I did a bunch of astrophotography in the Atacama desert last year, it was an absolutely phenomenal place. There are a lot of celestial objects you cannot image from the northern hemisphere and there aren't many other places in the southern hemisphere with weather conditions that good (maybe Namibia but it doesn't have the altitude advantage).
The only thing I wish is that some of the parks would be open after dark to shoot landscapes. Most of the parks closed before sunset, so I had to mostly image from roadsides, which was kind of sad.
Yeah the guards go home for the evening and then you can probably walk in after dark. You'd have to walk a really long distance in some of them though, because the vehicle gates are usually a very long walk from the scenery, and that's at high altitude.
The only thing I wish is that some of the parks would be open after dark to shoot landscapes. Most of the parks closed before sunset, so I had to mostly image from roadsides, which was kind of sad.