RC24 Focuser

The RC Optical Systems 24-inch Ritchie-Chretien telescope at Moore Observatory (MORC24) is focused with a motor-driven secondary. The original electronics was a TCC instrument control package that connected to the motor on the secondary, controlled the fans behind the primary, and read the thermistors on both primary and secondary mirror backs. The unit malfunctioned in August 2014 when it stopped reading the motor encoder and it was removed from service. This page documents our replacement.

We also note that a custom replacement part may be available from RA Limited LLC. We chose to develop a control that used hardware similar to that on our other telescopes.

Original Parts and Wiring

The telescope was wired through the primary mirror cell using an 18-pin Amphenol connector on the back plate. The pin assignments on this connector were not documented by RC Optical, but they iare known to include these functional groups:

• Secondary mirror motor power (12 V)
• Secondary mirror encoder power (5 V)
• Secondary mirror encoder phase A
• Secondary mirror encoder phase B
• Secondary mirror temperature thermistor
• Air temperature thermistor
• Primary mirror thermistor
• Primary mirror fan power

This connector and the fans are only accessible for rewiring by removing the primary mirror cell, which in the first phase of repair we did not do. The wiring to the fan motors is clipped into the motor supports and is accessible from the outside since the fans are not covered. There is a ribbon cable leading from behind the primary up one truss to the top ring where it crossed to the secondary. Behind the secondary mirror the cable terminated in connectors to the focus drive motor and what appears to be a thermistor taped to the back of the secondary. The only specific part that can be fully identified is the drive motor:

• Pittman 9237S011 24 VDC brushed DC motor

The secondary mirror motor power was divided over 3 lines for DC + and 3 more for ground in the original ribbon cable. The polarity determined the direction the motor rotates so that is ambiguous and effectively set in software. The encoder had 5 lines and 4 were used with these assignments and color codes from the Pittman DC motor documentation:

• Black - Ground (pin 1)
• Green - Index (pin 2, clipped at motor)
• Yellow - Channel A (pin 3)
• Red - Vcc for encoder (pin 4)
• Blue - Channel B (pin 5)

Pins on the motor encoder plut are numbered from the left looking on the back of the motor or encoder with the connector down.