Field Rotation
In any scope where an axis does not point exactly to the celestial pole, a slow rotation of the field of view occurs. Field rotation is independent of the size of the field of view, for instance, a large scope at high power will experience the same angle of
field rotation as a small rich field telescope at low power. Field rotation varies greatly over the sky, non-existent in the east and west, terrible at the zenith, and approximating the sidereal tracking rate when pointed at the meridian. A motor can be added to slowly rotate the focuser to compensate for field rotation.
For visual imaging, a field de-rotator motor is not needed. For CCD imaging, exposing for 5-20 minutes in many sections of the sky with typical chips, a field de-rotator motor is not needed. If you have a CCD chip with very large numbers of pixels, or planning to do prime focus astrophotography, then you will want to add the field de-rotator. The control program shows the field rotation in real time, so that you can plan exposures accordingly. Here's how I judge field rotation: for a CCD chip that is several hundred pixels on a side, the total number of pixels on the perimeter is roughly 1000. That means that we can tolerate a field rotation of 360 degrees/1000 resolution units, or about 1/3 degree. I watch the scope track in realtime, noting the amount of field rotation change over 10 seconds or so. If it looks like I can image for my desired exposure time, then I go ahead. If not, I wait until the object is better positioned in the sky, or adjust my exposure
time.
Since the field de-rotator unit need only rotate slowly with finite steps, a simple single chip driver circuit is all that is needed, with step and direction inputs. To keep vibration minimized, I suggest a field rotation per step size of 1 arcminute. This is
also fine enough to prevent field rotation from showing on fine grained 35mm film. The suggested field rotation circuit uses the SAA1042 chip. For more information on this chip, click here.
For a very nicely designed field de-rotator unit, check out Chuck Shaw's 14 inch.
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...by Mel Bartels