Chopper calibration: explanation and overall strategy

• There is evidence from the RCW 57(West) large-maps that the chopper throw is not really set to 0.65 degrees, as desired. The negative beams appear to be separated from the main beam by only 0.625 degrees. A detailed analysis by Giles and Hua-bai shows that this can be explained by postulating that the telescope beam-switch is correctly set to 0.65 degrees, but the chopper is really chopping only 0.60 degrees. The chopper throw was never calibrated carefully in summer, so it is possible that our calibration is slightly wrong.



• Paolo has provided the following information:
  

  
  setting of the peak-to-peak voltage on the HP 33120A signal generator is 4.446 V
  
  setting of the offset on the HP 33120A signal generator is -1.110 V
  
  




• According to the calibration derived in Summer, this should give a chopper throw of 0.65 degrees. (Calibration is 6.84 Volts per degree, see page 110 of logbook 7.)



• Accordingly, we want to look into this further, and possibly make an adjustment. The next steps in this process are:
  
  
  1. To really find out what is going on, we should make a set of large-maps of rcw57(West) that have the following characteristics: as all large maps, we want xstep=ystep=0.1625. But for these observations only we should set the sparo::throw parameter in Greg's scripts to be 0.325 instead of the usual 0.65. But we should leave the settings on the HP33120A set to the values given above (4.446 V, etc). This will make a weird map where the beam-switching is deliberately mismatched with the actual chopper throw. We should see four beams: two positive and two negative. The relative separations of these beams will directly give us the beamswitch distance (presumably 0.325), and the chopper throw (which we suspect is actually 0.60, not 0.65 as desired). These maps should be made with nx = 9, ny = 1.
  
  
  
  2. If we confirm that the chopper is really set to 0.60 as suspected, then the final step will be to increase the voltage settings on the HP33120 to make the chopper throw equal to 0.65 degrees.