Alonso won the F1 world title with Renault in 2005 and 2006

McLaren's Fernando Alonso said his first experience of Indianapolis was "fun" as he passed his orientation and began testing for the Indy 500.

Alonso is missing the Monaco Grand Prix this month, where Jenson Button will return to Formula 1 to substitute for him, to race at Indianapolis.

The Spaniard completed his mandatory 'rookie' test before starting his preparations for the event on 28 May.

"So far it is a good experience but now starts the real thing," Alonso said.

"It has been a very helpful day in terms of knowing all this different world and getting up to speed a little bit.

"There's still a long way to go but I am happy with this first step."

Button sent his former McLaren team-mate a good-luck message external-link on social media before the test session.

Why is Alonso doing a 'rookie test'?

All drivers who race at Indianapolis for the first time are required to complete an initiation test, no matter what their calibre or experience.

To pass, two-time F1 world champion Alonso had to complete three phases of running - 10 laps each at an average of 205-210mph; followed by 15 at 210-215mph; and 15 at 215-220mph. He completed the requirements in just 50 laps.

Alonso said: "It is a good way to start to build the speed. It was probably a little bit difficult at the beginning to reach the minimum but then in the phases it felt good.

"At the beginning, the right foot has its own brain and it was not connected to my brain. I wanted to go flat-out but the foot wouldn't let me. But after a few laps it was fine."

Alonso flew straight to the United States from Russia after Sunday's Grand Prix in Sochi

What else did he do?

After passing the rookie test, Alonso began a programme with his Andretti Autosport team to start learning the intricacies of IndyCars on an oval track where each 2.5-mile lap has four left turns that look identical but are each subtly different.

He ended the test with a fastest lap of 222.548mph. Last year's pole position time for the Indy 500 was 230.760mph.

"Everything went fine so far," Alonso said. "The circuit looks so narrow when you are at that speed. I was trying different lines but I was not as comfortable as I probably will be in a couple of weeks' time."

Alonso is racing in his home grand prix in Spain on 12-14 May before flying back to the States to start the official practice sessions for the Indy 500 the next day.

The competitors have a total of 30 hours of practice over five days before qualifying weekend on 20-21 May, with pole position decided on the Sunday.

Alonso's F1 team are fully involved in his Indy programme, with the car painted in the company's historic orange colour and given the McLaren name. It is the first time for 38 years that a car branded McLaren has raced at Indy.

He is taking part because McLaren are struggling in F1 this year as a result of a lack of performance in their Honda engine and Alonso has said one of his ambitions is to win the 'triple crown' of Monaco Grand Prix, which he won in 2006 and '07, Indy 500 and Le Mans 24 Hours.

McLaren executive director Zak Brown said he wanted to give Alonso the chance to win something after three difficult seasons since joining the team in 2015.

"We wanted to see Fernando running at the front because that's where he deserves to be," Brown said.

Brown revealed that Alonso had already watched about 25 Indy 500s in his preparations, including one entirely from an in-car camera on one particular car.

The test progressed so quickly that within four hours Andretti already had Alonso testing fuel saving and techniques for running behind a safety car.

But Alonso said he still had a lot to learn about fine-tuning the car for changing conditions on the track, a key aspect of driving at Indy.

"The guys make changes all the time to the car," he said. "On that aspect I am not up to speed. I am not yet able to to feel the car because at the moment I am not driving the car, the car is driving me around."

Mario Andretti, the 1978 F1 world champion, former IndyCar champion and father of ex-F1 and IndyCar driver Michael Andretti who runs the team Alonso is driving for, said: "He did a perfect job. He's the real deal and I think he's going to be strong this month."