DAYTIME OBSERVATIONS WITH ELTs IN THE THERMAL INFRARED USING LASER GUIDE STAR ADAPTIVE OPTICS
J. M. Beckers
Using Magneto-Optical Filters (MOFs; also called FADOFs = Faraday Anomalous Dispersion Optical Filters) it is possible to clearly see Sodium Laser Guide Stars in the daytime sky. This makes it possible to use ELT Adaptive Optics systems for diffraction limited observations 24 hours/day. Because of the bright daytime sky this LGS AO application is only of astronomical interest in the mid-infrared wavelength region (4 – 25 microns wavelengths) where the thermal radiation of the atmosphere-telescope system dominates the scattering of sunlight thus making the day- and night- sky background comparable. Incorporating MOFs in the LGS wavefront sensor thus would more than double the ELT observing time for mid-infrared astronomy and would make sources in almost the entire sky available for observation at any time of the year. Even though the AO would increase the brightness of point-sources, it would not compete with the James Webb Space Telescope in terms of detectability. The gain with respect to the JWST lies in the 5 to 6 times better linear angular resolution. The contrast gain in brightness at near-IR wavelengths is sufficient to give sufficient natural guide stars there for tip-tilt control. MOFs have been shown to function with Na lasers in LIDAR applications (see Beckers and Cacciani, Experimental Astronomy 11, 133, 2001). The main complication associated with incorporating MOFs in ELT AO system is likely the requirement to make the telescope and its enclosure robust in the daytime environment. I refer to SPIE Proceedings 6986 (2008) for a recent reference on this topic.