Performance of Subaru adaptive optics system AO188
Yosuke Minowaa , Yutaka Hayanoa , Shin Oyaa , Makoto Watanabeb , Masayuki Hattoria , Olivier
Guyona , Sebastian Egnera , Yoshihiko Saitoa , Meguru Itoa , Hideki Takamia , Vincent Garrela ,Stephen Colleya , Taras Golotaa and Masanori Iyec
a Subaru Telescope, NAOJ, 650 North Aohoku Place, Hilo, Hawaii 96720, USA;b Hokkaido University, Faculty of Science, Kita 10 Nishi 8, Kitaku, Sapporo, Hokkaido

060-0810, Japan;c National Astronomical Observatory of Japan (NAOJ), 2-21-1 Osawa, Mitaka, Tokyo

181-8588, Japan
ABSTRACT
Subaru adaptive optics system (AO188) is an 188-elements curvature sensor adaptive optics system that is operated in both natural and laser guide star modes. AO188 was installed at Nasmyth platform of the Subaru
telescope and it has been successfully operating in the natural guide star mode since October 2008. The performance of AO188 in the natural guide star mode has been well veriﬁed from on-sky data obtained with the infrared
camera and spectrograph (IRCS). Under normal seeing condition, AO188 achieves K-band Strehl ratio between
60% and 70% using R = 9.0 magnitude natural guide stars and it works well with faint guide stars down to
R = 16.5 magnitude. We measured the FWHM and Strehl ratio of stellar images in globular clusters and foundthat the isoplanatic angle is approximately 30 arcsec. In this paper, we describe an overview of the operation
procedure for AO188, as well as its performance such as angular resolution, Strehl ration, and sensitivity gain
for detecting faint objects.
Keywords: adaptive optics, telescope operation, image quality, strehl ratio

1. INTRODUCTION
Subaru AO1881 is a curvature adaptive optics system which consists of a 188 element wavefront curvature sensor
with photon counting Avalanche Photo Diode modules and a 188 elements bimorph deformable mirror. It is
installed at the IR Nasmyth platform of Subaru telescope and operated in both natural guide star (NGS) andsodium laser guide star (LGS) modes. The detail speciﬁcation of the AO188 is summarized in Table 1. The
science camera mainly used with AO188 is IRCS (Infrared Camera and Spectrograph),2 which provides imaging
and spectroscopy capabilities in near-infrared (0.9-5.3 μm) wavelength. HiCIAO (High Contrast Instrument for
the Subaru Next Generation Adaptive Optics)3 is also available with AO188 as a PI type instrument. HiCIAO
is equipped with a coronagraph and oﬀers the capability of high contrast imaging at 0.9-2.5 μm wavelength.
We are also planning to use Kyoto-3DII4 with AO188 as a PI-type instrument, which provides the capability ofintegral ﬁeld spectroscopy at optical wavelength range (0.65-0.95 μm). AO188 has successfully completed theﬁrst light observations with IRCS in late 2006. AO188 NGS mode has been oﬀered for open use observations
since 2008B. Currently, LGS mode is in commissioning phase.

2. AO188 OPERATIONThe operation of AO188 in NGS mode requires a guide star for the wavefront measurement. The guide starshould be a bright star located within 30 arcsec from your target. The target object for the science camera canalso be used as the guide star. To obtain reasonable performance improvement, the guide star should be brighter
than R magnitude of 16.5. Since we are using high-sensitive APDs for the wavefront sensor, the brightest limitof the guide star is around R ∼ 8, although the brighter star up to R ∼ −1 can be used as the guide star by
Further author information: (Send correspondence to Y.M.)
Y.M.: E-mail: minoways@subaru.naoj.org, Telephone: +1-808-934-5905

Adaptive Optics Systems II, edited by Brent L. Ellerbroek, Michael Hart, Norbert Hubin, Peter L. Wizinowich,Proc. of SPIE Vol. 7736, 77363N · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.857818

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Table 1. Summary of AO188 NGS mode speciﬁcations

Location of the system
Defomable mirror
Wavefront sensor
Real time computer
Control band widthField of view
Main optics throughput
Guide star magnitude
Science instrument

