6 Closed loop compensation of NAOS-CONICA static aberrations

6.1 Rendering of aberrations

Having explored in detail the application of phase diversity to calibrate NAOS and CONICA static aberrations in Paper I, we presented above the experimental results applying PD as a wavefront sensor. We described how the contributions of the different optical components in the light path are separated to create a complete calibration configuration table. For each possible configuration of the instrument the corresponding correction coefficients are rendered to NAOS and are used to adjust the AO system. In this manner the DM will take the shape needed for compensation of the static wavefront aberrations. To demonstrate the final gain in optical quality we compare the originally acquired images without correction for static aberrations with the images obtained after closed loop compensation. The gain will be quantified in terms of SR numbers.

6.2 Full AO correction

The 10 $\mu$m calibration source in the entrance focal plane of NAOS simulates a star without turbulence. The visible WFS is used to correct for the common path aberrations. Therefore, the loop is closed on the 400 $\mu$m source as described in Sect. 3.2. The light is separated by the dichroic VIS, thus the WFS sees the visible part and the near-infrared is directed towards CONICA.

Figure 10 shows two extreme cases of applying AO compensation. The upper pictures demonstrate the correction for a filter in J-band, the pictures below in K-band. In accordance with Figs. 4 and 5 the sensed aberrations in Jand K band are very similar - recall that the main contribution arises from the achromatic camera objective and the NAOS dichroic. But even if similar correction coefficients are rendered to the AO system, the effect on the image is strongly wavelength dependent. This is due to the fact that the influence of the applied Zernike coefficients scales with the wavelength. Thus, we achieve a striking correction in J-band visible with the naked eye on the images before and after correction. The most important aberration, the astigmatism, vanishes and the PSF is contracted. In K-band the non-corrected image is already very close to the optimum and the improvement is hard to see directly on the image. But calculating the SRs shows that even in K-band the performed correction is still significant (Table 5). Note that the given error arises from a maximum estimate of all error sources as described in Sect. 5.2. The nature of the error is mainly systematic (e.g., caused by background correction) and affects the calculated SRs for the image pairs in the same way. SRs determined on experimental data are intrinsically afflicted by rather high error bars, but a direct inspection of the images (central intensity, shape of the diffraction rings) shows the relative gain of 2 to 3% in K-band to be true. Even this rather small appearing gain in K is of high importance. On the way to scientific goals such as e.g. planet detection, the total error budget must be tackled to eliminate every percentage point of loss in SR.

Figure 10: Comparison of PSFs before and after closed loop compensation. Above a couple of J-band images and at the bottom a couple of K-band images are shown. Left side: without AO correction. Right side: with AO correction. Especially in J-band, the sharpening of the PSF can be clearly seen.

   

Table 5: Comparison of SRs for two selected filters before and after AO compensation for static aberrations. The maximum mainly systematic error is given. Statistical errors are significantly smaller.

Filter	SR no corr (%)	SR with corr (%)
Pgamma	60 $\pm$ 4	70 $\pm$ 4
Ks	91 $\pm$ 4	93 $\pm$ 4