We are proposing to pair a high order deformable mirror from ALPAO (468 actuators) with a pyramid wavefront sensor, and a no noise camera (NUVU 128) for the pupils imaging of the WFS. The PWFS flexibility and the no noise camera allows a very large optimization of the number of corrected modes and magnitude sensitivity. Extreme AO in good conditions, classical AO in moderate conditions, and Strehl improvement in bad seeing. This requires the simultaneous optimization of 5 parameters, as a function of the seeing, wind speed and NGS magnitude:
- The loop integration time
- The modes gain
- The GS beam modulation angle on the pyramid apex
- The spatial sampling of the detector
- The noise regularization parameter of the MMSE algorithm.
We propose the development of an over loop for the system continuous optimization, based on real time telemetry analysis. Because the behavior of the system wrt seeing and NGS magnitude is systematic, we think that machine learning might be very useful to speed up the optimization process, once the machine have been fed with enough knowledge.
The modeling of the system is done with the AO simulation tool YAO (Rigaut & van Dam, AO4ELT3). Even if the simulation was not fully optimized, it shows that the envelope of the Strehl, at 1650 nm, as a function of the NGS magnitude allows both an excellent correction for bright NGS and a really good limiting magnitude. This simulation was done for 0.9” seeing.
We use off axis parabolas (OAPs) for the common path (WFS + science) to avoid chromatic aberrations (compared to lenses). This choice implies that we must have parallel beam incident (or reflected) by each OAP in order to avoid spherical aberrations. The constant geometrical parameters are summarized below :
- exit pupil diameter : ØExtP = 727.4046 mm;
- distance from the exit pupil to the telescope focal plane: ExP-FP = 10338.74 mm;
- DM clear optical diameter : DM = 33.0 mm;