Although Canadian television personality Alan Thicke has been touted as a challenger for the late-night throne occupied for two decades by Johnny Carson, what Thicke appears to be, at least on tonight's premiere of his new "Thicke of the Night" series, is a cut-rate David Letterman. Compared to him, Letterman is Jack Benny, Will Rogers, Sen or Wences and the Harmonicats all rolled into one.

And let's throw in S.Z. Sakall for good measure. And let's throw out Alan Thicke for good reason.

Thicke's nightly 90-minute program, which makes its debut locally at 11:30 on Channel 5, opens with a direct steal from Letterman--a previously amusing bit in which a subjective handheld camera takes the host's point of view and we see various yes-persons doling out counsel and approval as Thicke makes his way to the stage. Letterman did it better. Letterman does everything better. What Letterman doesn't do better than Thicke, Carson does better than Thicke.

Producer Fred Silverman appears near the top of the show as himself--or rather, in his fantasy role as the Louis B. Mayer of television--to introduce his new bouncing bore. Silverman has lost weight, but this makes him look older. Freddie is thinning; Alan is thickening.

Ballyhooed incessantly as the host of Canada's most popular daytime TV show, Thicke challenges the viewer to figure out just what it is that he does so well that he should be on television. After 90 minutes, one is left at a loss for answers. Physically, he slightly resembles Carson--small nose, squinty eyes--but he's more of a dead ringer for Clu Gulager. He's just not quite as hilarious as Clu Gulager. Maybe Freddie should look Clu up.

"I'm not really as good as Silverman's been saying in the papers," Thicke says meekly as the show begins. "If I'd been that good, you would have heard of me before." Thicke is certainly the greatest Silverman discovery since Pink Lady, the Japanese singing duo that Freddie hired while busily destroying whatever was left of the National Broadcasting Company. Silverman didn't realize that the two young women who were the rage of Tokyo unfortunately spoke no English. Perhaps Silverman could interpolate that failure with a current one, and give America "Pink Lady and Thicke."

Thicke is a member of the put-on generation, as is Letterman, and there is a lot of we're-just-kidding material in the program, which is part prattlesome talk show and part vacuous comedy show. The scripted comedy on the premiere includes a routine in which Thicke sits at a desk and interviews zanies on a superimposed screen (in this case, the material is feckless, yockless stuff about artificial insemination and test-tube babies). This is the kind of comedy Steve Allen used to do on the old "Tonight Show," and other programs, many years ago.

Others have stolen from Allen, so Thicke might as well join the list. But Allen was funny. Louis Nye was funny. Tom Poston was funny. Don Knotts was funny. With Allen, even Skitch Henderson was funny. Thicke's crew is not funny.

The interview segments, later in the show, include an appearance from the criminally overinterviewed Joan Collins (in whom Thicke wisely evidences no interest whatsoever) and May Pang, author of a book about John Lennon. At the very end of the program, Thicke looks back on it and reviews it, concluding that once or twice he and his rag-tag repertory players wrongly wreaked a laugh by treating women as sex objects. This was due to "comedy pressure," he says apologetically, adding, "We'll occasionally make that kind of mistake."

To which a baffled viewer can only wonder, "What laugh? I don't remember any laughs." Actually, one segment of the show is rather funny. The Thicke pack assembles and re-dubs silly dialogue to clips from the old MGM version of "Dr. Jekyll and Mr. Hyde." But this is a borrowed idea, too--one that goes back to Woody Allen's "What's Up, Tiger Lily?" And while there was nothing but fun to be had from redubbing an obscure Japanese potboiler, Thicke and company get their laughs at the expense of Ingrid Bergman and Spencer Tracy.

MGM is a part owner and distributor of this program. Will the studio open its vault of classics so that Thicke and his troupe can get some cheap laughs at its expense? It's a discomforting thought.

According to voluminous press material on Thicke, he is a "multitalented" soul. He has worked as a producer ("Tony Orlando's First Special" among his glamoroso credits), a writer ("The Mac Davis Christmas Special," "The Miss Teen-age Canada Pageant") and the composer of theme songs for "Diff'rent Strokes" and "The Monty Hall Special." He even sings on the program tonight. Perhaps Gore Vidal understated it when he said, "Having no talent is no longer enough." If he saw Thicke, he might say, "Having no multitalent is no longer enough."

Taped in advance (the show totally lacks Carson's wonderful topical edge) so that Thicke can run back to Canada and continue his program there, and performed on overly decorated scaffolding that looks like the "Good Morning America" set gone wild, "Thicke of the Night" goes from a crouching position to a dead faint in no time at all. The skinny on Thicke is that he doesn't really seem to be there; he's the test-tube host, and he only serves to remind one how durable Carson is, how terribly bright Letterman is, and even how much more entertaining Ted Koppel is.

The "e" in Thicke is silent; Thicke should be, too. Perhaps it's time to increase security along the Canadian border.