Nasmyth focus (IR)
Bimorph mirror with 188 electrodes
Curvature sensor with 188 photon counting APDs
4 Xeon CPUs (2.0GHz) with real time Linux OS> 100 Hz, 1000 correction/sec
2’.7
∼ 70%
−1 < R < 16.5 (NGS)
IRCS, HiCIAO, Kyoto-3DII

using attenuation ﬁlters installed inside of the wavefront sensor. To feed the guide star to the wavefront sensor,
we used a guide star acquisition unit that consists of two gimbal mirrors and a linear stage. The gimbal mirrors
adjust the position and ray tilt of the guide star. The linear stage is mounted under the one of the gimbal mirrors
and adjusts the focus position of the guide star by changing the separation of two gimbal mirrors.5
Once we acquire the guide star to the wavefront sensor, we try to ﬁnd the optimum control parameters, namely
amplitude of the focus modulation for wavefront curvature measurement and servo loop gain. The modulation
amplitude is corresponding to the voltage applied to a vibrating mirror (VM) mounted in the wavefront sensor.
Higher voltage enables the measurement of higher order wavefront aberrations, but decreases the S/N of the
wavefront measurement because of the loss of photon counts detected on the APDs. To ﬁnd the optimum value,
we scan the VM voltage by checking the photon counts. We empirically found that the optimum value is thevoltage where the photon counts decreased by 10-20 % of the maxim um counts. Since AO188 is currently using
the classical zonal control algorithm, we scan a single servo gain by checking wavefront error estimated at the
wavefront sensor and the variance of the DM voltages. Figure 1 shows an example of the gain scan. Too low
gain gives worse wavefront correction (i.e. large wavefront error), while too high servo gain gives control loop.
We found that the DM variance and the wavefront error are good tracers for ﬁnding optimum gain that gives
the highest Strehl ratio.
Typical overheads for acquiring the guide star and tuning the loop performance are 10 minutes for brightstar (R < 14) and 15 minutes for faint stars (R > 14). If the guide star is an extended source (e.g., nucleus of
AGN), additional ∼5 minutes is required for the parameter tuning.

Figure 1. Example of the servo gain scanning. The images of PSF obtained with IRCS during the scanning are also shown.
The optimization is performed by ﬁnding the gain at which the wavefront error is close to the smallest value before theDM control becomes unstable due to the oscillation. In this case, the gain of 3 gives the best performance.

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3. PERFORMANCE EVALUATION
AO188 was extensively tested during the engineering run held from 2008 to 2009. The performance was evaluatedin the near-infrared bands (zJHKL M ) using IRCS image mode with a pixel scale of 20 mas/pix. We took point
sources (stars) with various R-band magnitude from 8 to 17. All exposures were longer than 10 seconds to getstatistically meaningful data. To characterize AO188, we used the Strehl ratio and FWHM extracted from the
IRCS images.

3.1 Strehl ratio calculator
The Strehl ratio is often used to characterize the error of AO system. It is deﬁned as the ratio between thepeak intensity of a PSF divided by the peak intensity of a diﬀraction-limited PSF with the same total ﬂux. To
calculate the Strel ratio, we need to ﬁnd the peak value of the observed and ideal PSFs normalized by their
total ﬂux. However, it is diﬃcult to ﬁnd the peak value of the high Strehl ratio (> 0.5) PSFs, because the pixel
scale (20 mas/pix) of the IRCS is not small enough to well-sample the diﬀraction-limited PSFs whose FWHM
is smaller than 60 mas. Moreover, it is diﬃcult to ﬁnd the total ﬂux from the observed images from short (∼ 10
seconds) exposure images. The ﬂux from the outer part of the PSF halo cannot be measured due to the noise
and thereby underestimating the total ﬂux (or overestimating the Strehl ratio). To overcome these diﬃculties,
we have developed a code for measuring the Strehl ratio. At ﬁrst we ﬁts the observed PSF to ﬁnd its peak value
and center coordinates and measure the ﬂux using a circular or elliptical aperture. The size and shape of theaperture are determined depending on the S/N of the observed image using SExtractor software.6 Then a newimage which only contains the ideal diﬀraction-limited PSF at the same coordinates with the same pixel scale(20 mas/pix) as the observed one are created. The ideal PSF is created by the fourier transform of the telescope
pupil. A linear phase is applied on the fourier transfrom to shift the PSF. Figure 2 shows the example of the
PSF pupil, the linear phase, and the calculated PSF. We measure the peak value from this “under-sampled”
ideal PSF using the same ﬁtting procedure for the observed PSFs. In order to take into account the eﬀect of thetotal ﬂux underestimation, we use the same aperture as the observed PSF for measuring the ﬂux of the ideal
PSF. Finally, we can get the Strehl ratio by dividing the peak to ﬂux ratio of the observed PSF by that of theideal PSF with less aﬀected by the PSF under sampling and the ﬂux underestimation.

