Here's the deal - we wouldn't have been able to get telescope time for this project if we didn't have our two collaborators. However, they didn't take the data, nor did they do any of the data reductions/analysis or writing of any portion of the paper. They are not in my particular research field, so, although they will read the paper before it is submitted, they won't be able to give any comments on the actual science.
So, do I put them as co-authors (because we wouldn't have the data without them) or in the acknowledgments section (because they didn't do any work on the actual data or paper)?
Please help - I don't want to get in a situation that could be thought of as academic fraud!
Thanks for the comments so far!
I asked a couple of professors here about the etiquette regarding this situation, and both gave me some really great advice. They both agree that, basically, it's always better to err on the side of caution - and if we couldn't get the data without our collaborators, then it's safe to put them as co-authors.
I was actually quite surprised by the answers they gave - they didn't just brush me off or give me a one liner. They both took the time to really explain the situation (and even gave personal examples from both sides) and gave thoughtful, and useful answers.