Spectrophotometry with Hectospec, the MMT's Fiber-Fed Spectrograph

We describe techniques for photometric calibration of optical spectra obtained with the MMT's fiber-fed spectrograph, Hectospec. The atmospheric dispersion compensation prisms built into the MMT's f/5 wide field corrector effectively eliminate errors due to differential refraction, and simplify the calibration procedure. The procedures that we describe here are applicable to all 220,000+ spectra obtained to date with Hectospec because the instrument response is stable. We estimate the internal error in the Hectospec measurements by comparing duplicate measurements of $\sim$1500 galaxies. For a sample of 400 galaxies in the Smithsonian Hectospec Lensing Survey (SHELS) with a median z=0.10, we compare line and continuum fluxes measured by Hectospec through a 1.5 arcsec diameter optical fiber with those measured by the Sloan Digital Sky Survey (SDSS) through a 3 arcsec diameter optical fiber. Agreement of the [OII] and H alpha SHELS and SDSS line fluxes, after scaling by the R band flux in the different apertures, suggests that the spatial variation in star formation rates over a 1.5 to 3 kpc radial scale is small. The median ratio of the Hectospec and SDSS spectra, smoothed over 100 Angstrom scales, is remarkably constant to ~5% over the range of 3850 to 8000 Angstroms. Offsets in the ratio of the median [OII] and H alpha fluxes, the equivalent width of H delta and the continuum index d4000 are a few percent, small compared with other sources of scatter.