|STIS Instrument Handbook for Cycle 25|
The E140M grating is used with the FUV-MAMA and provides echelle spectra at a resolving power of 45,800 from ~ 1144–1710 Å.A single E140M echellogram covers ~600 Å, however at wavelengths longward of 1649 Å, the echelle orders overrun the width of the detector in the dispersion direction, so there will be three small gaps in the wavelength coverage. One gap of ~0.2 Å, one of ~0.4 Å, and one of ~0.7 Å occur between 1649 and 1700 Å.The separation between orders is ~15 pixels (or 0.44 arcseconds) at 1150 Å and ~33 (0.96 arcseconds) at 1700 Å.Also note that while the spectral format of the E140M extends down to 1123 Å, when combined with the HST primary and secondary, the total system throughput for external targets becomes extremely small below about 1144 Å.
λ/91,700 Figure 13.54: Wavelength Range for the E140M Grating SettingTable 13.15: E140M Throughputs & Sensitivities for a Point Source
Figure 13.55: E140M Point Source (left axis), and Diffuse Source (right axis) Sensitivities. Throughput curve traces the peaks of the blaze function for each order.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 seconds.