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

The Pathfinder Dragonfly Spectral Line Mapper: pushing the limits for ultra-low surface brightness spectroscopy

“…The pathfinder Dragonfly Spectral Line Mapper is a 3-lens version of the Dragonfly Telephoto Array with Dragonfly Filter-Tilter instrumentation that implements ultranarrow-bandpass imaging capability on the telescope (as described in Ref. 15). The pathfinder Dragonfly Spectral Line Mapper was built at New Mexico Skies Observatories in Mayhill, New Mexico, in the spring of 2019.…”

“…This sensor has 2750 × 2200 pixels at 4.54 microns square, resulting in a 14.6 mm × 12.8 mm chip size and a 1 • .4 × 1 • .9 field of view. In addition, each OTA is equipped with Dragonfly Filter-Tilter instrumentation, 15 which holds a 152 mm diameter ultranarrowband filter at the entrance pupil of each OTA. The Dragonfly Filter-Tilter mechanically tilts the filter with respect to incident light, effectively changing the incidence angle and thus the central wavelength of the filter bandpass.…”

“…See Ref. 15 for further details on the theory and instrument design of the Dragonfly Filter-Tilter. A custom designed baffle was built for each OTA to block stray light contamination from entering the optical path.…”

“…13, and discuss in detail below, it should be possible for an upgraded Dragonfly Telephoto Array 14 (Dragonfly) with ultranarrowband imaging capability to reach this low surface brightness limit and directly detect visible wavelength line emission from gas in the circumgalactic medium. 13,15 Dragonfly is a telescope specially designed for low surface brightness imaging. Its design is based on the innovative concept of building an extremely fast refracting telescope using lenses instead of mirrors to reduce scattering of light in the optics and multiplexing together an array of high-end commercial lenses to synthesize a larger effective aperture.…”

“…The pathfinder Dragonfly Spectral Line Mapper uses the Dragonfly Filter-Tilter instrumentation described in Ref. 15 to implement ultranarrowband filters on the telescope at the front of the optics. This is important for two main reasons: 1) to prevent degradation of the filter transmission profile that inevitably occurs when interference filters are placed in converging beams and 2) to incorporate a rotational mechanism to tilt the filter, which smoothly shifts the bandpass in wavelength space.…”