  The pathfinder Dragonfly Spectral Line Mapper is a distributed aperture
telescope based off of the Dragonfly Telephoto Array with additional
instrumentation (the Dragonfly "Filter-Tilter") to enable ultranarrow bandpass
imaging. The pathfinder is composed of three redundant optical tube assemblies
(OTAs) which are mounted together to form a single field of view imaging
telescope (where the effective aperture diameter increases as the square-root
of the number of OTAs). The pathfinder has been on sky from March 2020 to
October 2021 equipped with narrowband filters to provide proof-of-concept
imaging, surface brightness limit measurements, on sky testing, and observing
software development. Here we describe the pathfinder telescope and the
sensitivity limits reached along with observing methods. We outline the current
limiting factors for reaching ultra-low surface brightnesses and present a
comprehensive comparison of instrument sensitivities to low surface brightness
line emission and other methods of observing the ultra-faint line emission from
diffuse gas. Finally, we touch on plans for the upcoming 120-OTA Dragonfly
Spectral Line Mapper, which is currently under construction.
