First, I was writing about a different chord, F#A C E, which logically would be the F# dim7-and then I wrote an A# where I meant A- mistake again--didn't mean to confuse--

As per the definitions above, even though a diminished chord features notes that are a minor third apart, because of the funny way that the note names in the scale are set up, it is not possible to spell them so that the notes reflect that--you have to squeeze the extra half step out somewhere, and that would vary, depending on which chord and which key you were using--

In F#, you would probably write it as F# A C D#, which is to say, your sharps are in the key signature, so you'd put natural signs next to the A and the C--in F, with one flat in the key signature, you'd put a accidental sharp next to the F and an accidental flat next to the E--

One thing to remember is that that in a major scale, the diminished triad does occur naturally, extending from the 7th step (B D F) but the four note diminshed chord(B D F Ab) has to be constructed with an out of scale note, written as an accidental--

This means that, in many cases where a diminished chord is used, the accompaniment actually passes into another key, and the notation has to reflect that-