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Celestron Focus Motor driver for LAST

See the project where all this started from: NexStarMatlab.

This is a trimdown of the former, controlling only the Focus Motor and not the whole NexStar mount assembly. Keep in mind that if all such a mount is to be controlled via a single serial port, this class needs to be augmented with all the functionality of the former project. Augmenting, rather than duplicating, is IMHO the way to go, as all the communication infrastructure (e.g. the infamous AUX protocol) is common.

The AUXmsg class (and its ancillary enumerators, and the basic communication methods) would deserve being part of an independent package, if it is to be shared with a general class for the NexStar mount. Here, as a rule, files starting with capitals denote classes.

It is left to the responsibility of the LAST integrator to copy or link the files of this project into an +obs\+instr tree. I couln't think of any viable directory structure satisfying both git project naming constraints (even considering subtrees and submodules) and Matlab/MAAT/LAST organization requirements.

Note on connection: On the focuser motor I tried, connecting the computer on to the USB port of the focuser looked problematic. In many cases we have to use long, amplified cables and USB hubs (e.g. the hub embedded on the telescope mount for convenient routing), which are a recipe for failure. On most of the patched connections which I tried, the USB/serial chip disconnected itself within just a few seconds of continuous motor operation, which definitely happens when commanding long focus strides, or in calibration. It may be for a large part a matter of weak engineering of the communication module and non-immunity to EMI too. Disconnects and OS reconnects are easily monitored by dmesg -wH; they cause the OS assigning a new /dev/tty device each time, are problematic for Matlab as they leave stale serial resources, and in general for stable operation. One may be forced to issue again and again delete(instrfind) to release ALL open resources and scan again all existing serial devices to find the one the focuser reattached to.

other notes on the focuser itself:

  • The focuser remembers its last position and its calibration limits at poweroff.
  • If the focuser receives a new position command while it is moving, it immediately changes its target to the new position. If many commands are received, the focuser will go to the last of the positions. If a new position is command is received while moving in one direction, the focuser will decelerate a bit before moving in the opposite direction, if asked to.
  • Calibration is performed by searching for the lower point of high resistance (high drain current, probably) first. Then this is set as zero of the index, and the motor searches the upper high resistance point. Once these two points are found, the operating range is set as the value of the upper index. The range limits are then defined, by Celestron choice, to be operating_range*[1,19]/18. The motor is then moved somewhere mid range.
  • the motor runs at ~400steps/second. 1000 steps are a full turn. Increasing counts mean CCW rotation.
  • The step counter wraps around at 60000 (not 2^16). This is seen when attempting calibration of a mechanically disconnected from focus screw.
  • On the RASA telecope, 1 turn equals 1mm. CCW rotation pushes inside the mirror, i.e toward the corrector plate, i.e. it focuses the telescope farther. The total range is seen to be ~35000 steps, i.e. the tick limits may result in ~[1500,36500].
  • The focuser is not made for working when stuck. Among the rest the overcurrent probably causes a voltage drop also on the communication circuits, which cause USB-serial drops and disconnects.
  • Forcing the motor to turn when powered, using a wrench, causes permanent damage to the gearbox.