Performance of Subaru AO188
Performance of AO is characterized by a Strehl ratio (a peak intensity normalized by that of PSF without any turbulence) and FWHM of the PSF. These depend on a the AO guide star magnitude, seeing, sky background and angular separation between the target and guide star. In general, performance of the AO system is better with a brighter guide star, at a better seeing, or longer wavelength.
On-source Strehl ratio and FWHM (NGS mode)
The plots below show Strehl ratio and FWHM as a function of AO guide star magnitude and observed wavelength. Please note that the performance shown here would be the best cases since these data were taken under good natural seeing condition of 0.4 arcsec. We also show a FWHM as a function of seeing based on the data obtained by AO36 (Oya et al. 2004, SPIE Proc., 5490, 409), for your information. Seeing statistics at Subaru is shown here.
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Encircled energy (NGS mode)
The plot below shows the encircled flux plot of the point sources obtained by AO188 with R=9.0 guide star at 0.5-0.6 arcsec seeing condition. Half-flux radius, within which half of the total flux is contained, is 0.1 arcsec for JHKL'M' band, 0.2 arcsec for shorter z band, and 0.5 arcsec for normal seeing condition. The higher flux concentration due to the AO correction resulted in much higher sensitivity. The expected sensitivity gain by AO188 for point sources is about 2.0 mag for K band in best case (SR~0.5).
Isoplanatic field
Performance of AO correction degrades gradually with increasing distance from a guide star because the angular correlation of atmospheric turbulence becomes lower as increasing distance from a guide star (anisoplanatism). The isoplanatic angle (in which atmospheric turbulence should be considered approximately uniform) of AO188 would be around 30 arcsec in radius, although these values may vary from night to night, or even during a night (see figures below).
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10 August 2009