FRIDA@IAC

Observations with Adaptive Optics (AO) have the strong limitation of requiring the presence of a suitable bright star, or point-like source, (R ~11 mag is ideal, but not fainter than R~16 mag) relatively close, within a few arcsecs, to the science target. This distance is referred to as thse anisoplanetic patch, it depends on the site atmospheric conditions and observation wavelength. Under good atmospheric conditions, seeing with FWHM ~ 0.6" in the IR, a distance r <~ 12 arcsec from the science target in K-band, r <~ 6 arcsec in J-band, are recommended. The closer the AO star to the science target, the better the improvement in image quality. Partial atmospheric correction might still be achieved if the AO star is located at larger distance, but this should not exceed more than 30 arcsec radius. Of course, the best atmospheric correction is achieved when the target and the reference star are the same.

The implementation of a Laser Guide Star in GTC will somewhat easier the above requirements. Adaptive Optics with a laser still requires the presence of a star -so called tip-tilt star - near to the science target but it could now be fainter, R <~ 17 mag, and further, ~40", with decreasing performance for further distances up to 1 arcminute radius. The graphics bellow show a direct measurement of the number/probability-of-finding suitable stars for Adaptive Optics observations with and without a laser system. The presented calculations, by Fermenia, Devaney, Castro and Garcia, 2003 (GTC document NNTE/OPTI/0235-R), make use of the Guide Star Catalog (GSC-II), which is completed up to R=18.5 mag and samples the whole sky. It is shown the number of available stars within a field of view of 2 / 1.5 arcmin diameter, for different Galactic latitudes and longitudes, as a function of the magnitude of the star.

As it can be seen, the probabilities are rather low in particular at high latitudes, which mostly affects extragalactic astronomy. However, it should be note that for example, nucleus of galaxies, quasars, if within the magnitude limits quoted above, are also perfect targets for AO observations - as shown in FRIDA science drivers.

Probability of finding a reference star of a given magnitude, m(R), and different Galactic longitude and latitude (l,b), within a FoV of 45 arcsec radius from the science target, suitable for natural AO systems, and within a FoV of 60 arcsec, suitable for laser guide systems. Different curves within the same panel refer to the I-bands in which the data are binned. For the sake of visualization only the error bars associated with l = 0 degrees band are plotted, yet all curves within the same panel (i.e. same b) exhibit the same level of dispersion. For b = 90 degrees there are no associated error bars as that bin was generated with only 1 file per Galactic Pole (from B. Fermenia, GTC).

Cumulative star number densities as a function of the star magnitude. Each panel displays for a given Galactic latitude the number of stars in a 1.5' FoV with magnitude less or equal than m(R) (from Fermenia et al. 2003).

Cumulative star number densities as a function of the star magnitude. Each panel displays for a given Galactic latitude the number of stars in a 2' FoV with magnitude less or equal than m_R (from Fermenia et al. 2003).

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