Effect of the Detector Technology on a XAO System

Figures 1 and 2 show the Strehl Ratio (SR) evolution as a function of the visible magnitude of the guide star. The SR was optimized by varying the frame frequency for a given detector type, a given noise and a given magnitude: between 250 and 2500 Hz for CCD1, 250 and 1500 Hz for the CCD2. Results for two types of guide stars are shown, a blue type A0 and a red type M5. These figures show that for the VLT-PF instrument and the typical conditions at Paranal, the EMCCD always gives higher SR, except for visible magnitude below 8.5 with an A0 type guide star and below 10.5 with a M5 type guide star. The gain of SR for a good classical CCD (CCD1 with 3e- read noise) at low magnitude is just 1% whereas the value of an EMCCD (CCD2) is clear at high magnitudes (faint guide star). Moreover, building a classical CCD with < 3e- of read noise at 1.5 kHz of frame rate is extremely challenging. These simulations lead to the following conclusions:

• An EMCCD (CCD1) with a read noise lower than 1e- is the best option, especially if the QE can be improved in the red by using deep depletions devices.

• A classical CCD (CCD2) may be interesting (in particular because of its very good red QE) if a readout noise lower than 2e- at high frame rate could be achieved.

1 oo r—'—■—1—i—■-■—'—h-'—1—1—i—1-■—■—

OS magnitude

Figure 1. Strehl Ratio comparison of CDD1 (classical) and CCD2 (EMCCD) for an A0 guide star type (blue guide star).

OS magnitude

Figure 1. Strehl Ratio comparison of CDD1 (classical) and CCD2 (EMCCD) for an A0 guide star type (blue guide star).

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