A fixture at the Audubon Aquarium of the Americas for almost 20 years, the giant-screen movie house on the riverfront that locals simply called the IMAX theater quietly closed in September for a $1.2 million technological overhaul.

What emerged with the reopening a month later was the Entergy Giant Screen Theater — whose screen, oddly enough, is about 600 square feet smaller than the 4,125-square-foot screen it replaced. The new screen is about 9 feet shorter than the old one; the width is almost the same.

“It still is big, and it’s still the largest one in Louisiana,” said theater manager Rodney Daniels. In fact, the theater, owned by the nonprofit Audubon Nature Institute, has the largest screen along the Gulf Coast between Houston and Mobile, Alabama.

During the monthlong renovation, Audubon parted ways with IMAX, the company that pioneered the high-resolution 70mm celluloid projection system the theater used.

In place of the IMAX system is a 4K digital projection system, a more compact machine that began appearing in commercial theaters nationwide more than 15 years ago.

Although some filmmakers still use celluloid reels, the migration to digital was inevitable, Daniels said.

“One of the reasons we made the conversion is Hollywood said, ‘We’re not doing it anymore,’ ” he said of celluloid, whose picture quality degrades over time.

The switch to digital will also allow for a broader range of uses of the theater, such as for live concerts and theatrical events, in addition to movies, Daniels said.

During the renovation, Audubon installed a Dolby Atmos sound system, delivering a chest-thumping, multi-dimensional audio experience.

Before the change, about six speakers behind the screen were augmented by two large speaker boxes in the back corners. Now, 46 speakers deliver sound to the 327 seats in the steeply graded theater; many of them hang from the ceiling and line the walls, Daniels said. “Now you have speakers all the way around you,” he said.

Additionally, the color scheme was changed. Tans in the checkerboard wall patterns were replaced with blue hues that match the blue vinyl seats that were installed in 2012, the date of the last major renovation. “The darker, the better,” Daniels said. “You can focus more on the screen.”

Although the equipment has been updated, the theater will stick with its mission of airing primarily educational movies.

From the business perspective, digital technology is less expensive. Producing digital movies is cheaper than celluloid, Daniels said.

The theater rented celluloid films for up to $40,000, paid royalties and coughed up a percentage of ticket sales, he said. The IMAX brand also got a cut, he said.

Digital movies don’t come with such heavy expenses, and the movies can be kept and stored locally. Digitized movies are encrypted, so distributors charge to provide a key that allows the theater to show a movie for a period of time, Daniels said.

Savings also are seen in freight costs. A 45-minute length of celluloid comes in a reel that’s about 3 feet in diameter. For a 3-D movie, that could mean as many as eight massive reels, Daniels said.

Using digital technology, a 45-minute length of movie takes about 50 gigabytes of storage space and is delivered in a device about the size of a mobile phone, projectionist Richard Houidobre said while preparing for a noon screening of “Journey to Space,” one of four movies now playing.

While the technology has changed, the projection room lexicon has not.

Scrolling through files on a computer monitor mounted on the two Christie digital projectors, Houidobre still used old-school terminology.

“I just call them films,” he said. “Old habits.”