To cite the service as such, we suggest the following BibTeX entry:
@MISC{vo:hpp_web, year=2016, title={Historical Photographic Plate Image Archive}, author={{GAVO} Data Center}, url={https://dc.zah.uni-heidelberg.de/hppunion/q/im/info}, howpublished={{VO} resource provided by the {GAVO} Data Center} }