DAzLE is a near infrared narrowband differential imager being built by the Institute of Astronomy, Cambridge, in collaboration with the Anglo-Australian observatory. It is a special purpose instrument designed with a sole aim; the detection of redshifted Lyman-α emission from star forming galaxies at z>7. DAzLE will use pairs of high resolution (R=1000) narrowband filters to exploit low background 'windows' in the near infrared sky emission spectrum. This will enable it to reach sensitivities of ~2 x 10-21Wm-2, thereby allowing the detection of z>7 galaxies with star formation rates as low as a few solar masses per year. The design of the instrument, and in particular the crucial narrowband filters, are presented. The predicted performance of DAzLE, including the sensitivity, volume coverage and expected number counts, is discussed. The current status of the DAzLE project, and its projected timeline, are also presented.
To understand the factors involved in the cost and risk of manufacture of ultra-narrowband imaging filters in the near IR, we have carried out detailed design work for a range of different filter specifications. This work was essential to the success of the DAZLE near-IR imaging spectrograph, a project led by the Institute of Astronomy at the University of Cambridge. The necessity for very narrow passbands (approximately 1 nm), for observing in the windows between the OH emission lines in the near-IR bands, requires coatings of more than 200 layers and greater than 30 μm thickness. Broadband filters to block OH and thermal emission over wide near-IR spectral ranges (1.4 - 2.8 μm) also require many thin film layers in the filter coatings. In most cases, the large number of layers is the most significant factor in determining manufacturing cost, as such coatings require long periods of time for deposition (days to weeks), with a resultant high risk of failure during manufacture.
Advances in thin film technology in recent years are providing new possibilities for interference filters. The isolation of spectral bands for astronomical imaging is no longer limited to single segments of the electromagnetic spectrum. Multiple passband, or 'multi-band' filters can now be manufactured for simultaneous narrowband imaging at several widely separated wavelengths. Such filters provide major benefits to differential imaging with tunable filters since systematic uncertainties arising form atmospheric variations can be averaged out. A multi-band thin film interference filter has been manufactured specifically for passbands at H(alpha) and H(beta) . In tandem with the AAOs TAURUS Tunable Filter and charge shuffling techniques, this double-band filter allows for accurate mapping of the local dust extinction and star formation rates in nearby spiral galaxies. Additional filter designs for a variety of emission-line combinations and their scientific applications are also discussed.