On Regula/Orbital Office design starbases (we see them in the various Star Trek films and even in some of the series), what is that X-shaped thing we see on the side of the upper structure? See my picture below.I've been curious about this since I first watched Star Trek years ago.
I'm trying to find a better picture of a ship leaving a Federation starbase, but what you're looking at is basically the (lit) interior of the dock, seen through the partially closed doors.
You can see a similar looking entrance here (from TNG: Relics).
On the FASA RPG 'Regula-1 Orbital Station deckplans' (non-canon but based on conversations with the show's model designers), you can see that this entrance leads directly into the Deck 8 shuttlebay.