The parts of ARGOS are colored (not complete on DX).
functional scheme of the whole ARGOS system is not very different to any other
Rayleigh guided adaptive optics system:
A main time base
in conjunction with electronic delay generators synchronizes the whole process
of laser firing and detection. After the main trigger and appropriately tuned
delays, all laser heads will fire simultaneously a pulse with ~20-30ns
duration. With the pulses propagating through atmosphere a small percentage is
scattered by molecules on its path. Opening the Pockels cell shutters exactly
after twice the time of flight to 12km distance, and closing after 2Ás again,
collects the photons on the detector out of a 300m long column at that
distance. With the lasers being triggered at a 10kHz rate, the shutter will be
opened in front of the CCD ten times before the accumulated image is
transferred to the frame storage area of the detector. Reading the CCD thus
occurs at a framerate of 1kHz. The transferred image then will be pre-processed
for dark and line-to line variations, and sent then to the slope computer,
measuring the centroids in the subapertures. Those are then transferred to the
adaptive secondary for re-construction and mirror setting.
A laser launch telescope (LLT) associated with each eye of the LBT
serves to expand three laser beams and relay them from the laser box to
Left: Launch mirror above of the secondary mirror. The right
figure shows the large lens and the folding mirror on top of
the beam expander tube.
Layout of lasers, pre-optics, diagnostics and entrance of the launch telescope.
As the light path from the laser up to the emission level and back down
to the telescope is almost identical, the tip-tilt signal will cancel
out. Consequently it is important to have a rapid tip-tilt signal
available to take out this low order, but potentially high power
effect. The tip-tilt sensor will be placed on the on-axis first light
wavefront sensor board, the so called “W-unit”, developed
by the Arcetri group.
Two critical components of the wavefront sensor. The wavefront sensor
detector (upper right, pnCCD) and the gating units (upper left, Pockels Cell).
Middle: Draft of the sensor design.
Software and real time control
Sketch of the control loop design.