Celestron Compustar Manual de Instruções descarregar pdf (Página 39)

have counterweights in the left fork arm to counterbalance the declination drive box on the right

fork arm. Most locations where equipment can be added to the Compustar are intentionally

placed along the longitudinal axis of the telescope halfway between the right and left fork arms,

like imaging equipment at the center of the rear cell, or the mounting brackets on the center top

and bottom of the optical tube. Adding weight in these locations will only shift the center of

gravity above or below, or forward or backward, along the longitudinal axis. It will not shift the

center of gravity sideways along the Dec axis. Thus, with the left and right fork arms balanced at

the factory, you will only need to balance the telescope vertically and horizontally. However, any

equipment mounted on either side of this center line, like a new finderscope, a Telrad, or a guide

scope, can move the center of gravity to the left or right of the triple intersection along the Dec

axis. If this is the case, then the final step you need to take will be to release the RA axis lock and

see if the telescope wants to swing east or west. Add weight to the opposite fork arm until the

tendency is counteracted and you will have put the center of gravity back on the triple

intersection point.

Self-Guiding SBIG CCD Cameras

There is another option for autoguiding besides using a separate guide scope or an off-axis

guider that has not yet been discussed. The SBIG CCD cameras, including the ST series and the

STL Research series, contain an additional small CCD chip, just off one edge of the main

imaging chip. This chip is for autoguiding the CCD. It entirely replaces the separate guiding

cameras discussed in other sections. Furthermore, because the guiding chip is in a fixed position

relative to the imaging chip and is illuminated by the same telescope optics as the imaging chip,

this system is a form of off-axis guiding. However, because everything is built into the CCD

camera body, no off-axis guider device is required. Because it is a valid form of off-axis guiding,

it too completely eliminates problems of guiding errors due to differential flexure and mirror

flop. In this context, the autoguiding is called self-guiding because the CCD camera guides itself.

Self-guiding is patented by SBIG, so it is currently only available on some of their CCD

cameras, which is why it was not discussed earlier, as not all readers will be using these

particular SBIG CCD cameras.

Self -guiding has tremendous advantages over traditional off-axis guiding. With no pick-off

prism, the image of the guide star will be of much higher quality, even when a focal reducer-

corrector is being utilized. Because the guiding chip is in the same focal plane as the imaging

chip, the guide star will always be in focus when the telescope has been focused on the imaging

chip. The extra step of focusing the guiding camera is never required. Furthermore, because the

guiding chip is a sensitive, cooled CCD chip, it is much more sensitive than the other types of

guide cameras mentioned earlier. Therefore, it can guide on considerably fainter guide stars.

With such sensitivity, odds are very high that whenever a subject is framed on the imaging CCD

chip, a star suitable for guiding will fall on the guiding CCD chip. SBIG has calculated that the

odds of this occurring at f/6.3 or lower ratios are 95% in the sparsely starred regions of the sky

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Celestron Compustar Manual de Instruções Página 39