West Germany's popular president, Walter Scheel, announced officially today that he would not be a candidate for another five-year term when a parliamentary assembly meets on May 23 to elect a new president.

The statement by the 59-year old Free Democrat comes one day after the combined conservative opposition parties nominated Christian Democrat Karl Carstens, 64, speaker of the lower house of Bonn's parliament, as their candidate for the largely ceremonial but nevertheless important office of president.

Barring some unexpected development, the controversial Carstens now is assured of being chosen West Germany's next president.

The controversy surrounding a Carstens presidency stems from several factors. carstens is generally viewed being on the right wing of his already conservative party and thus for many years, has ben a target for liberals and the left both inside West Germany and in other countries. He would become the first conservative opposition leader to enter the top circle of power in Bonn since the current center-left ruling coalition of Social Democrats and Free Democrats came to power in 1969.

In the course of the sometimes bitter maneuvering for the presidency in recent months, it also has been disclosed that Carstens was a wartime member of the Nazi Party, beginning in 1940. Although a post-war Allied deNazification panel ruled he was only a nominal, and not an active, member, the past association is widely viewed as certain to continue haunting Carstens and the presidency.

Finaly, Carstens has had to battle in and out of court a series of recent allegations, thus far unproved, that he lied to a parliamentary investigating committee in 1974 regarding his knowledge of illegal arms sales by West German intelligence agencies.

Carstens' victory is assured because, while the ruling coalition of Chancellor Helmut Schmidt holds power at the federal level, the special assembly that elects the president also includes delegates from state government. The conservatives hold the edge in six of West Germany's 10 federal states and a clear majority in the special assembly.

The ruling coalition parties have not yet indicated wheter they will be fielding another candiate to replace Scheel.

Scheel, according to all opinion polls, remains by far the most popular choice among the general population, even though it was also disclosed during the campaign that Scheel also was a nominal Nazi Party member during wartime.

Scheel, foreign minister under former chancellor Willy Brandt, has won considerable respect both here and internationally as a thoughtful statesman who frequently reminds West Germans of the special role and responsibility that their history demands.

Just last week, Scheel jolted a large group of German scientists commemorating the birth here 100 years ago of Albert Einstein by observing that Einstein probably would have objected to such a ceremony in Germany had he lived.

Recalling the famous physicist's Jewish heritage, Scheel reminded the audience that Einstein may not have survived had he remained in Nazi Germany.

How was it, he asked, that one of the century's most powerful intellects should come to shun his own people? "Why was it that most German intellectuals, with a few exceptions, were blind to the evil spirit that came over their country?" he asked.

A few months ago, at Bonn University, Scheel challenged another audience, warning them that West Germans -- and especially members of parliament -- tend too frequently to turn issues into arguments over principles, that never lead to solutions.

He warned that too few people are prepared to defend unpoplar positions.

"We fight so long and hard for the outward forms of democracy," he said, "that we hardly have time and energy left for democratic content."