CFHT Information Bulletin, number 38, First Semester 1998

Introduction

For those unfamilar with PUEO, it is based on a 19 subaperture curvature wavefront system (Roddier et al. 1991). A complete description of the system is given by Arsenault et al. (1994), and a comprehensive discussion of its performance by Rigaut et al. (1997). If the target is to be observed in the infrared, a dichroic is used to reflect the visible light to the wavefront sensor (WFS) while transmitting the IR light to the science detector, but if the target is to be observed in the visible, a beamsplitter must be used to send a percentage of the light to the WFS. The delivered image quality depends on the brightness of the reference star, its distance from the target, the seeing at the time of observation and the wavelength of observation. To gain an appreciation of how these variables affect the images and what can be expected, the reader is encouraged to visit the "performance meter" at http://www.cfht.hawaii.edu/Instruments/Imaging/AOB/psf.html@. To achieve FWHM ~ 0.12" imaging in the near-IR (diffraction-limited) and at J and I under median seeing conditions, the reference star must be brighter than R~14 and within ~30" of the target. However, the system will provide good correction on stars as faint as R = 17 under good seeing conditions, and the reference star can be located anywhere within the 90" diameter field. AOB also offers considerable advantages in terms of operational efficiency, since it is literally a push-button operation that eliminates all the overhead of focussing and guiding associated with conventional observing.

Since WFPC-II on HST delivers excellent images in the visible spectral region, most of the observations with AOB to date have been carried out in the near-IR where there is less competition and also where it produces the best images. Hence, the observations reported here were mostly made with the University of Montreal infrared camera MONICA (Nadeau et al. 1994) which was modified to give a pixel scale of 0.034" and hence a field of 8.8"x 8.8". This very small field is a significant handicap for many observations, as is the fact that the detector suffers from persistence problems, i.e., a bright source leaves a residual image that slowly decays with time. Since the site seeing is variable, observations of relatively bright stars usually have to be carried out in order to monitor the PSF (point-spread function). The sequence of observations of very faint sources thus has to be carefully planned to minimize such problems while frequently observing the PSF star to attain the scientific goals. Veran et al. (1997) have recently demonstrated that statistics of the wavefront sensor signals can be used to derive an excellent model of the average PSF (as long as the guide star is m = 13 or brighter). This is obviously ideal in that the PSF estimate is simultaneous with the target observation, and it obviates the requirement of directly observing the bright star (saving time and avoiding persistence problems). A comparison demonstrating how well the model reproduces the observed PSF is reproduced in Figure 18. For each observation, statistical data on conditions at the time of observation are recorded in "psf" files and the atmospheric-corrected PSF is stored in "dph" files. The PSF corresponding to each exposure can then be calculated with the CFHT "DPH2PSF" package (see http://www.cfht.hawaii.edu/~veran/cfht_dph2psf.html@)

Examples of Self-referencing Targets

AGN are obviously also easy to guide on if they are sufficiently bright. Several AGN and Seyfert 1 type galaxies with point-like nuclei and magnitudes in the range m = 11-14 (e.g., bright ones like NGC 4151 to fainter ones like NGC 6814) were observed during commissioning. However, guiding was not successful on more diffuse objects such as the V = 13 nucleus of the Seyfert 2 galaxy Mrk 266, or the large bright elliptical galaxy which hosts 3C296. Numerous "AGN" projects are currently underway and a few results have already been published or are in press, e.g., NGC 7469 (Lai, 1997), Mrk 273 (Knapen et al. 1997) and NGC 1068 (Rouan et al. 1997)

In general, observations of these targets was hampered by the small MONICA field since neither the sky nor PSF stars are included in the frames. An accurate estimate of the PSF is essential for these sources in order to adequately subtract the effects of the bright point source from the nuclear region, so both the sky and a nearby reference star of similar brightness must be monitored throughout the sequence of observations. PSFs derived from the WFS data using the Veran et al. method are obviously valuable in this context.

