Optimal LGS pointing with a faint tip - tilt NGS

Experience with the cmrent gelleration of astronomical single laser guide star (LGS) adaptive optics (AO) systellls has demoIlstrated systelll perforlIlance that is often lilllited by residual tip-tilt errors illduced by the paucity of bright tip-tilt natural guide stars (NCS). To overcollle this lilllitation, we are developillg a nt'w gelleration of tip-tilt sensors that will operate at near-illfrared wavelengths where the :"JGS is sharpened to the diffraction limit. To optimi2e performance, single LGS AO systcIlIs utilizing sharpem~d tip-tilt NCS should generally not point their LGS directly toward their sciellce target .. Rather, optilllal perfOrInallCe for widp sky coverage is obtained b~' offsel.ting LGS pointing alollg a r;l(lills connect illg 1.1](' sci(~llc(, targct, ilnd t hc t,ip-Cilt, :\GS. We demons/rate that determinatioll of the joilltly optilllized LCS pointillg allgle amI tip-tilt wavefront scnsor (\VFS) integration tiIlle Call illlprove pCrfOrlllalICP nlPtrics by factors of scveral, particularly for failltest NGS o]H'ration. \Ve filld tlw LCS offsl'\, should be as IlllICh as 1/2 the dist;lllcp to the NGS to maxilllize Strehl ratio at npar-infrared wawlengths and;::::; 1/4 thc distance to the NGS to lllHXilllizc ('nsqllarrd ('lwrgy, with lesser off-poillting for bright.er NGS. Future AO systems may benefit from predictive dctcrnlinat.iOlI of optinral LGS ofrsetting, hased upon cirangillg at.lllospheric cOllditions and ohservational geoIllPt.ries.