filter_on_positions behaving very differently for Swift depending on whether BAT or BAT+XRT are selected

[DavidT3]

This must be a consequence of how I select data when there are multiple simultaneous instruments with dramatically different fields of view. For instance, a recent selection of clusters dotted around the sky went from ~3500 obs selected (which seemed dramatically low) when the chosen instruments were XRT and BAT to ~360000 when it was just BAT.

The second number is essentially the entire archive, which given the FoV of BAT is much more what I expected.