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. Followings are summary of the AO188 performance in the NGS and LGS modes. Please note that the LGS performance has not been well determined at this moment. It will be updated as the engineering runs for the performance verification are conducted during S10B and S11A.

NGS mode

1. On-source Strehl ratio and FWHM
2. Encircled energy (NGS mode)
3. Isoplanatic field

LGS mode

1. LGS brightness

Performance in the NGS mode

Reference

"Performance of Subaru adaptive optics AO188": Minowa et al. 2010, Proc. SPIE in press.

1. On-source Strehl ratio and FWHM

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.

FWHM as a function of R-magnitude of the NGS and the observed wavelength under good seeing condition of 0.4 arcsec.

Strehl ratio (SR) as a function of R-magnitude of the NGS and the observed wavelength under good seeing condition of 0.4 arcsec.

Natural seeing vs. FWHM of AO corrected PSF. The data points are color coded along the guide star magnitude. Note that this plot is based on the data obtained by AO36.

2. Encircled energy

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).

3. 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).

Variation of stellar FWHM and Strehl ratio within IRCS 52mas mode field of view as a function of distance from a guide star. The results are based on snapshots of a globular cluster M15 taken under good seeing condition. The color difference shows the difference between frames that were continuously taken at approximately 20 seconds intervals.

Performance in the LGS mode

Performance in the LGS mode has not been well determined yet. We will perform the engineering runs for verifying its performance during S10B and S11A semester. The performance information will be updated as it is well determined in S10B and S11A semesters. It should be noted that the performance of the LGS mode become worse as decreasing the brightness of tip/tilt guide star (TTGS) and as increasing the distance between the science target and TTGS.

1. LGS brightness

The brightness of the LGS is around 10.5 mag in R band in best case. This brightness might become fainter depending on the condition of the sodium layer and telescope elevation. The performance of the LGS mode should be worse than that of the NGS mode with a similar guide star brightness.

2 August 2010