1) Is a southern-hemisphere one. I am going to Chile in a few weeks so this is what it will be mainly used for. If it also has a northern-hemisphere side then that is fine, but I'm not fussed.
2) Not too big or expensive - must fit in a rucksack!
3) Durable/waterproof.
4) Includes some of the brighter objects of the night sky. The skies should be spectacular - we're in the desert