TELESCOPE CONTROL SYSTEM

Control flow diagram

 

Telescope control software functionality

  • Establish and maintain asynchronous serial communication with controller hardware.
  • Interpret and respond to Meade Telescope Serial Command Protocol (MTSCP) messages.
  • Support functionality beyond that which MTSCP offers through an extended command set.
  • Calculate local sidereal time.
  • Calibrate telescope mount with data (local time, local sidereal time and observation site information).
  • Retrieve and display data from telescope mount (right ascension, declination, azimuth, altitude etc.).
  • Provide peltier cooler, focuser and handset controls.
  • Provide a means of sending and displaying messages for diagnostic purposes.

TSControl.exe - 94,208 bytes

 

Compatibility of controller with commercial astronomy software

Adopting the Meade Telescope Serial Command Protocol (MTSCP) ensures compatibility with most commercial software without the need to develop additional driver software. This enables the system to be tested, debugged and used in a simulated environment. Depending on the required functionality, only limited compatibility with Meade telescopes is necessary.

 

Controller hardware

The telescope controller hardware consists of a Microchip PIC18F452 microcontroller with open collector TTL buffered outputs, a MAX232CPE serial port driver and a 5V regulator. The circuit is constructed on 0.1" matrix board to accommodate experimentation and easy modification.

 

Controller software functionality

  • Maintain telescope positioning through the support of customised, King, sidereal and solar tracking rates.
  • Establish and maintain asynchronous serial communication with controlling device.
  • Interpret and respond to MTSCP messages.
  • Maintain accurate local sidereal time and local time through use of a timer interrupt.
  • Generate pulse sequence and precision timing for focuser, declination and right ascension drive stepper motors.
  • Respond to handset commands for local operation.
  • Provide a slow-changing pulse width modulated power supply to the peltier cooler to prevent thermal shock to CCD sensor.

 

Tracking rates

The default tracking rate used is the sidereal rate where the target right ascension is maintained through movement of the mount. The other rates are realised by deliberately introducing drift in the target right ascension and, by using the customised rate setting, in the target declination.

The customised tracking rate is used for tracking celestial bodies such as comets and the moon. The apparent motion of the moon to an observer on earth is not constant as a result of its inclined elliptical orbit, the motion parallax between it and the observer and the latitude from which it is observed (least apparent motion seen from the equator). For these reasons a lunar rate is not directly supported and the customised rate is used instead. Positional data of a celestial body a solar hour apart is obtained from an ephemeris and entered using the customised settings. This results in the correct amount of drift for that solar hour period.

The King rate of 99.97% the sidereal rate compensates for the refraction from the atmosphere. This rate does not allow for the non-linear characteristics of atmospheric refraction and the sidereal rate would be a better choice when observing high altitudes.

The solar rate is set at 99.73% the sidereal rate.

The Meade Telescope Serial Command Protocol (MTSCP) does not support all the above tracking rates and therefore implementation is supported through the extended command set.

 

Notes

PIC18F452 code written in C and compiled with CCS, Inc. PCH compiler.

 

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