MOC Facets: For Spectra and Images

We are all well aware of the success of the Hierarchical Progressive Surveys (HiPS) (see Fernique et al. 2015) for images surveys and catalogues. We can now search using positional parameters many hundred thousands (even millions) of surveys and observational results (HST, SDSS, etc). But what about searching using values on the temporal or energy axis? This paper is proposing a way to implement these capabilities through a specialized usage of the MOC system (MOCs are footprints of individual or grouped sets of images/spectra expressed as HEALPix cells (see Fernique et al. 2014)) which then could be used to search and select using other parameters than what is available. The system is hybrid and not ready for release. It is presented here mostly to trigger discussions around the concept.