Over a period of ∼7 to 8 months, using dark time to minimize background contributions (2 weeks/month), we will make deep observations of the northern-sky. With a FOV of ∼30-degree diameter, we can cover the northern hemisphere with ∼75 individual pointings, with adequate overlap between fields for calibration of baselines. For each pointing, we will require at least ∼10 of 20 min exposures to provide deep ( ≥3 hour) observations using narrow-band (∼10-20 A) Hα, Hβ and [OIII] filters, and shorter observations in continuum bands (∼100 Å wide) to remove stellar contamination. The total integration time required is ∼700 hours, which should be readily achieved on a timescale of ∼7 to 8 months. We will use the ratio of Hα and Hβ brightnesses to estimate the dust extinction at Hα (see e.g. Casassus et al. 2004) and also the possible contribution from dust-scattered Hα light, which may be significant at high latitudes (Adolf Witt, priv. comm.). Absolute calibration will be achieved using standard nebular sources (e.g. the California nebula) or via the publically available Wisconsin H-Alpha Mapper (WHAM) Fabry-Perot data on large angular scales (Haffner et al. 2003). The images will then be combined, with appropriate background corrections, to make a large mosaic map. Combining this with other surveys (WHAM and SHASSA) will allow an accurate full-sky map of Hα, with an angular resolution of ∼1 arcmin. This will be complementary to high-resolution Galactic plane surveys in Hα such as the IPHASS/VPHAS surveys, and will become a Legacy Survey to be used for many years to come for studying diffuse Galactic emission (e.g. Dobler, Draine \& Finkbeiner, 2009). The calibrated sky maps will be made publically available. A possible future extension to the survey would be to map other lines (e.g. [S ΙΙ] etc.) or to map the Southern sky with particular emphasis on the environment of the Magellanic Clouds.
The full-sky map of Hα can be used for a variety of scientific investigations (see e.g. Valls-Gabaud (1998) for a review), including:
- Component separation for CMB data – template fitting and cross-correlation
- Separation of synchrotron and free-free emission in low frequency data (e.g. C-BASS, GEM)
- Estimating the electron temperature and ionization of the Warm Ionized Gas in our Galaxy
- Estimating the distribution of dust grains along the line-of-sight
- Discovery of large-scale filaments/nebulosities and association of Hα and [OIII}] emission with the giant radio loops and spurs.
- Ionization of High Velocity Clouds (HVCs) and the Magellanic HI Stream and how much ionizing radiation escapes.
- Galactic plane survey – low surface brightness diffuse HII regions, PNe and molecular clouds (complementing higher resolution surveys such as IPHAS).
- Comparison with Fermi/WMAP haze/bubbles.