  With TESS and ground-based surveys searching for rocky exoplanets around
cooler, nearby stars, the number of Earth-sized exoplanets that are well-suited
for atmospheric follow-up studies will increase significantly. For atmospheric
characterization, the James Webb Space Telescope will only be able to target a
small fraction of the most interesting systems, and the usefulness of
ground-based observatories will remain limited by a range of effects related to
Earth's atmosphere. Here, we explore a new method for ground-based exoplanet
atmospheric characterization that relies on simultaneous, differential,
ultra-narrow-band photometry. The instrument uses a narrow-band interference
filter and an optical design that enables simultaneous observing over two 0.3
nm wide bands spaced 1 nm apart. We consider the capabilities of this
instrument in the case where one band is centered on an oxygen-free continuum
region while the other band overlaps the 760 nm oxygen band head in the
transmission spectrum of the exoplanet, which can be accessible from Earth in
systems with large negative line-of-sight velocities. We find that M9 and M4
dwarfs that meet this radial velocity requirement will be the easiest targets
but must be nearby (<8 pc) and will require the largest upcoming Extremely
Large Telescopes. The oxyometer instrument design is simple and versatile and
could be adapted to enable the study of a wide range of atmospheric species. We
demonstrate this by building a prototype oxyometer and present its design and a
detection of a 50 ppm simulated transit signal in the laboratory. We also
present data from an on-sky test of a prototype oxyometer, demonstrating the
ease of use of the compact instrument design.
