2C = both majors
2D = spades and a minor
2H/2S/3C/3D = natural
Double is normally for penalties. (A possible variation by a passed hand, or perhaps against a strong NT opening, is to play double as showing hearts and a minor.) And a 2NT bid would probably show the minors, though there are other possibilities for this as well.
The advantage compared to other Astro variants is that it is easier to play. In particular, overcaller's partner is is a better position when one of the two-suited bids comes up. For example, we can compare it to "Asptro" where 2C shows hearts and another suit and 2D shows spades and another suit, showing the weaker suit with both majors. Asptro is good at finding the right major to play in when overcaller has both majors (unlike "Astro" or "Aspro"). The problem is that for both 2C and 2D there are three different possibilities for the second suit, and it is not always easy to cater for all three. It is particularly difficult over 2D, where overcaller's partner can be faced with problems like these:
- Holding 3 spades and 3 or 4 hearts, and no interest in game. Here we want to play in 2H if overcaller has hearts. But bidding 2H invites partner to bid 3 of his minor if he has a 5-card minor. This may not be what we want: we would often prefer to play in 2S in that case (for example with 3=3=5=2 shape).
- Holding a decent hand with 4+ good hearts, where we are interested in game if partner has hearts as his second suit but want to stop in a part-score otherwise. This is impossible to handle because if we bid 2H (or 2S) then partner will pass when he has hearts, which is not what we want.
Playing Half-Astro there are not so many hand-types to worry about: instead of three possible two-suiters in each bid, we have only one (for 2C) or two (for 2D). And after either of these bids, overcaller's partner can use the next step to ask which suit is longer.
Of course, what Half-Astro does not have is a way to show hearts and a minor. With these hands we have to bid naturally (or pass if we do not have a suit good enough to overcall). But this is not such a huge disadvantage, since showing two-suited hands without spades is less important - sometimes when we could find a heart fit opponents might be able to bid spades over it.
I play this defence because it is effective and yet very simple. You could agree it with a new partner and expect not to have any mishaps. It's one of the very few conventions I've come up with which has been taken up by people who aren't my partners!