Image Plane Derotation System

The motor is an SAIA 48 step,12v stepper motor with a 50:1 gearbox, turning a 240:1 worm gear (HDPE Gear made by Andy Saulietis).  The focuser is an Astro Systems, Phase 4 focuser.  Each step of the motor rotates the image plane gear 2.25 arcseconds.

The worm gear can swing downward, out of mesh with the 8" dia HDPE gear, to allow manual re-positioning of the camera to adjust the framing of targets, or the software can drive the motor to reposition the camera for fine tuning of the framing.

The normal camera I use is a CB245.  The camera in the photo is a greyscale QuickCam (for more info, click here)

The field rotation assembly is basically a flange on the rear (inside) of the gear that rides on two 4" ID bearings.  The bearings are held by a ring mounted in a plate screwed to the rear of the wooden slat in the secondary cage.  The flange has a rear plate that is screwed to the end of the flange and squeezes the two bearings together in the bearing plate, capturing the gear/flange on the plate.

(click on any of the following images to see a large image)

	The mounting plate and bearing ring (backside, away from focuser)
	The mounting plate and bearing ring (Top side, towards focuser)
	Flange attached to rear of gear
	Rear Plate (attaches to inner end of flange on gear to capture bearings and bearing ring onto flange)
	Disassembled FR system
	FR system together with the secondary cage slat that the plate attaches to (on the rear) and the worm/motor assembly.
	Sec cage slat with the assembled FR system mounted to it, ready to be mounted in the sec cage.
	View of inside of the assembled sec cage slat, ready to be mounted to the sec cage.  Silver screws attach the plate to the rear of the sec slat.

The inside of the secondary cage has a "closeout" panel made of aluminum flashing.  The following photo show the inside of the secondary cage, the closeout panel, and the adjustment screws and their locking clamps on the Rear Plate of the Field Rotation bearing assembly.

(Click on the image to see a larger view)

The motor is controlled by the same software that drives the altitude and azimuth motors (Mel Bartel's "Scope.exe").  Mel's circuitry incorporates a Motorola SAA1042 stepper motor driver chip to provide the full step or 1/2 step pulses for the field rotation stepper motor.   This chip is actually designed to drive a Bipolar stepper motor that draws less than 500ma.  In order to use a 12v unipolar stepper motor that can draw up to 1 amp per winding, the outputs from the SAA1042 must be first sent to a daughterboard circuit to adapt them to drive a unipolar motor.  For a .gif file of a schematic of the daughterboard circuit I use, click HERE.  To download a slightly "cleaner" Powerpoint drawing of the schematic, click HERE.