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

TL;DR

This paper details calibration techniques for optical spectra obtained with the Hectospec fiber-fed spectrograph at the MMT, demonstrating stable instrument response and minimal spatial variation in star formation rate measurements.

Contribution

It introduces calibration procedures that leverage atmospheric dispersion compensation prisms, applicable to over 220,000 spectra, ensuring stable and accurate flux measurements.

Findings

Calibration procedures reduce systematic errors in spectral flux measurements.

Spectral flux ratios are consistent within ~5% across 3850-8000 Å.

Star formation rate indicators show minimal spatial variation over 1.5-3 kpc scales.

Abstract

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.