Chopper Testing

1
Chopper Testing

• 8/3/06

2
Chopper Information

• We chose to use the Thorlabs chopper with two
  7-slot wheels offset to give approximately 728µs
  (18.7º) of open time at 500Hz chop rate (534µs
  for return pulse 194µs dark time). The delay is
  given to the function generator before the signal
  is sent to the chopper. The delay was determined
  experimentally on the sky with the laser. The
  current best delay is 36000cts.
• Chris changed the electronics so that we can stop
  the wheel in either the open or closed position.
  The control is accomplished through the ext sync
  button. When the chopper is stopped Ext Sync on
  chopper open and Ext Sync off chopper closed.
  The chopper always stops in one of these
  positions.
• All functions can be accessed via a serial line.
  The setup is in a hyperterminal configuration on
  the WFS PC labeled chopper.ht.
• There was some instability in the chop rate due
  to the addition of the second wheel. Using a
  scope to check the synchonization of the pickup
  signal to the external sync, we adjusted the
  trimpots in the electronics that determine the
  control loop parameters in order to stabilize the
  wheel. The trimpots were moved by I 3 turns
  CCW P 1 turn CCW TC2 6 turns CCW. The
  jitter in the sync between the two signals is now
  about 50µs P-V.

3
Chopper PSDs

• Testing was done on the white light
• HOWFS was running at 2kHz

4
TTM Residuals with and without Chopper

• Testing was done on the white light
• HOWFS was running at 2kHz
• Frames with partial or no illumination have been
  filtered out of the 500Hz chopper data

5
Centroid Movements
RMS 500Hz 0.028 pixels Stopped 0.025
pixels Off 0.025 pixels Maximum Motion
500Hz 0.12 pixels Stopped 0.098 pixels
Off 0.097 pixels

• The maximum centroid motion with the chopper
  running is comparable to the motion with the
  chopper off and it is only a bout a 1/10 of a
  pixel, so there should not be a problem with the
  chopper due to vibration.

6
Comparing Background Methods

• We took three background images with the chopper
  running one with the laser shuttered, one with
  the laser detuned and one with the uplink TT
  (llt_b) pointed off by 20
• These images show the shuttered image subtracted
  off of the detuned and shifted images
• We are blocking the Rayleigh (and other scatter?)
  that we normally see in the detuned images, but
  when we point off we are picking up some other
  light (scatter?).
• NOTE This testing was done before we tried to
  stabilize the chopper rate.
• HOWFS was running at 100Hz

7
Conclusions and Further Testing

• The chopper on and stopped does not increase the
  noise in the WFS due to vibration.
• The residuals in the HOWFS are low enough that
  the vibration due to the chopper running should
  not be a problem for us.
• We should re-take the background data to verify
  the results with a stabilized chopper wheel
• We should check the effect of the chopper on the
  LOWFS residuals