Figure 2. Examples of the telescope pupil (left) and linear phase (middle). The ideal PSF (left) was created by the fouriertransform of the pupil and phase images.

3.2 NGS on-source Strehl ratio and FWHM
Figure 3 shows the achieved Strehl ratio and FWHM as a function of the R-band magnitude of the natural guidestar and the observed wavelength. The Strehl ratio that we obtained for a bright guide star (R < 10 mag) was
1.0, 0.85, 0.65, 0.40, 0.20 and 0.08 at M (4.86μm), L (3.77μm), K (2.20μm), H (1.63μm), J (1.25μm), and z(1.03μm) -bands, respectively, under seeing conditions of 0.4−0.7 arcsec. We achieved the Strehl ratio of ∼ 0.1
at K-band even for R ∼ 16.5 faint guide star. The FWHM for all near-infrared bands were less than 0.1 arcsec
except for the faintest guide star with R ∼ 16.5, which is very close to the diﬀraction limit. Note that the
FWHM of the shorter wavelength images might be overestimated because of the insuﬃcient pixel sampling (20mas/pix) with respect to the size of diﬀraction-limited images.

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Figure 3. Strehl ratio as a function of R-band magnitude of the NGS and the observed wavelength under normal seeing
condition of 0.4 − 0.7 arcsec. Dashed lines show the FWHM of the ideal diﬀraction-limited (SR=1.0) PSF.

3.3 Fried length versus Strehl ratio
Figure 4 shows a plot of the Strehl ratio versus Fried length (r0 ) at the image wavelength. Solid lines show themodel plot of the residual wavefront variance (Δ). The residual wavefront variance after removal√ of the N thlower degrees of Zernike modes from Kormogorov phase screen is given by Δ(N ) = 0.2944N − 3/2(D/r0 )5/3
[rad2 ].7 We found that the best ﬁt models are Δ/(D/r0 )5/3 = 0.01 to 0.02 for bright stars (R < 10) and 0.08
for faint stars (R > 15). These are corresponding to the correction up to the Zernike order of 20 − 50 for brightstars and 5 − 6 for faint targets. Although we achieve the Strehl ratio of more than 0.7 in best case at K-band,our best performance is still low for the curvature sensor with 188 elements. According to the Racine 2006,8 thecurvature sensor with more than 100 sub-apertures is expected to achieve Δ/(D/r0 )5/3 < 0.01. This discrepancy
could be cause by the non-common path error due to the diﬀerence in optical path between science and WFS
paths and/or the limitation of the control bandwidth due to unexpected turbulence.

3.4 Encircled energy and sensitivity gain
Figure 5 shows the plot of the encircled ﬂux as a function of the aperture radius in z  JHK-bands with a typicalR = 12th magnitude guide star. We found that the aperture radii that contains 50% of total ﬂux are 4, 3, and 2
times smaller than typical seeing PSF (FWHM∼0.4) at KH, J, and z  -bands, respectively. These mean that theProc. of SPIE Vol. 7736 77363N-4
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Figure 4. Strehl ratio versus Fried length (r0 ) at the near-infrared wavebands. Solid lines show the model predictions withthe residual wavefront variances of 0.01, 0.02, 0.04 and 0.08.

sensitivity gain for detecting faint objects are increase by 0.75 mag, 0.60 mag, and 0.37 mag at KH, J, z  -bands,
respectively.

Figure 5. Encircled energy of the PSF as a function of the distance from the center of the AO corrected PSF at eachnear-infrared band. The encircled ﬂux of a seeing PSF (FWHM∼0’.4 arcsec) is also plotted for comparison. Total ﬂux
was normalized to unity.

3.5 Isoplanatic Field
Performance of AO correction degrades gradually with increasing distance from a guide star because the angularcorrelation of atmospheric turbulence becomes lower as increasing distance from a guide star (anisoplanatism).

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Figure 6 shows the Strehl ratio as a function of the distance from the guide star estimated from the image ofglobular cluster M5 and M15 obtained with IRCS 52 mas/pix mode. The Strehl ratio decreases with increasing
the distance from the guide star (Δθ) by SR ∝ exp[−(Δθ/θ0 )5/3 ] where θ0 is an isoplanatic angle deﬁned as theseparation angle at which the Strehl ratio has degraded by a factor of e with respect to the on-axis value. We
found that the isoplanatic angle of AO188 was around 30 arcsec in radius.

