Made-for-television opera comes of age tonight with a $1-million production of "The Marriage of Figaro" at 9 o'clock on Channel 26.

The 3 1/2 hour production of Mozart's ultra-sophisticated comedy, which might aptly be renamed "Upstairs, Downstairs at the Court of Count Almaviva," will be simulcast on WETA-FM, enabling the audio portion to be received on home-stero radio equipment.

The performance, featuring Dietrich Fischer-Dieskau as the Count and Karl Boehm conducting the Vienna Philharmonic, is as fine as one would expect from any opera house of a work called by Stendahl "a sublime blend of wit and melancholy such as there is no other example."

Productions of this caliber have been televised from stages before, but never has the dramatic and visual potential of opera freed from such constraints been so ambitiously addressed.

The result is eye-opening, and despite an occasional overuse of the new tools, the production establishes beyond doubt that the TV screen can add an important new dimension to the operatic experience.

Close-ups, zooms, dissolves and the other techniques of the cameraman intensify the action of this "Figaro," and clarify many of the subtleties of the drama.

For instance, we are staring right at his face, not sitting 30 rows away, when Count Almaviva finds the young roue, Cherubino, hidden in the countess' bedroom. The hard look that comes over Fischer-Dieskau's face is chilling - and through the camera we find that the world's preeminent base-baritone is as consummate an actor as he is a singer.

Few artistic compromises have been made in this production. The only cuts are ones generally made in stage productions. A star cast was assembled last December in the Shepperton Studio in England. Sets derived from the version staged at the Salzberg Festival by the director, the noted Jean-Pierre Ponnelle, were constructed.

The production, financed by a German company named Unitel, was shown on German television last spring and is scheduled soon on both Austrian and Swedish television. The company expects to get a return on its investment with the eventual advent of the video disc.

At such cost this "Figaro" is probably the most expensive opera production in history. Even the Met's controversial cost override on the "Anthony and Cleopatra" that opened its new house went to only about $800,000.

Money to acquire the production for American showing came from grants by the National Endowment for the Arts, the Corporation for Public Broadcasting, PBS member stations and Exxon. It is the first of five weekly operas to come in the "Great Performances" series. None of the others present remotely the challenge of "Figaro," One, a "Cavellerias [WORD ILLEGIBLE] from La Scala under Karajan, is new. The others, "Salome," "Pagliacci," and "Madame Butterfly," are repeats from previous years. On the sixth week, PBS will bring us Massanet's. "Manon" with Beverly Sills in the "Live from Lincoln Center" series.

The circumstances that made possible a million-dollar "Figaro" are part of the growing presence of opera on the screen, both here and in Europe. The opera boom is in response to audience demand.

"We have consistently found that operas outdraw other cultural offerings, including drama and dance," says David Griffiths, head of music programming for WNET, the New York public television outlet that is putting on the series. "The audience ranges from six million to 10 million, and last year's live "La Boheme" from the Met drew 11 million." As a result, the Met will present three operas live this year.

In "The Marriage of Figaro," the Count does not necessarily have the most or even the best music. But he is the head of the household, and he is the central figure in the net of intrigues. He is also, many times, the weak spot - allowing Figaro (his valet), the Countiess, or Susanna (the Countess maid and Figaro's bride-to-be), to become dominant. But Fischer-Dieskau restores the Count to his place of command.

Hermann Prey is an earthly, likeably Figaro, Mirella Frenl is perhaps the leading Susanna of the day and here is a permanent record that shows why. If Kiri Te Kanawa's Countess and Maria Ewing's Cherubino are not quite in this league, they carry their weight, musically and dramatically.

Television is at its operatic best in the wonderful interrogation in the finale of Act 2, where the Count interrogates Figaro about who wrote the anonymous love note to the Count, and who is was who jumped out the window to keep from being caught. The sternness of the Count is delightfully countered by Prey's calculated, and successful, bluffing.

The Philharmonic playing Mozart under Boehm is one of music's most delectable experiences.

The boon of filmed-for-TV-opera (this is not video tape) is its effect because it can zero in on physical and emotional nuance. It works best here in "Figaro's" countless and complete ensembles.

It could work just as well in the arias, but Ponnelle gets too cut, Major arias, like the Countess "Dove sono," are done with the singer not mouthing the words but leaning back in a moody trance, and the melodic line being sung by the singer in a voiceover. This is a bad idea, and spoils some marvelous moments.

An even worse result of Ponelle's camera fetish comes when Figaro vents his rage against the supposedly unfaithful Susanna in the last act. He seats the real Figaro, quite properly, on a bench in the garden, and he starts to sing; and then suddenly a second image of Figaro, in a different costume, is superimposed standing near and he assumes some of the vocal line. The idea is to turn a monologue into a debate with one's self, and it never should have been done, because nothing in the score justifies it.

But we share Ponelle's enthusiasm for the camera.His "Figaro" is an operatic turning point.