Tonight, ABC finally gets David Letterman onto its late-night schedule.

But while ABC suits were willing to sacrifice Nightline newsman Ted Koppel for the honor last year, it is Koppel who brings CBS' tormented late-night funnyman to the network for his first sitdown interview since 1997. 

When it airs tonight at 12:05 a.m. on the debut episode of Koppel's new show Up Close, it will have been more than 12 days since the highly anticipated interview was conducted, shortly after the June 26 edition of Letterman's Late Show finished taping. 

"It was very casual," a source told The Post

Koppel, dressed like a war correspondent in a khaki jacket and pants, conducted the interview at the Ed Sullivan Theater on the set of Letterman's show, while Letterman — wearing a navy sweater and khaki pants, reclined comfortably in a director's chair. 

The source said the two spoke for more 40 minutes, but the interview itself will be edited down to less than 22 minutes when it airs tonight on the half-hour show. 

It is not known what they talked about, but it's a good guess that last winter's fiasco - when ABC tried to steal Letterman away from CBS to displace Koppel's Nightline at 11:35 p.m. — was a hot topic. 

In the end, Letterman agreed to a deal under which CBS will reportedly pay him $31.5 million annually for about 5 years, Koppel got Up Close, and Bill Maher's Politically Incorrect was canceled.

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