror may experience mirror “flop” as the mount

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Celestron Compustar Bedienerhandbuch Seite 6