Celestron Compustar Manuale delle Istruzioni Scaricare il pdf (Pagina 4)

With all their features, it is no wonder that Compustar owners lovingly care for and maintain

these telescopes rather than buy some of the newer Go -To telescopes. It is also little wonder that

some owners want to do long-exposure astrophotography with their Compustars.

Tracking Errors During Astrophotography and Their Sources

Unfortunately, one cannot simply attach a camera to the Compustar (or any other telescope),

frame the subject in the camera, take a long exposure, and expect good results. Rather, the stars

in the image will be trailed and the subject blurred. The reason for this is that no telescope clock

drive and mount are perfect. They all make tracking errors. Most of these are what are known as

periodic errors caused by the imperfections in the gear mechanism of the clock drive. At some

times, the gears will lag behind the stars in pushing the telescope toward the west. Other times, it

will push it just a little too far. Because the drive gear is typically a rotating gear, like a worm

gear, the pattern of errors will repeat after one full revolution, or period, of the gear; hence the

name, periodic error. Periodic error will not bother you during visual observing, as these errors

are fairly small and the subject generally stays quite well centered in the eyepiece, but they will

ruin all time exposures of any significant duration. Other errors arise from inaccurate polar

alignment of the mount. This causes the image to drift north or south in the field of view over

time. Still other tracking errors come from flexure of the mount. If the telescope is tracking

something from the east side of the meridian, the mount may be flexing or bending in that

direction under the weight of the telescope and camera. At the same time, the clock drive is

having to push the telescope “uphill” and may have difficulty doing that. As the telescope

crosses the meridian, the problems reverse. The mount will flex toward the west and the drive

may begin accelerating “downhill” toward the west. These problems can all cause tracking

errors.

Schmidt-Cassegrain telescopes are prone to one other type of tracking error, namely that caused

by mirror flop. Schmidt-Cassegrain telescopes focus by moving the position of the primary

mirror forward or backward along the axis of the central baffle tube. Thus, the primary mirror is

not fixed, but can wobble, or flop, slightly from side to side around the baffle tube. A slight flop

can cause a significant shift in the position of the image. It can even change the collimation of

the telescope. The author‟s Compustar C14 sometimes requires re-collimation for imaging near

the poles compared with more southerly declinations. Also, regardless of recollimation, SCTs

may be particularly prone to mirror flop as they cross the meridian. Fortunately, that type of

mirror flop can be minimized by the simple technique of always approaching focus by turning

the focus knob counterclockwise. This is because turning the knob counter clockwise results in

the mechanism pushing the mirror away from the rear cell. Thus, the push pins of the focusing

mechanism are always in contact with the rear surface of the mirror, stabilizing it somewhat and

reducing its ability to flop from side to side. Conversely, turning the focus knob clockwise

moves the pins of the focusing mechanism toward the rear cell, requiring the mirror to follow

them under tension from a mechanism. Friction and other factors may prevent the mirror from

moving all the way against the pins, leaving it prone to mirror flop.

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Celestron Compustar Manuale delle Istruzioni Pagina 4