Space Telescope Imaging Spectrograph Instrument Handbook for Cycle 17

Echelle Grating E140M

---

Description

The E140M grating is used with the FUV-MAMA and provides echelle spectra at a resolving power of 45,800 from 1123-1710 Å.

Special Considerations

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.

Grating	Spectral Range		Average Dispersion (Å per Pixel)	Dispersion, Cross Dispersion Plate Scale (arcsec / pixel)	Tilts	Central Wavelengths
	Complete	Per Tilt
E140M	1123-1710	620	/91,700	0.036,0.029	Prime	1425

Figure 13.54: Wavelength Range for the E140M Grating Setting


 

E140M Sensitivity

Table 13.15: E140M Throughputs & Sensitivities for a Point Source

	Sensitivity	% Throughput
1200	1.0E11	0.3
1250	2.1E11	0.5
1300	2.6E11	0.6
1350	2.6E11	0.6
1400	2.4E11	0.5
1450	2.0E11	0.4
1500	1.7E11	0.3
1550	1.3E11	0.2
1600	9.3E10	0.1
1650	7.4E10	0.1
1700	6.0E10	0.1

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.


 

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 (cross dispersion) plate scale in units of arcseconds per pixel and by the width of the desired slit in units of arcseconds.

E140M Signal-to-Noise

Note:

The top axis displays constant F values corresponding to the STMAG units (V+STMAG) on the bottom axis. Recall that STMAG=0 is equivalent to F = 3.63E-9 erg cm^-2 s^-1 Å^-1. The curves are labeled with exposure times in seconds.

Figure 13.56: Diffuse Source Signal-to-Noise as a Function of STMAG for E140M


 
Figure 13.57: Point Source Signal-to-Noise as a Function of STMAG for E140M