Figure 6. Variation of the Strehl ratio within IRCS 52mas/pix mode ﬁeld of view as a function of distance from a guidestar. The results are based on snapshots of globular clusters M15 (crosses) and M5 (asterisks) taken under normal seeing
condition. The solid and dashed lines show the Strel ratio derived from the model of the residual wavefront variances asa function of the distance from guide stars with the isoplanatic angle of 26 arcsec and 35 arcsec, respectively.

4. SUMMARY
In this paper, we presented the procedure for AO188 operation such as the guide star acquisition and loop
performance tuning. We also described a method for calculating the Strehl ratio which takes into account theeﬀect of uncertainties in ﬁnding the peak and ﬂux values from under-sampled short exposure images. Finally,
AO188 NGS mode has been characterized from the on-sky performance such as FWHM and Strehl ratio of the
PSF extracted from the IRCS near-infrared images.

ACKNOWLEDGMENTS
We would like to thank all staﬀ of Subaru telescope, National Astronomical Observatory of Japan (NAOJ), whokindly support our research and development.

REFERENCES[1] Hayano, Y., “Progress of the laser guide star adaptive optics at subaru telescope,” in [Exopnalents and disks:
their formation and diversity], T. Usuda, M. Tamura, . M. I., ed., American Institute of Physics Conference
Series 1158, 385–386 (2009).

Proc. of SPIE Vol. 7736 77363N-6
Downloaded From: http://proceedings.spiedigitallibrary.org/ on 02/08/2013 Terms of Use: http://spiedl.org/terms

[2] Kobayashi, N., Tokunaga, A. T., Terada, H., Goto, M., Weber, M., Potter, R., Onaka, P. M., Ching, G. K.,
Young, T. T., Fletcher, K., Neil, D., Robertson, L., Cook, D., Imanishi, M., and Warren, D. W., “Ircs:
infrared camera and spectrograph for the subaru telescope,” in [Optical and IR Telescope Instrumentation
and Detectors], Iye, M. and Moorwood, A. F., eds., Proc. SPIE 4008, 1056–1066 (2000).
[3] Hodapp, K. W., Suzuki, R., Tamura, M., Abe, L., Suto, H., Kandori, R., Morino, J., Nishimura, T., Takami,
H., Guyon, O., Jacobson, S., Stahlberger, V., Yamada, H., Shelton, R., Hashimoto, J., Tavrov, A., Nishikawa,J., Ukita, N., Izumiura, H., Hayashi, M., Nakajima, T., Yamada, T., and Usuda, T., “Hiciao: the subaru
telescope’s new high-contrast coronographic imager for adaptive optics,” in [Ground-based and Airborne
Instrumentation for Astronomy II], McLean, I. S. and Casali, M. M., eds., Proc. SPIE 7014, 42 (2008).
[4] Sugai, H., Hattori, T., Kawai, A., Ozaki, S., Hayashi, T., Ishigaki, T., Ishii, M., Ohtani, H., Shimono,A., Okita, Y., Matsubayashi, K., Kosugi, G., Sasaki, M., and Takeyama, N., “The kyoto tridimensional
spectrograph ii on subaru and the university of hawaii 88 in telescopes,” PASP 122, 103–118 (2010).
[5] Watanabe, M., Takami, H., Takato, N., Colley, S., Eldred, M., Kane, T., Guyon, O., Hattori, M., Goto, M.,
Iye, M., Hayano, Y., Kamata, Y., Arimoto, N., Kobayashi, N., and Minowa, Y., “Design of the subaru laser
guide star adaptive optics module,” in [Advancements in Adaptive Optics.], Domenico B. Calia, B. L. E. andRagazzoni, R., eds., Proc. SPIE 5490, 1096–1104 (2004).
[6] Bertin, E. and Arnouts, S., “Sextractor: Software for source extraction.,” A&AS 117, 393–404 (1996).
[7] Noll, R. J., “Zernike polynomials and atmospheric turbulence,” Journal of the Optical Society of America 66,
207–211 (1976).
[8] Racine, R., “The strehl eﬃciency of adaptive optics systems,” PASP 118, 1066–1075 (2006).

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