13.1	Introduction

The information in this chapter will help you select your grating configuration and observing slit and determine your observing plan (e.g., total required exposure time, and number of exposures). This chapter is basically organized by grating and slit (corresponding to Spectral Element and Aperture in the Phase II Proposal Instructions.)

For each grating mode the following are provided:

•	A brief description of the grating mode’s specifications, including recommended uses and special considerations.

•	The central-wavelength settings and range of wavelength covered at each setting, together with the dispersions and plate scales.

•	Plots and tables of sensitivities and throughputs as a function of wavelength.

•	Plots of signal-to-noise as a function of V+STMAGλ (the color-dependent correction from V magnitude to STMAG at wavelength λ), Fλ, and exposure time.

For each slit the following are provided:

•	A brief description of the slit’s specifications, including recommended uses and special considerations.

•	Plots and tables of relative throughputs as a function of wavelength.

For representative grating-slit combinations we provide:

•	Tables of encircled energy for the flux in the default pipeline extraction aperture, and the percent flux contained in the peak pixel.

•	Plots of representative spatial (perpendicular to the dispersion) profiles at selected wavelengths.

•	Line spread functions and FWHM as a function of wavelength.

In addition,

•	Section 13.8 provides a summary of the screening brightness limits for the MAMA spectroscopic modes.

The sensitivities and dark currents adopted for calculating the figures and tables used in this chapter were projected before SM4 for a date of August 2008. Actual values for Cycle 21 will differ slightly. For the most up-to-date values, please consult the STIS ETC.

The next section explains the plots and tables found in the grating sections in this chapter.