In a first of a series of studies of the H \alpha + [ N II ] emission from nearby spiral galaxies , we present measurements of H \alpha + [ N II ] emission from H II regions in M81 .
Our method uses large-field-CCD images and long-slit spectra , and is part of the ongoing Beijing-Arizona-Taipei-Connecticut Sky Survey ( the BATC survey ) .
The CCD images are taken with the NAOC 0.6/0.9m f/3 Schmidt telescope at the Xinglong Observing Station , using a multicolor filter set .
Spectra of 10 of the brightest H II regions are obtained using the NAOC 2.16m telescope with a Tek 1024 \times 1024 CCD .
The continua of the spectra are calibrated by flux-calibrated images taken from the Schmidt observations .
We determine the continuum component of our H \alpha + [ N II ] image via interpolation from the more accurately-measured backgrounds ( M81 starlight ) obtained from the two neighboring ( in wavelength ) BATC filter images .
We use the calibrated fluxes of H \alpha + [ N II ] emission from the spectra to normalize this interpolated , continuum- subtracted H \alpha + [ N II ] image .
We estimate the zero point uncertainty of the measured H \alpha + [ N II ] emission flux to be \sim 8 % .
A catalogue of H \alpha + [ N II ] fluxes for 456 H II regions is provided , with those fluxes are on a more consistent linear scale than previously available .
The logarithmically-binned H \alpha + [ N II ] luminosity function of H II regions is found to have slope \alpha = -0.70 , consistent with previous results ( which allowed \alpha = -0.5 \sim - 0.8 ) .
From the overall H \alpha + [ N II ] luminosity of the H II regions , the star formation rate of M81 is found to be \sim 0.68 M _ { \odot } { yr } ^ { -1 } , modulo uncertainty with extinction corrections .