Targets with guide stars

The investigation of quasar host galaxies provides an example of a project where knowledge of the off-axis PSF is vital. Studies of QSO host galaxies are notoriously difficult due to the faintness of the galaxy itself plus the superposition of the bright nuclear source. The current quasar catalogues have been cross-correlated with the HST Guide Star Catalogue, yielding a sample of a few hundred quasars with suitable bright guide stars. A number of these have now been observed with AOB, both in the visible region (at I) with a CCD detector and in the near infrared (mostly at H). Unfortunately, our project has so far been plagued by poor weather and the data for only one, 1055.3+019, has been completely analysed and published (Hutchings et al. 1997). This z = 1.1 quasar is well-resolved in both I and H. The inner region (r < 0.65") of the galaxy exhibits a r^1/4 profile, while the outer region has an exponential scale length of 0.35" (1.8 kpc) in the H band. The data suggest that the host galaxy has recently been subjected to a merger event.

The components of multiply-imaged gravitational lenses formed by galaxies in the line of sight to distant quasars typically have subarcsecond separations and consequently their study and monitoring could significantly benefit from the improved resolution offered by adaptive optics. Unfortunately, only a few of the ~35 such lenses currently known (according to the list maintained by Chris Kochanek http://cfa-www.harvard.edu/glensdata ) have nearby guide stars that are sufficiently bright to give good correction. One exception is SBS 1520+530 (Chavushyan et al. 1997), a doubly-imaged BAL quasar with a separation of 1.6 which happens to be only 13" from a m~12 star (see Figure 20). H band images with FWHM = 0.15" taken with AOB reveal the lensing galaxy 0.40" from the fainter component, offset 0.12" from the line joining the components (Crampton, Schecter and Beuzit 1997).

Registering invisible targets

Shift_x = -0.3588 × (WFSX - 1900) - 0.0714 × (WFSY - 2150)

Shift_y = -0.0825 × (WFSX - 1900) + 0.3715 × (WFSY - 2150)

where the WFS coordinates 1900, 2150 correspond to the approximate centre of the detector. The above equations can be used to determine the relative shifts required to stack a series of images. Unfortunately, the WFSY stage has developed more backlash than when tested in the lab, so that application of the above calibration leads to a dispersion in y of 2.9 pixels compared to 1.7 pixels in x. In practice, since the same dither pattern is normally used for sequences of exposures, empirical corrections for the backlash can be determined and applied. If this is done, repeated observations of the same point source (e.g., a quasar) over several hours show that the WFS positions can be used to register frames with a dispersion of 1.4 pixels or 0.05". Since the delivered images in the near-IR are usually diffraction-limited with FWHM ~0.12, this is clearly inadequate, so future systems should be designed to minimize this. One solution (which we have adopted for Gemini instruments) is to incorporate a tip/tilt/focus sensor on each instrument to eliminate the effects of flexure, backlash, etc. Ideally, one would like diffraction-limited images to be registered to better than ~10% of their FWHM; for 8m telescopes this will be of the order of only a few mas.

The 3C 273 jet provides a good example of a target which was barely visible, even on background-subtracted 300s exposures, and certainly no features are present which can be used to register the images. The jet is located between 11 - 21" from 3C273 which, of magnitude R = 13, was used as the guide star. Given the small field of the detector, the jet was observed at two different locations along the jet centered at 15 and 20" from 3C273. Repeated observations of point sources in other fields and in M13 were used to calibrate the WFS positions for this run. The (preliminary) result of stacking the exposures is shown in Figure 21. During these observations, the site seeing was poor on average and very variable (from 0.5" to 1.2") and so the compensated image quality is also variable. Although the results of the AOB commissioning observations indicate that images do not degrade as quickly with distance off axis as initially expected, the image quality at the end of the jet (~20" from the guide star) is not as good as on axis, particularly when the seeing is poor. Including registration errors (0.05"), the spatial resolution of the jet in Figure 21 is probably not better than ~0.3". The appearance of the jet in the H band is very different from that in the visible (Bahcall et al. 1995, Roser et al. 1997), but is similar to that seen at K' by Neumann et al. 1997.

