|STIS Instrument Handbook for Cycle 24|
Like G750L, the G750M grating is used with the CCD and has a spectral range from 5500–10,000 Å. With a resolving power R~5000, a single exposure with this grating covers only 570 Å, and the grating must be scanned, with a series of exposures taken at 9 distinct settings to cover the full range of the grating.This grating is designed for relatively high spectral resolution work centered on selected wavelength regions of the optical to near-infrared (NIR).There is a partial ghost spectrum present in the λc=5734 setting which is ~1.2% of, is inverted with respect to, is offset by ~ 70 pixels from, the prime spectrum, arising from back reflections between the CCD and the order sorter.Fringing in the CCD compromises the realizable signal-to-noise longward of 7500 Å if contemporaneous fringe flats are not obtained (see Section 7.3).The secondary central wavelengths at 6581 & 8561 Å have had direct sensitivity and wavelength calibrations performed during Cycle 7 to support the large number of observations using these positions.
Figure 13.6: Wavelength Ranges for the G750M Grating Settings
Point source sensitivity assumes full transmission (zero slit losses). Diffuse source sensitivity assumes a 0.1” wide slit. To convert point source sensitivities to diffuse source sensitivities multiply the point source values by the grating spatial (cross dispersion) plate scale in units of arcseconds per pixel and by the width of the desired slit in units of arcseconds.The top axis displays constant Fλ values corresponding to the STMAG units (V+STMAGλ) on the bottom axis. Recall that STMAG=0 is equivalent to Fλ = 3.63E-9 erg/cm2/s/Å. The curves are labeled with exposure times in secondsFigure 13.8: Diffuse Source SNR as a Function of STMAG for G750MFigure 13.9: Point Source SNR as a Function of STMAG for G750M