  We describe plans for adding a wide-field narrow-band imaging capability to
the Dragonfly Telephoto Array. Our plans focus on the development of the
`Dragonfly Filter-Tilter', a device which places ultra-narrow bandpass
interference filters ($\Delta\lambda \approx 1$ nm) in front of each of the
lenses that make up the array. The filters are at the entrance pupil of the
optical system, rather than in a converging beam, so their performance is not
degraded by a converging light cone. This allows Dragonfly to image with a
spectral bandpass that is an order of magnitude narrower than that of
telescopes using conventional narrow-band filters, resulting in a large
increase in the contrast and detectability of extended low surface brightness
line emission. By tilting the filters, the central wavelength of the
transmission curve can be tuned over a range of 7 nm, corresponding to a
physical distance range of about 20 Mpc of extragalactic targets. A further
benefit of our approach is that it allows off-band observations to be obtained
at the same time as on-band observations, so systematic errors introduced by
rapid sky variability can be removed with high precision. Taken together, these
characteristics should give our imaging system the ability to detect extremely
faint low-surface brightness line emission. Future versions of the Dragonfly
Telephoto Array may have the sensitivity needed to directly image the
circumgalactic medium of local galaxies. In this paper, we provide a detailed
description of the concept, and present laboratory measurements that are used
to verify the key ideas behind the instrument.
