Wide-field ultra-narrow-bandpass imaging with the Dragonfly Telephoto Array

TL;DR

This paper proposes a novel ultra-narrow-bandpass imaging system for the Dragonfly Telephoto Array, enabling highly sensitive detection of faint low-surface brightness emission with tunable wavelength and simultaneous off-band imaging.

Contribution

Introduction of the Dragonfly Filter-Tilter device that places ultra-narrow filters at the entrance pupil, allowing for high-contrast, tunable, wide-field narrow-band imaging.

Findings

Design concept and laboratory verification of the filter system.

Achieved tunable wavelength range of about 7 nm.

Potential to detect extremely faint low-surface brightness emission.

Abstract

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.