Is it legal? – 1

Your left hand opponent opens 1N and your partner bids 2D (ASTRO, showing 5-4 in spades and another suit).   Over your response, partner bids 3D.   You have agreed that this now shows only diamonds and is to play, so you pass.

Prior to 1st April 2005, this was not permitted at OB level 3, but now you may play any defence to 1NT at this level.

However  -  If you want to have the agreement in the question you should make this very clear on your convention card and you must be particularly careful in answering any opposition questions on your methods. You must not describe the 2D bid as ASTRO without emphasising "or a desire to play in 3D".