PERCHED high in the Rockies in an old mining town 8,500 feet above sea level, the Central City Opera is all that still glitters in a town that once ran with gold and silver. Sterling names like Beverly Sills, Samuel Ramey, and more recently J. Patrick Raftery and Debra Lynn Cole have trod the boards of the gracious old Opera House.Today, the CCO's principal artists are rising stars, already firmly launched on their careers. Many of them have been apprentices in the past at Central City. As the talent search begins for the '92 season, it's clear how much summer programs like Central City's - with its built-in educative function - contribute to the mainstages of the opera world. Quaint as the 1878 Opera House is, the tough-hombre old West ethic survives in a strong-minded discipline. The apprentice program is a high-level advanced training program for young professionals. Only 17 of the 1200 singers who apply will be accepted each year. "There's no rule book," says artistic director John Moriarty of the road to success in opera. "It takes brains, hard work, talent, and perseverence.... And one of the strong steps is for a young singer to get into an apprenticeship program with summer companies like Wolf Trap, Santa Fe, Chautauqua, or Central City." Opera appren- ticeships are springing up, too, in winter as opera becomes an ever healthier art form in the US. Says apprentice Leslie Shull, "Agents won't even consider you until you've done an apprenticeship and a lot of companies won't hear you if you aren't managed." After reorganizing the Santa Fe apprenticeship program in the early 1960s, Maestro Moriarty, who is chairman of the opera department at the New England Conservatory of Music, assumed artistic directorship of Central City Opera in 1978 and introduced the prototype for today's apprenticeship program. Apprenticeship is by no means the lowest rung on the ladder to opera success. Most apprentices have already finished graduate studies in voice, and university voice training has become very sophisticated. But the road to grand opera is a long and difficult one. By the time singers are ready for Central City, they have already had experience in opera chorus, in small roles in medium-sized companies, and secondary roles in small companies. Most are in their mid- to late-20s (it takes years to perfect their "instruments") and they are already members of the American Guild of Musical Artists. Apprenticeship, says Moriarty, is meant to help singers make that difficult transition from young artist to professional. A typical day starts at 9:30 a.m. and runs late into the night. Moriarty stresses diction, and all apprentices take daily diction classes in French and Italian. Moriarty, after all, wrote the diction textbook used in universities and conservatories across the country. Then for a bracing morning of stage combat, movement, and courtly dance. Finally, everyone takes a weekly aria class d esigned to teach interpretation and practice in all sorts of music. The aria class, most importantly, teaches them the art of the audition. Says apprentice Mark Schnaible, "There is no end to how much you can learn in the aria class on how to present yourself, how you hold yourself, how you deliver the title of the aria, how to perform the roles." Moriarty points out that in an audition a singer has three minutes to establish a whole character a constellation of mental and physical idiosyncracies" that make up character. In addition, AGMA apprentices understudy all the major roles, sing in the chorus, and perform minor roles in the mainstage productions. In order to insure the understudy's mastery of the role and to give the fledgling singers the most important experience of all, the apprentices are given several of their own full-dress stagings of the operas they understudy. These apprentice productions have a secondary function - they are outreach shows meant to help educate the next generation of operagoers in the very special delights of the form. Central City Opera accentuates careful theatricality - every production stresses realistic acting and visual form as well as excellent musicianship. The same care is lavished on apprentice productions. Just as the house lights dim for an apprentice production of Gounod's "Romeo and Juliet," conductor Francis Graffeo walks out on stage and, Leonard Bernstein-like, addresses the audience of young people and their parents. He is charming, witty, and he understands just how much needs to be said to quiet the youngsters and intrigue their interest. He tells them what to expect in the first act and how sword fights are made to look real on stage, and when the curtain goes up a hush falls across the historic auditorium and the children sit entranced. They giggle mischievously when Romeo kisses Juliet, but are otherwise perfectly good for the whole three hours of the performance. "If you give them something good, they'll behave," says Moriarty. "You can't sing at them in a foreign language and expect them to pay attention. We've been doing this for nine years and we've never had any trouble. They enjoy it." Condescension with children is a mistake, Moriarty points out. Still, Mr. Graffeo's funny introduction helps them understand what they see. The process of education in the arts has never been more important as pop culture threatens so many art forms with extinction. But opera is burgeoning. And while Moriarty now embarks on his search for his next season's apprentices in Denver, Chicago, New York, and Boston, he will look for more than beautiful tone. Vocal health and that ineffable something that makes the performer reach out to the public, and the public reach back, will help determine the brightest stars in grand opera.