I think (ra, dec, zenith) has two solutions, since the object could be on either side of the meridian (this would manifest as a flip about the NS axis of the image).  On the other hand, I believe (HA, dec, latitude) is unambiguous.

Here's my math (for the Southern hemisphere):

![img_0682](https://f.cloud.github.com/assets/3650485/2342544/673e430c-a4f9-11e3-8764-d75f0d9a9240.JPG)

It's quite possible I've made a sign error or two...

Is it reasonable to assume that the image is North=up and East=left ?  or should this (maybe optionally) interact with a WCS somehow?