A derotating Piggy-back-Mount

A derotating piggy-back camera mount

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 the section about 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.

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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.

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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.

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• This image shows the piggyback mount attached to a newer version of the telescope mount. You can also see the box to connect the stepper motor to the PC and a heatsunk resistor to limit the stepper current.

First Testshots of Polaris

Both photographs are 5 min. exposures with a 135mm lens at F5.6 on TMAX 100

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• Photo 1 has been taken without engaging the derotating drive.

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• For Photo 2 the camera has been rotated.