Further examples of "invisible targets" are observations of high redshift galaxies. A pair of CFRS galaxies near a R = 15.3 guide star were observed during commissioning, and one of the z = 3.0 Steidel et al. (1996) galaxies was also attempted. In both cases, the goal was to examine the morphology of high redshift galaxies at more normal restframe wavelengths than is possible in the visible (H band corresponds to restframe I and B at z = 1 and 3 respectively). Observations of these very faint targets were significantly hampered by the detector remanence effect discussed above and also by the reduced sensitivity (three times lower than anticipated) of MONICA during the latter half of 1996. For example, it was initially estimated that each z = 3 galaxy (R ~ 24) would require a total exposure of ~8h, but when it became obvious that the object was barely visible after a few hours (because of the reduced sensitivity), the project had to be abandoned.

Devising observing strategies to minimize the effects of detector persistence yet still be confident of the registration in these tiny blank fields is quite a challenge. However, since the field of the new KIR detector for AOB is four times larger (and its sensitivity is three times higher) future observations of this type of target will be much easier. Although initial reports (Doyon, private communication) indicate that the detector remanence effects are absent or much reduced for KIR, it will still probably be wise to centre the observations of bright stars (for PSF and WFS calibrations) at different locations on the detector than faint targets.

Summary

Given a suitable guide star, AOB delivered near-IR images worse than ~0.1" on only 1 of the ~30 AOB nights that I've been involved with so far. Figure 22 shows typical results, from ~350 observations taken on the first 10 commissioning nights (Rigaut, Lai & Veran, 1996). It feels like we're really entering a new era: from now on ground-based astronomers will complain when the images are significantly worse than 0.1" rather than 1"!

Acknowledgment: While the above discussion is the result of my personal experience with AOB, the fantastic performance of Pueo is due to the combined efforts of teams from CFHT, Meudon, Laserdot, UH and DAO. Francois Rigaut deserves much of the credit for this superb implementation of Francois Roddier's curvature concept. It is a very exciting instrument to use.

References

Bacon, R., Emsellem, E., Monnet, G. & Nieto, J.L. 1994, A&A, 281, 691

Bahcall, J.N., Kirhakos, S., Schneider, D.P., Davis, R.J., Muxlow, T.W.B., Garrington, S.T., & Conway, R.G. 1995, ApJ, 452, L91

Chavushyan, V.H., Vlasyuk, V.V., Stepanian, J.A., & Erastova, L.K. 1997, A&A, 318, L67

Crampton, D., Schecter, P. & Beuzit, J.-L. 1997, submitted to AJ

Davidge, T.J., Rigaut, F., Doyon, R. and Crampton D. 1997, A.J., 113, 2094

Hutchings, J.B., Crampton, D., Morris, S.L., & Steinbring, E. 1997, submitted

Knapen, J.H. et al. 1997, astro-ph/9709234

Kormendy, J. & Richstone, D. 1995, ARAA, 33, 581

Lai, O. 1997, These, Observatoire de Paris-Meudon

Lai, O., Arsenault, R., Rigaut F. et al. 1995, in OSA/ESO Topical Meeting on Adaptive Optics, ed. M. Cullum (Garching, ESO), p. 491

Nadeau, D., Murphy, D.C., Doyon, R. & Rowlands, N. 1994, PASP, 106, 909 Neumann, M., Meisenheimer, K. & Roser, H.-J. 1997, A&A, in press

Rigaut, F. et al. 1997, PASP, in press, astro-ph/9712196

Rigaut, F., Lai, O. & Veran, J.-P. 1997, Internal CFHT report

Roddier, F., Northcott, M. & Graves, J.E. 1991, PASP, 103, 131

Roser, H.-J., Meisenheimer, K., Neumann, M., Conway, R.G., Davis, R.J. & Perley, R.A. 1997, A&A, in press

Rouan, D. et al., 1997, submitted to A&A

Steidel, C., Giavalisco, M., Pettini, M., Dickinson, M. & Adelberger, K.L. 1996, ApJ, 462, L17

Steinbring, E. 1997, in preparation

Tremaine, S. 1995, AJ, 110, 628

Veran, J.-P., Rigaut, F., Maitre, H. & Rouan, D. 1997, J. Opt. Soc. America, A, 14, 11