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From the point of view of an altaz mount or a poorly aligned equatorial, the photographic image rotates over time and longer exposures will cause star trails to appear, pinwheeling in arcs around the guide star. These arcs will ALWAYS be ZERO length in the direct proximity of the guide star but will appear longer and longer further away from the guide star - just as if you poked a pin on a star in any photograph and rotated the photo on that pinpoint - this is field rotation. There are actually 3 ways to combat these star trails. One way is to cancel the image rotation by rotating the tube or camera the same rate in the opposite direction. Another is to keep exposures short or for medium length exposures, place an 'eyepiece-view' circular mask over the finished print to cover the edges & corners of the print where the arcs are most apparent. But there is another far better technique... 'Rotacalc' addresses the third possibility. Since the rotation RATE changes over time (and can even reverse direction in most instances!), calculations can be made to find when these rotation rates are minimal - optimizing for longer exposures - sometimes very long exposures - measured in hours! All by finding the right time to begin exposure. The best time is when rotation is slowing to zero and then slowly reverses direction - doubling the available exposure time without increasing arc length! It is this knowledge that makes TECH2000 the leader in Dobsonian photography equipment - the rest is up to you the user! |
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TECH2000, 69 Ridge St. South, Monroeville, OH 44847 |
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