The Kennedy Center's summer chamber opera will move into its second phase this week and next with a switch in repertoire. This afternoon's performances of the strange version of Mozart's "The Impresario" and Webber's "Abu Hassan" will be the last for those two, while last Tuesday night was the finale for the much-admired production of "Postcard From Morocco" by Argento.

This coming Tuesday night the gifted company will open its new production of Christopher Columbus," which is aptly, though perhaps misleadingly, called "the best operetta Offenbach never wrote."

After running for five nights, through Aug. 5, "Columbus" will begin, on Aug. 8, to alternate with what promises to be a smash surprise, Donizetti's "Il Furioso all'Isola di San Domingo," or "The Madman of the Island of San Domingo."

The "best operetta" Offenbach never wrote is a pastiche, a potpourri, a hodgepodge of bits and pieces from the more than 80 operettas Offenbach left lying around when he died in 1880. It's like this: In 1970 an American musicologist named Patric Schmid and a London translator and librettist, Don White, put their heads together and decided to revive - or fix up - some neglected operas of the 19th century.

(It seems highly legitimate to wonder: Might not "Patric Schmid" be another name for the eminent authority on opera librettos, Patrick Smith, who is now also the able president of the Music Critics Association?)

In any case, the Schmid-White team picked up a forgotten Offenbach piece called "La boite au lait," or "The Milk Box," which Offenbach wrote in 1877. Producing opera in London under the company name of Opera Rara, Schmid-White presented "Christopher Columbus" in 1976 as a tribute to the American Bicentennial. And why the Bicentennial?

Because, by adding to some items out of "La boite au lait," the team of discoverers constructed a chamber opera with this for a story line: Columbus is about to marry the beautiful Beatriz when he is caught by three other ladies he has already married in Switzerland, Italy and France. In the town square of Cordova, everyone is arrested and hauled before Queen Isabella. (You remember Ferdinand and Isabella?)

Isabella takes one look at Columbus and is, shall we say, hooked? Shortly thereafter, she and C.C. are found by Ferdinand "in flagrante," as the courts would have it. However, Ferdinand offers Columbus the position of Lord Admiral if he will sail away and find a new route to the Indies.

As the Nina, the Pinta and the Santa Maria are westward-ho-ing, the crew of the last-named throw the Lord Admiral overboard, claiming he has brought them nothing but bad luck and disaster. They do, however, loan him a longboat. Now this you are not going to believe: When the three ships land and their crews are captured by Indians, they find Christopher Columbus is already there, has married the local princess, and been made chief of the tribe. What's more, Columbus discovers Coca-Cola! (That's what it says.) So everyone sails away for good old Spain.

Brian Salesky will conduct the score, with Lois Bewley as director, and a cast that includes Elaine Bonazzi, Karen Hunt, Dana Krueger, Myra Merritt, Erie Mills and Neil Rosenshein.

Donizetti's "Il Furioso" is a different kettle of fish. "Vandalistically slashed, castrated, profaned, against the canons of the musical code of good manners, nothing but a blood-stained corpse," is what a writer in the Rivista Teatrale in Rome said about a pirated version of Donizetti's "Anna Bolena," that was sung in that city on the same night "Il Furioso" had its world premiere. The date was Jan. 2, 1833.

But one of the interesting things about the version of "Il Furioso" at the Kennedy Center beginning on Aug. 9 is that the opera was given only last summer at the Spoleto Festival in Charleston, S.C., in a corrupt, severely cut version. When John Mauceri, who is music director of the Kennedy Center's operatic summer, got in touch with the Milan publishers, Ricordi, last fall, he found out that a complete, absolutely authentic edition of "Il Furioso" was only then becoming possible, thanks to the collating of all the genuine materials.

"I thought we would have the score and parts ages ago," Mauceri said recently. "But actually they only arrived about two months ago. We will be giving the first genuinely authentic, complete performances of this opera ever in the United States."

Donizetti's opera is the story of a Spaniard who goes insane when he finds out that his Portugese wife is unfaithful, flees to a West Indian island where he becomes like a savage in the forest, living on plants and refusing all human companions. (Will the Kennedy Center be able to use some of the same scenery for its San Domingo scenes in Donizetti and its West Indies, which were after all where Columbus landed? You would think so.)

The late Herbert Weinstock, who was one of Donizetti's great biographers, says "Il Furioso" was one of the great Donizetti successes, attaining performances in about a hundred Italian and foreign opera houses within its first decade. "It stands very high on the roster of Donizetti operas that should be revived," he wrote.

The Kennedy Center production will be conducted by Mauceri, directed by Richard Pearlman, and sung by Judith Christin, Marianna Christos, Rockwell Blake, William Dansby, Charles Long and James Tyeska.Its performances, beginning on Aug. 9, will alternate with the "Columbus" through Aug. 19 when the summer season will end.