As head maestro of the Milwaukee Symphony Orchestra, Lukas Foss seldom ventured south to Chicago. Now, several years after returning to the Brooklyn Philharmonic, the distinguished musical polymath is back to conduct his first concert at Grant Park. Go figure. Himself an estimable composer–probably the most adroit of the European Americans at fusing the traditional and the experimental–Foss has been an avid promoter of new music, and for this occasion he’s picked a recent work worthy of attention: The Protecting Veil for Cello and Strings by John Tavener. A convert to the Russian Orthodox Church, the British Tavener is respected for the skillful eclecticism and profound mysticism of his music; like Estonian Arvo Part, he reaches back to the tonal systems and instrumentation of the Middle Ages for inspiration, and often strives for purity and transcendence, as he did with The Protecting Veil, which was requested in 1987 by the BBC on behalf of cellist Steven Isserlis. The title of this eight-section work refers to a vision of the mother of God in a Constantinople church in the early tenth century, which spurred the Greeks to turn back the intruding Saracen army. Isserlis will be the soloist. The remainder of the program includes Tchaikovsky’s Francesca da Rimini, which depicts the stormy affair between two adulterers who end up sweating in the Second Circle of Dante’s Inferno, and two less spiritual selections: exceprts from Copland’s Billy the Kid and the Pas de Deux from the second act of The Nutcracker. Saturday, 8 PM, and Sunday, 7 PM, Petrillo Music Shell, Grant Park, Columbus and Jackson; 294-2420 or 819-0614.