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 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. 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

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