STIS Instrument Handbook for Cycle 24

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First-Order Grating G140M

Description

The G140M grating mode is used with the FUV-MAMA and has a spectral range from 1150 to 1700 å.

With a resolving power R~10,000, a single exposure with this grating covers only 55 å. The grating must be scanned, with a series of exposures taken at 12 distinct settings to cover the full spectral range.

Recommended Uses

The G140M grating mode is designed for observations where spatially resolved, long slit, spectroscopy is desired at relatively high spectral resolution over a limited region of the UV spectrum.

 

Grating	Spectral Range		Average Dispersion (å / Pixel)	Plate Scale (arcsec / pixel)	Tilts	Central Wavelengths
	Complete	Per Tilt
G140M	1140-1740	55	0.05	0.029	Prime	1173, 1222, 1272, 1321, 1371, 1420, 1470, 1518, 1567, 1616, 1665, 1714
					Secondary	1218, 1387, 1400, 1540, 1550, 1640

Figure 13.42: Wavelength Ranges for the G140M Grating Settings

G140M Sensitivities

Table 13.12: G140M Throughputs & Sensitivities for a Point Source

λ	Sensitivity	% Throughput
1200	1.7E12	1.2
1250	2.9E12	2.0
1300	3.1E12	2.1
1350	2.6E12	1.7
1400	2.1E12	1.3
1450	1.6E12	1.0
1500	1.2E12	0.7
1550	8.2E11	0.5
1600	5.3E11	0.3
1650	3.9E11	0.2
1700	3.2E11	0.2

Figure 13.43: G140M Point Source (left axis), and Diffuse Source (right axis) Sensitivities.

Note

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 plate scale in units of arcseconds per pixel and by the width of the desired slit in units of arcseconds.

G140M Signal-to-Noise

Note:

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.

 

Figure 13.44: Diffuse Source Signal-to-Noise as a Function of STMAG for G140M

Figure 13.45: Point Source Signal-to-Noise as a Function of STMAG for G140M