A bright torch: it doesn't need to be big, and its best if it is small enough to fit in your pocket and can be attached to a key ring. If you are on the mountain more than a week, bringing spare batteries would be sensible.
Power adaptors: at CTIO, and in the compound at La Serena, the power is on the US system. In the rest of Chile it is on the European system.
If taking a laptop: an ethernet dongle (if you have a Mac air or similar - although there is wifi in various locations, so its certainly not necessary), and a back pack (briefcases would be awkward to carry up to the summit).
A hoodie/fleece to wear when observing (its very warm, even on the mountain top at night, but when you are tired you will feel the cold more).
T-shirts: its hard to pack for summer when you are starting off in winter, but pack t-shirts even if that seems counter intuitive. Take at least one t-shirt with your Uni name - your department will appreciate a photo of you in it to use on its webpages.
Coat, gloves, hat (if you don't need one when you leave home, you will probably need one at the mountain, and vice versa!).
High factor suncream; sunglasses; aftersun (especially if you are the sort of person who forgets to put on suncream); possibly also a sunhat, the UV index is very high.
Lip balm; moisturiser
Sensible shoes - e.g. trainers, for walking up and down between the telescope and the accommodation - the ground is stony and steep and there are biting bugs in the scrub. If you are going to be here over a week, consider your fellow observers, i.e. take another pair, so you can rotate (your feet do get sweaty on planes and walking to/from the summit).
Some mechanism to take photos (a camera phone is fine for the novices, but the experts might want to consider bringing the full kit to take time lapses of the night sky)
Food: chocolate bars; your favorite ground coffee/tea; other dry treats (your favourite biscuits or cereal). Don't bring anything fresh or with a moisture content (e.g. cheese). You won't need more than you'd usually use of any such item in a day, and probably less (since there is food and drink a plenty on the mountain). You MUST tick the food to declare box on the customs form, but you don't need to list what you have brought on that form. If everything is dry then your bag probably won't be looked into. If it is, and they end up confiscating something, don't argue, just apologise sweetly. Everything must be in original, sealed packages - i.e. don't take 20 tea bags from the box at home, buy a new package of 40. If you take fruit for the journey, dispose of it before you get to the customs hall, in fact its best to leave it behind on the plane.
Binoculars for star gazing (the sky is lovely enough without magnification, so its probably unnecessary unless there is a comet up).
Basic medicines/first aid: paracetomol and ibuprofen - headaches are common at altitude and when working nights; Rennie (sleep disruption, and travel stress can lead to indigestion); a few plasters (switching from winter shoes to sandals at La Serena beach can give your winter softened feet a blister). If you are prone to certain ailments (sore throats, nausea, diarrhea etc.) then take medicines for those just in case - but I don't think its necessary to plan for ailments you don't normally get (otherwise you'll end up with a bag of stuff you won't use when here or when you get home). I haven't seen any flying biting insects, such as mosquito's, but it might be worth bringing something to treat bites just in case. Oh and there are jelly fish in the water at La Serena: lots of families had young kids in the water, so it can't be too dangerous, but be aware of where you are paddling! Finally, a small hand sanitiser would be a good idea for traveling and the mountain top. There is a first aid station on the mountain if you have unexpected, or serious, medical problems.
One smart(ish) outfit: its fine to wear casual, even scruffy, clothes on the mountain and when doing tourist stuff. But you might well want to go out for a nice meal (or even dancing!) in La Serena, and people in Chile tend to be better dressed than in UK or US. That said, its not formal in La Serena (its a beach resort), so nice jeans would be fine. Posh restaurants in Santiago might be more fussy, but you probably can't afford to eat in those!
Some downloaded movies/tv shows: there may be some times, especially in La Serena, when you are on your own and don't want to go out, so having a film to watch will make it feel a bit less lonely.
Music: It seems like the old days of having music on the stereo in the control room are gone. You can still listen to your own music, but use headphones that allow you to listen with only one ear (i.e. not Dr Dre's!) are a good compromise, since you have to be able to hear instructions in the control room.
Toiletries: There is nice soap and basic shampoo in the CTIO motel. But that's it.
Laundry powder/tabs: There is a washer/drier set there and washing powder is usually available, but sometimes there isn't any. If you are going for a length of time and will definitely need to do laundry, it might worth taking some with you.
Swimming costume - in case you want to go surfing (don't assume that just because that sounds like an insane idea from the comfort of your home that it won't appeal to you once you get to La Serena!)