One big drawback of Alt-Azimuth mounted telescopes is that fieldrotation is not compensated for as it is in equatorial mounted telescopes. While this is no problem for visual use, the effect is easily visible as star dashes instead of dots on long exposure photographs. We all know this effect from Polaris photographs with a fixed mounted camera.
To compensate for the occuring fieldrotation one has to rotate the
photographing device with the object in question. This is normally
accomplished by rotating the eyepieceholder for prime focus
photography, but the same can be done on a piggyback mount if we
rotate the camera body.
Mel Bartels' computerized Dob system includes the output to
control a steppermotor driven FR-unit. (See also my fielrotation daughterboard
and Chuck Shaw's excellent page )
 
 
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My FR-Piggyback unit consists of a camera-carriage that rotates
at the proper rate, driven by a 12V - 0.9 deg/Step Stepper
motor. (Click on the small icon to see larger image) |
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The motor works in halfstep mode and motorspeed is further
reduced with a selfmade gearbox to give ~7arcseconds/Step
rotating speed of the carriage. First images of Polaris show that the unit works, even though the values for the tracking rate still have to be tuned a little. (Click on the small icon to see larger image) |
 
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Initially the finderscope came in the way of the camera field,
so I had to mount the finder on hinges to move it out of the
way. (Click on the small icon to see larger image) |
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