Motorized Collimation System

e-mail Carl Anderson

The motorized collimation system consists of mounting stepper motors on a reflecting telescope to control the primary mirror tilt.  This system was originally designed for use on a large Yolo telescope, however it can also be used on any large Dob mounted telescope.  This would be especially handy for large Dobsonians where the user might be up on a tall ladder, and wishes to tweak the collimation, while viewing at the eyepiece. Key features: 1.  Manual motor control from a hand remote switch box. 2.  Motors are not energized while sitting still.  This elimates troublesome air currents from warm motors. 3.  Automatic correction for a sagging structure.  As the telescope is tilted to various elevations, the motors automatically tweak the collimation.

The hand control has four manual push buttons.  These would tilt the primary mirror in two axis so that the view through the eyepiece would move up and down, or side to side.  The middle green LED indicates that the power to the system is on. Automatic control: First the telescope is positioned to view a star near the zenith.  Tilt adjustments are then made for best collimation.  In and out of focus star images are used to judge the best position.  When the collimation is correct,  the top left button is pushed.  The corresponding LED now remains lit, indicating that the stepper motor settings for viewing near the zenith are stored in memory. Second, the telescope is positioned to view a star lower in the sky, near the horizon.   Tilt adjustments are again made for best collimation.  When the collimation is correct, the top right button is pushed.  The corresponding LED also remains lit, indicating that the stepper motor settings for viewing near the horizon are stored in memory. As both position LEDs are lit, the system is in automatic mode.  From now on, the motors will change depending on how the telescope is tilted.  If either top control button is pushed again, its corresponding LED turns off and the system is back in a manual mode.

Tilt sensor and circuitry: The key to the control system is a little micro electronic machined structure (MEMS) device that measures incline.  This a SCA61T inclinometer made by VTI Technologies.  Basically, 5 Volts is applied to the device.  It has an analog voltage output that changes with incline,  and also a digital output serial peripheral interface (SPI) signal.  Either output method can be used. A microcontroller (PIC16F877) by Microchip, Inc. is used to measure the tilt signal, monitor the push buttons, and then control the stepper motors.

Close up view of the tilt sensor.  For more information about this device, click this link to view the data sheet.