To finish up extragalactic star formation rates, we need H$\alpha$ images of the Galactic plane. Here, the IPHAS survey does the best job of providing coverage thanks to the VTSS going quietly into the night. The latest release is DR2. The data are photometric, but they are in optical people units. Specifically, every image has a well defined PHOTZP such that, in the usual way,

$$m_{\mathrm{Vega}} = -2.5\log_{10} I + \mathrm{PHOTZP}$$

The DR2 page gives that magnitude zero in the H$\alpha$ filter corresponds to

$$F_0 = 1.56\times 10^{-7}~\mathrm{erg/s/cm^2}$$

As such,

$$\begin{eqnarray} -2.5\log_{10} \left( \frac{F_{\mathrm{H}\alpha}}{F_0} \right) & = & -2.5\log_{10} I + \mathrm{PHOTZP} - 0.0 \\ F_{\mathrm{H}\alpha} & = & 10^{-0.4\mathrm{PHOTZP}} F_0 I \end{eqnarray}$$

The final issue is pixel size. Flux ain’t surface brightness so when we rebin by a factor of 64 in pixel area, we end up needing to scale up the flux by the rebin factor.