PUPIL SLICING ADAPTIVE OPTICS: MAKING EXTREMELY LARGE TELESCOPES DIFFRACTION LIMITED AT SHORT WAVELENGTHS

Extremely Large Telescopes (ELTs) will have aperture diameters up to 42 meters. Adaptive Optics (AO) at short wavelengths (<1 micron) will be very hard to implement at these wavelengths because of the limited number of actuators on state-of-the-art deformable mirrors and because of the limited brightness of Laser Guide Stars (LGSS). For 1 arcsec seeing at 500 nm wavelength deformable mirrors (DMs) with about 150000 actuators will be needed and LGSs of a brightness of V = 8 to 9. That exceeds our present capabilities by a factor of about 100 and 2 magnitudes respectively. One might expect both to improve with time. We propose to combine the techniques of "pupil slicing" and AO to sharpen the telescope images at short wavelengths to the size of Airy disk of the pupil slices. I refer to this technique as "Pupil Slicing Adaptive Optics" or PSAO. At 500 nm wavelength that would correspond to the Airy disk of an approximately 5 meter diameter aperture, or a FWHM of 0.02 arcsec. As DMs increase in their number of actuators, the size of the pupil slices increases thus improving the angular resolution. Ultimately (he full angular resolution of, for example, a 42 meter aperture would be reached (0.0024 arcscc at 500 run). Of course, this does not resolve the issue of the limited brightness of LGSs. For it one has to wait for more powerful lasers and the development of perspective elongation correction techniques. Alternatively one would accept limited sky coverage (0.1%) when using natural guide stars (NGSs). Particularly interesting is the PSAO technique for high resolution spectroscopy where the smaller image sizes even for many slices results in a significant decrease in spectrograph dimensions.