The F sharp diminished chord is a 3-note chord consisting of the notes F#, A and C.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as F#dim.
Chords that are a superset of F#dim. The chords include more notes but always F#, A and C.
In case you prefer a non-interactive variant of the chord chart, we've embedded a PNG image below that shows the notes for the the F#dim chord. Feel free to save or share the image as needed.