HBO PIRACY INCIDENT STUNS OTHER SATELLITE USERS

A so-called ''video pirate'' using the name Captain Midnight, who briefly interrupted a Home Box Office movie with his own signal over the weekend, has sent concern through the satellite communications world, prompting warnings that all users of satellite signals - the television networks, banks, even the Federal Government - are vulnerable to similar sneak attacks.

''Sure, it's scary,'' one Government official, who asked not to be named, said yesterday. ''Think of the possibilities of people interfering with banking records, telephone calls, national defense communications. Almost all video programming is going off the satellites. There are lots of people up there.''

However, military sources said it would be vastly more difficult, though not impossible, to override military transmissions in the manner that Captain Midnight overrode HBO's signals. The military employs more frequencies and much more complex signals than the television networks, including what are called multiple access methods.

''Military satellites are designed to be extremely resistant to such interference,'' said an F.C.C. expert, although he added that nothing is impossible. Another expert said the newer American military satellites are very jam-resistant. Movie Interrupted

What prompted the worry was the sudden interruption early Sunday morning of HBO's airing of the movie ''The Falcon and the Snowman'' by the image of a ''color-bar'' - that five-color test pattern used by broadcasters to adjust their equipment. The image contained the five-line message, printed in white letters:

''Goodevening HBO ''From Captain Midnight ''$12.95/month? ''No way! ''(Showtime/Movie Channel beware.)''

The message was an apparent reference to HBO's policy, instituted Jan. 15, of scrambling its satellite signal to prevent viewers with receiving dishes from viewing HBO programming for free. HBO provides a descrambler for $12.95 a month. Showtime/The Movie Channel, an HBO competitor, is expected to begin scrambling its signal this year.

In normal operation, satellite distribution works this way:

The programmer - a television network, a banking concern, any user of satellite distribution - transmits a signal from an earth station, or satellite dish, to a communications satellite hovering 22,300 miles above earth. Each programmer has an assigned transponder, or channel, on the satellite, which is basically a mirror. It receives the signal from earth, turns it around and beams it earthward again. There, receiving dishes aimed at the satellite receive the signal and, in the case of cable operators, for example, transmit the signal to viewers' homes. Overpowered HBO's Signal

Captain Midnight, apparently equipped with a transmitter, managed to overpower HBO's signal, transmitted from Hauppauge, L.I., with his own - a process called ''double illumination.'' So the HBO transponder on the Hughes Galaxy I satellite received Captain Midnight's signal, rather than HBO's, and sent that message along the normal path to HBO viewers.

Officials at all three television networks, which distribute programs to their affiliates via satellite, acknowledged that a determined individual with a powerful enough transmitter could knock regularly scheduled programming off the air, at least temporarily, and replace it with his own.

''He interfered with our satellite transmission,'' said David Pritchard, a vice president of HBO, ''which means that he could do it to Dan Rather's 'Evening News' or he could do it with a Department of Defense communication, or any other broadcasting communication using satellite.''

''It could happen to anyone, sure,'' said Curt Block, a spokesman for NBC.

Stephen J. Petrucci, a vice president of Hughes Communications, which operates the Galaxy I satellite used by HBO, said in a statement yesterday: ''We're taking steps to insure the integrity of our satellite transmission.''

''This is not a matter of just an interruption to HBO,'' he added. ''It represents a threat to any company that uses satellite transmission, including the Federal Government, phone companies, business and data users and broadcasters.'' Professional Equipment

Most broadcasting and satellite communications industry observers interviewed yesterday agreed that Captain Midnight was probably no mere backyard satellite hacker, but had access to professional equipment. ''It was a pretty sophisticated transmitter - you're talking at least $60,000 to do that,'' said William H. Tillson, chairman of Netcom International, a California-based company that distributes video programming through satellite signals. ''Somebody in his backyard can't just go out and do this.''

The information required to override HBO's signal, such as the identity of HBO's transponder and the frequency HBO uses, is readily available because all legitimate users of HBO's signal, such as cable operators and private dish users who pay for the signal, are provided that information by HBO. A Backup Signal at CBS

George F. Schweitzer, a spokesman for CBS, said the network felt its signal was more secure against video pirates because CBS had a backup satellite signal it could employ the moment its primary satellite signal showed interference. However, someone with two transmitters could try to overpower both CBS signals, he conceded.

''What if Qaddafi wants to send a message to the American public?'' asked Mr. Tillson of Netcom. ''He could, sure.''

The Federal Communications Commission has begun an investigation into Captain Midnight's exploit, on the ground that it was, at the least, a violation of the Communications Act's provision barring broadcasting without a license, said William Russell, an F.C.C. spokesman. Mr. Russell said that investigators were pursuing the possibility that Captain Midnight's broadcast was also a violation of the criminal code.

Apparently, there is no limit to what can be done to civilian communications satellites by a determined individual with access to a powerful transmitter. Paul Heinerscheid, a vice president of the United States Satellite Broadcasting Company, said a signal pirate could send phony commands to the satellite itself, causing it to spin out of position.

''If someone decides there are too many porn movies coming from the Galaxy I satellite, he could say, 'I'll take care of them' by turning Galaxy I upside down,'' Mr. Heinerscheid said. ''It is a very serious issue.''