Celestron Compustar Instrukcja Obsługi Pobierz pdf (Strona 43)

shot color) by taking many, many 30 second sub-exposures. This was because 30 seconds was

the longest period that the Compustar could ever track an exposure without noticeable trailing.

However, with the periodic error of the Compustar drive, only one-third of all exposures were

actually useable; the other two-thirds had unacceptable trailing and had to be discarded. This was

very inefficient. In order to obtain two or three hours of useable exposures, one had to collect

exposures for a total of six to nine hours! Furthermore, for deep sky objects requiring such long

exposures, one ultimately had to stack 240 to 360 of the 30 second sub-exposures. While

stacking multiple exposures totaling 2-3 hours does increase the signal to noise ratio of the deep

sky object in the final image to something very similar to that obtained with a single equivalent

long exposure, each sub-exposure also adds a certain amount of imprint noise that cannot be

subtracted with calibration frames. Thus, when stacking sub-exposures, one reaches a point of

diminishing returns where any further gain in signal to noise ratio begins to be offset by an

increase in imprint noise from the large number of sub-exposures. However, trying to obtain

better imaging results by acquiring longer exposures with a cooled CCD camera would be futile

without autoguiding. The images would certainly be trailed and unusable.

The author first verified that excellent autoguiding results could be obtained throughout the 30

second sub-exposures with his DSLR. Using the autoguider, an off-axis guider, and a Meade DSI

as the guider camera, virtually every 30 second sub-exposure was adequately guided and

completely useable, resulting in much greater imaging efficiency. The sub-pixel accuracy of

autoguiding also produced much sharper images (Figures 21 and 22) than could be achieved by

stacking only the very best “unguided” images.

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Celestron Compustar Instrukcja Obsługi Strona 43