Half-wave plate control and sensing

The hardware, electronics, and computers involved in controlling and checking the SPARO half-wave plate ("hwp") are as follows:

• Hwp motor. This is mounted onto the SPARO dewar. This is a stepper motor that drives the hwp. It is heated via Masterman's system.



• Indexer. This small electronics module lives in the blue SPARO data-system enclosure on the telescope. It converts "move commands" that come in on a serial line from the SPARO data-system Mac into pulses that travel to the SPARO hwp stepper motor. In 1998, one of these indexers died due to sparking during plugging-and-unplugging of the AC cord. So we don't power it on and off in this way anymore. Instead we use an AC switch, such as the one in the control room that controls the power to the blue data-system enclosures. Also, three spares for the indexer are in SPARO closet on shelves labelled "computer stuff". Links are now available to the Indexer Software Manual and a brief description of the Indexer.



• SPARO Data-system Macintosh. The "SPARO" program that runs on this Mac can generates serial "move commands" for the Indexer at the push of a (graphical software) button on the "hwp window", or automatically when under computer control and in polarimetry mode (see Mac's "nod window" for buttons to switch between auto and manual mode). There is a menu option to reinitialize the Indexer from the Mac. It used to be the case that this was done automatically on startup of the "SPARO" program on the Mac, but this is no longer the case in the latest versions of the code. The reinitialize command makes both the Mac and the Indexer think that the hwp is at zero degrees and also sets the velocity and other indexer motion-control parameters such as maximum stepper-motor current and off-state stepper motor current. There is also a menu option that sets the "positive direction of motion" of the hwp. In the past, we have never needed to use this option.



• hwp encoder. This is a resistive element that is mounted directly adjacent to the half-wave plate. It serves as an absolute encoder: the resistance measured across the device (accessed via a BNC connector in the control room labelled "encoder signal") is linearly related to the encoder angle. The actual definition of what point is defined to be zero is arbitrary but it is a good idea to pick a convention and stick to it. Several choices of convention are written on a piece of paper taped to the wall in the Viper control room.
  
  There are two ways to read the hwp encoder:
  • The direct method is to hook an Ohm-meter to the BNC labelled "encoder signal" and read the resistance in kOhms. In this case nothing should be connected to the BNC connector directly above the "encoder signal" BNC. (I think this one is labelled "encoder power" or "+5 Volts to encoder" or somthing like that.)
  • The computerized method for reading the angle involves using the cryocontroller computer (a.k.a. "national") to temporarily apply a +5 Volts TTL signal to the "encoder power" BNC (using one of the cryocontroller computer's TTL outputs: specifically, using the one that is labelled as being dedicated to this purpose) and then using the cryocontroller computer's A/D inputs to read two voltages: the voltage on the "encoder signal" BNC and the voltage on the "encoder power" BNC directly above it. The ratio of these two voltages is linearly proportional to the hwp angle, and the cryocontroller computer can do the conversion to hwp angle using parameters stored by this computer (and displayed by the "cryocontroller program"). There are two distinct programs, both resident on the cryocontroller computer, that use this computerized method:
    • The "cryocontroller program", that is also used to monitor cryogenic temperatures and pressures. This program, in addition to all the cryo-monitoring features, has a graphical button that will temporarily set the appropriate TTL level high, then make the required voltage measurements and output the hwp angle in degrees.
    • The labview program that Matt Newcomb wrote, called "hwpclient.vi". This program also lives on the cryocontroller computer, (e.g. "national") in "C:\hwp". (Dale modified this program in Feb. 2000.) "hwpclient.vi" simply sits there waiting for the control computer to ask it what the hwp angle is. It then measures the angle using the hwp encoder in precisely the same fashion as when the "cryocontroller progam" does this measurement - except that "hwpclient.vi" does this measurement on command from the control computer, instead of at the push of a graphical button. "hwpclient.vi" cannot be run simultaneously with the "cryocontroller program" that we use to monitor the cryogenic temps and pressures. The command that needs to be sent out by the control computer to "hwpclient.vi" is:
      tell $s hwp "measure"
      and the response is:
      hwp angle: (less-than-sign)floating point number(greater-than-sign)
      
  
  
  
• Cryocontroller computer ("national"). As described above the cryocontroller computer can read the hwp encoder.



• Control computer . This computer can be scripted so as to accomplish the important job of measuring the hwp position after every move. Then this angle can be passed to the Macintosh for storage with the data. This functionality can be implemented via modifications to the polarimetry script. Details will be added here later.