In such situations it makes sense to consult Google - there are 180 000 hits for "восходящая звезда" and only 943 for "восходившая звезда." So clearly, the latter is not used very often in Russian.

Here is one example:

В ходе праймериз выясняются все основные слабые места соперников, а сами они проверяются на прочность, способность противостоять критике со стороны прессы и конкурентов. Характерна судьба сенатора Гэри Харта — восходившей звезды Демократической партии, который был документально (именно с ясными и четкими доказательствами, а не на основе неясных слухов, как сейчас происходит с Маккейном) уличен в супружеской измене именно на этом этапе избирательной кампании 1988 года.

Here the idea is that Mr. Hart was in the process of rising up in the Democratic party until he was caught cheating on his wife.
In this case, восходящая звезда would not be suitable because that would indicate that Mr. Hart was still a rising star within the overall timeframe of the article. восшедшая звезда wouldn't make sense either, because he never reached the top, but fell before getting through the primaries.

Anyway, it is obvious that you would never use the phrase
я наблюдал восходившую звезду
without some very specific context that would justify using this weird form of the verb - but you can't say that it's grammatically incorrect to say that, the thing is that there are no rules in languages - because the number of things that we might want to talk about it bigger than the number of expressions in our language, so that's why people are always coming up with new words and phrases every day.

So just keep trying to express yourself as best you can, and hopefully things will work out alright. If people don't understand you, just try to change your point of view a little bit.