ADONIS Adaptive Optics System

PSF calibrator Star - Deconvolution methods

Note: The ADONIS adaptive optics system is controlled by the telescope operator.

Field Performances

A recent paper presented in the San Diego SPIE meeting, summarizes the Adonis performances. Get the gzip file (284Kb).

The Strehl Ratio, the Full Width at Half Maximum (FWHM) and the Encircled Energy are the performance parameters we consider.
A Strehl Ratio of 1 corresponds to the full diffraction limit performance of the telescope at the wavelength of observation. Any adaptive optics system achieves partial correction only, with demonstrated Strehls up to 0.8.
Note that the diffraction limit spatial resolution is achieved very closely already at Strehls of 0.15 - 0.2, which allows for post-processing deconvolution techniques to restore the full diffraction limit in the images. Higher Strehls do not add to the spatial resolution, but add to the photon concentration and SNR of the observed features.
The plot of experimental Strehl Ratios vs FWHM in diffraction limit units is shown below.

The Point Spread Function [PSF] is variable in time if the atmospheric seeing varies with time.
We have observed on average slow [tens of minutes] drifts of the average seeing conditions, however on short time scales [seconds] we have rapid variations of the seeing and of the AO PSF. Those who need short exposures may seriousely consider post-facto frame selection before starting the data reduction and analysis.

Due to the strong dependence of an adaptive optics system on atmospheric conditions (seeing, averaged wind speed of the turbulent layers), the following characteristics are given for average atmospheric conditions (seeing=0.8 arcsec, averaged wind speed=10 m/s). We distinuguish the case when the WFS reference star is on-axis and the case when it is off-axis:

The object observed is used for wavefront sensing, i.e. on axis reference star.

The Strehl vs FWHM recorded data represent the performance of the system.

We have measured the performances which can be expected observing at 2.12 microns, with the Reticon Wavefront Sensor. These have been published in the ESO technical report (330 Kb, Gzip Postscript). We also show from the same report some plots:

Strehl Ratio vs the magnitude of the reference star (NGS), Figure 2.
Strehl Ratio vs FWHM in arcsec, Figure 4
50% Encircled Energy diameter vs reference star magnitude, Figure 5
Strehl Ratio variation vs time, Figure 6

If the spectral type of the object observed is higher than M0, one magnitude should be added at fig.2 x-scale.

EBCCD

The number of photons in the wavefront sensor is related to the color-index of the reference star. Redder stars provide more photons at a given magnitude, as shown for the ebccd in Figure 7

2. The reference star has an angular offset from the science target:
The larger the separation between program source and reference star is, the less efficient the compensation will be.
Figure 3 is a plot of theoretical Strehl ratio behavior versus angular separation. This Strehl value has to be multiplied with the Strehl achievable by the system, with the star on axis. It has been established with a theoretical Cn² profile and checked for some separation angles. The maximum field angle between the object and the reference star we recommend is 30 arcsec.

Numerical simulation performances: Reticon and EBCCD

A numerical simulation of the Adonis Adaptive Optics system has been done to provide the observers with simulated PSFs and expected performances. The performances have been checked with our average results during Adonis runs. The results allow users to forecast or simulate their observations.

The results have been published and the reference will soon be made available here (18\08\00).

The D/ro value (ratio of the telescope diameter to the Fried parameter) taken is 18. This choice corresponds to a seeing of 0.62" at 0.6 µm which is the weigthed WFS wavelength.
We report results for Adonis using the Reticon (200 Hz) and the EBCCD (100 Hz) wavefront sensors. The PSF images as well as the encircled energy are given, for each natural guide star (NGS) V-magnitude. It has been assumed a K0 spectral type NGS, the magnitude may be rescaled via the measured color dependance of the WFS response. For the ebccd this has been measured experimentally and reported.
The PSF represent an IR long exposure of 10 seconds.

Limitations

One of the most important limitations of an adaptive optics system is the lack of sufficiently bright reference stars close to the observed object. It is recommended to use the object itself whenever possible to sense the wavefront in the visible.
The degree of correction depends on the visible magnitude on the reference star, on the angular distance from the object and on the seeing conditions. The angular distance from reference to science target lowers the PSF quality due to field anisoplanatism. This effect is shown in Fig. 3, however one should bear in mind that it can be weaker or stronger, depending on the altitude of the turbulent layers. We have observed 30% changes with respect to the theoretical curves shown in Fig.3

Other limitations

Recommended max. zenith angle: 50° (above this zenith angle the correction efficiency decreases rapidly); Max. zenith angle: 60°.
Maximum reference object diameter for wavefront sensing: 2 to 3 arcsecs depending on the seeing conditions.

PSF calibrator star - Deconvolution methods

If desired, a point spread function (PSF) measured with an unresolved calibrator star specified by the astronomer might be recorded in order to carry out an a posteriori deconvolution. We strongly recommend this. We also recommend to use the data reduction routines we distribute, at least if the user has not experience with AO data. The purpose of observing a calibrator star is to obtain a good estimated PSF in the IR for adaptive optics correction identical to astrophysical program source. It will not be however exactly identical to the PSF used in your science acquisition, since the seeing changes continuosely. Therefore, linear or direct deconvolution methods should be used with extreme care to avoid artifacts, especially if faint structures are searched for.

The PSF calibrator star should be measured as closely in time as possible (before and after each exposure) to the program source observation in order to limit the effects of seeing evolution. This condition can be achieved if the flux (number of photon per m²) at the input of the telescope in the bandpass of the wavefront sensor ([450-850]nm) is the same for the reference star and for the calibrator star (with a tolerance of 20%). This may lead to the following guidelines to choose the calibrator star:

For reference star fainter than m_v=10 the optical magnitude of the calibrator star should be equal to that of the reference star to ensure identical correction efficiency.
Have the same spectrum type than the reference star used for wavefront sensing:
A set of neutral density filters are available in front of each wavefront sensor to attenuate the flux of the reference star. Those density filters can help to adjust the flux of the PSF calibrator star equal to that of the reference star (+/- 1 magnitude).

Additional conditions should be fulfilled:

The calibrator star should be bright enough in the IR to obtain a good quality PSF in a relatively short time. In case the observed object is very faint, requiring for instance exposures longer than 30 min, an image of the calibrator should be recorded often enough, say every 10 min. The PSF chosen for deconvolution will then be the more representative of the seeing conditions during the exposition.
Be located within 10 deg. maximum around the object observed.

The equivalent integration time for the PSF should be long enough to integrate the correction variation (t > 3 mn).

Please note:
If it is not possible to find a psf calibrator close to the object with the right flux in the IR and visual, at least both object and psf must be suitable for the same wavefront sensor camera. If the astronomer chooses the psf according to the visual magnitude only (because there is no IR magnitude available) he/she should come here with 2 or 3 psf candidates, to be sure than one of them will be okay in the IR.

References

[1]: Hubin N. et al., New adaptive optics prototype system for the ESO 3.6m telescope: Come-On-Plus, SPIE 1780-87 (ESO pre-print no 48)
[2]: Rousset G. et al., First diffraction limited astronomical images with adaptive optics, Astron. and Astrophys., 230, L29-L32, (1990)
[3]: Rigaut F. et al., Adaptive optics on the 3.6m telescope: results and performance, Astron. and Astrophys., 250, 280- 290, (1991)
[4]: Hofmann R. et al., SHARP and FAST: NIR Speckle and spectroscopy at MPE, ESO conference on Progress in Telescope and Instrumentation Technologies (1992)