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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