Bravo for 'Candide' and 'Idomeneo'; To come up with great evenings at the opera, there are no shortcuts

This city's two opera houses unveiled new productions on consecutive evenings that were not only triumphs, but proof that the faith the public has in both the Metropolitan and New York City Operas is not unfounded.

The City Opera has needed a major boost for quite some time. Things have been better there this year, but not consistently. Hence, the world premiere of the opera-house version of Leonard Bernstein's ''Candide'' found the company casting from strength, and inviting someone in to stage the work who had something imaginative to say - Hal Prince.

Across the plaza at Lincoln Center, the Met has followed its dazzling ''Les Contes d'Hoffman'' of last season with an equally stunning staging - the Met premiere of Mozart's ''Idomeneo,'' with Luciano Pavarotti's first attempt at a Mozart role on these shores, and the return of the gifted Jean-Pierre Ponnelle as designer-director.

The message is loud and clear. Great evenings at our opera houses happen only with great directors, designers, and superior vocal talent. When both houses cut corners, the evenings fall short of the mark. Granted, it is harder to ensure success in this jet-age day, where low risk is the norm, but both houses proved it is still possible.

It has been argued that ''Candide'' is more Broadway than opera. And it has been argued that ''Ido-meneo'' and all operas in the form known as ''opera seria'' are too static for today's audiences. But clearly, the City Opera gathering adored every minute of its evening, and even the so-often restless gala-night patrons witnessing the Mozart stayed for the four-hour duration to enjoy and applaud the artists. Both houses resonated with thundering ovations (and inexcusable booing for Mr. Ponnelle, who has become a ''controversial'' figure), not unknown in either house but quite scarce of late at the City Opera. 'Candide'

''Candide'' is more music theater than it is opera or even operetta, but it is rip-roaringly American. The edition that conductor John Mauceri patched together from the various versions of the opera kicking around should be welcome in any opera house around the world.

Bernstein's score is irrepressibly entertaining, tuneful, witty, magical. The Hugh Wheeler revised book incorporates a certain un-Voltairian cutesiness and flip quality I suspect was not a part of Lillian Hellman's original book, but that original is, apparently, gone forever. And the version of ''Candide'' seen at the City is unquestionably more reverent, less shallow, than the one Hal Prince and company devised in Brooklyn, and moved later to the Broadway Theater for some 740 performances.

Scenes deleted for the Brooklyn-Broadway presentation (which was done in one act) have been restored, and the orchestrations have been fleshed out for a full orchestra rather than a 13-piece pit band. If there is a flaw in the evening, it is that even Mr. Prince's incessant inventiveness cannot hide the slump that inhabits the second act. His directorial decisions will be familiar to Broadway theatergoers, but in the opera house they are novel and refreshing. He has the unusual ability to get a show to burst out of the frame of a proscenium arch and explode into a huge playing space.

General director Beverly Sills has cast it to perfection. David Eisler (Candide) gives an ardent performance full of candor and simplicity. His voice is ideal here, as is his boyish handsomeness - in all the finest work he has done in this house. Erie Mills, debuting as Cunegonde, was fighting the ever-present shade of Barbara Cooke, whose singing on the original cast album is the stuff of legends.

Fears were utterly groundless. Miss Mills is a consummate performer and singer in her own right - funny, engaging, capable of filling a large theater with gesture and tone. The singing was dazzling, even in the fiendish frills and roulades of ''Glitter and be Gay.'' She is that rare combination, comedi-enne and opera star.

John Lankston assumed the multiple roles of Voltaire-Pangloss-etc. with style and presence. Deborah Darr, Scott Reeve, Muriel Costa-Greenspon, and Jack Harrold stood out in a large and excellent cast.

In the pit, Mr. Mauceri conducted with fervor. The audience cheered it all wildly, and if it could run for weeks rather than just a few performances, it would remain what it has become already - a standing-room-only event that restores one's faith in the City Opera's ability to put on something super. 'Idomeneo'

The Met's new production is a Salzburg Festival type of event at considerably cheaper prices. And I doubt even Salzburg could have persuaded Pavarotti to try his first Idomeneo there. On Ponelle's unit set with changing backdrops - sepia Piranesi-like etchings of semi-ruined classic structures - the production looked stunning.

The story concerns the Cretan king who agreed to sacrifice the first mortal he sees on his homeland to Neptune in return for safe passage there. That mortal turns out to be his son, and the opera concerns his attempts to get around the bargain and also involves the story of the women who love that son (Idamante) - the Greek princess Elettra and Trojan captive Ilia.

The director sees the presence of the god Neptune as pivotal to the action, and a huge head with a cavelike entrance for its mouth dominates the set at crucial junctures. Ponnelle has decided that to provide the flavor of a late-Baroque evening in the theater - through baroque costumes and settings - would allow the artifices of the plot to have some meaning for a contemporary audience used to seeing the opera in Greek costuming. And the stylized gesturing , posturing, and movement fit impeccably into a total ensemble production.

Pavarotti was somewhat nervous opening night, but even with that, his way with the recitatives, and with the forthright approach to the arias, was substantial. And to have a voice of that quality caressing Mozart's noble phrases was a special treat for the ears one all too rarely encounters anywhere today.

Idamante (a male role given to a castrato in Mozart's age) was sung at the Met by Frederica von Stade, whose physical presence and impact proved ideal, but whose voice sounded thin and colorless most of the night. As Ilia, Ileana Cotrubas was all femininity and longing - for peace and quiet, for Idamante's love - and despite passing hoarseness she sang affectingly.

Elettra is the show-stopping role of the opera, and Hildegard Behrens gave it her formidable all. The first two arias, despite rough patches, impressed for their conviction and clear delivery. The final aria - a tempestuous mad scene - brought the house down for its seemingly self-destructive intensity. In an evening dominated by fine actors, Miss Behrens's performance would be a unanimous Tony winner if operadom had such an award.

James Levine has had some patchy days in the Met pit of late, but there was nothing less than world-class magnificence in his account of Mozart's sublime score. The music brings out the very best in him and reminds us anew what a treasurable musician he can be in a score that he finds stimulating.

Perhaps most remarkable in this remarkable evening - a taste of Salzburg in the US cultural capital - is how Mozart's sublime score holds the stage so rivetingly. It shows what a brilliant cast, a dedicated conductor, and a director for whom the slightest detail is of utmost importance in his quest of theatrical excellence can achieve when all work together - a magical evening in the grandest, yet most theatrical, of Met traditions.

Bravo!