The Grism Lens-Amplified Survey from Space (GLASS). I. Survey overview
  and first data release
  We give an overview of the Grism Lens Amplified Survey from Space (GLASS), a
large Hubble Space Telescope program aimed at obtaining grism spectroscopy of
the fields of ten massive clusters of galaxies at redshift z=0.308-0.686,
including the Hubble Frontier Fields (HFF). The Wide Field Camera 3 yields near
infrared spectra of the cluster cores, covering the wavelength range
0.81-1.69mum through grisms G102 and G141, while the Advanced Camera for
Surveys in parallel mode provides G800L spectra of the infall regions of the
clusters. The WFC3 spectra are taken at two almost orthogonal position angles
in order to minimize the effects of confusion. After summarizing the scientific
drivers of GLASS, we describe the sample selection as well as the observing
strategy and data processing pipeline. We then utilize MACSJ0717.5+3745, a HFF
cluster and the first one observed by GLASS, to illustrate the data quality and
the high-level data products. Each spectrum brighter than H_AB=23 is visually
inspected by at least two co-authors and a redshift is measured when sufficient
information is present in the spectra. Furthermore, we conducted a thorough
search for emission lines through all the GLASS WFC3 spectra with the aim of
measuring redshifts for sources with continuum fainter than H_AB=23. We provide
a catalog of 139 emission-line based spectroscopic redshifts for extragalactic
sources, including three new redshifts of multiple image systems (one probable,
two tentative). In addition to the data itself we also release software tools
that are helpful to navigate the data.
