Space Telescope Imaging Spectrograph Instrument Handbook for Cycle 14

First-Order Grating G230M

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Description

Like the G230L grating, the G230M grating is used with the NUV-MAMA; it has a spectral range from 1650-3100 Å, however, with a resolving power R~10,000 a single exposure with the G230M grating covers only 90 Å, and the grating must be scanned, with a series of exposures taken at 18 distinct settings to cover the full spectral range.

Recommended Uses

The G230M grating mode is designed for observations where spatially resolved, long slit, spectroscopy is desired at relatively high spectral resolution covering a limited region of the near-ultraviolet spectrum.

Special Considerations

Notice that the CCD G230MB grating mode also covers the near-ultraviolet with comparable resolution. See Comparison of G230MB and G230M for a detailed comparison of these two grating modes in that wavelength regime.

Grating	Spectral Range		Average Dispersion (Å per Pixel)	Plate Scale (arcsec / pixel)	Tilts	Central Wavelengths
	Complete	Per Tilt
G230M	1640-3100	90	0.09	0.029	Prime	1687, 1769, 1851, 1933, 2014, 2095, 2176, 2257, 2338, 2419, 2499, 2579, 2659, 2739, 2818, 2898, 2977, 3055
					Secondary	1884, 2600, 2800, 2828

Figure 13.34: Wavelength Ranges for the G230M Grating Settings


 

G230M Sensitivities

Table 13.10: G230M Throughputs & Sensitivities for a Point Source

	Sensitivity	% Throughput
1700	5.2E11	0.1
1800	9.8E11	0.3
1900	1.6E12	0.4
2000	2.2E12	0.5
2100	3.2E12	0.7
2200	4.3E12	1.0
2300	4.7E12	1.0
2400	4.9E12	1.0
2500	5.1E12	1.0
2600	4.9E12	0.9
2700	5.0E12	0.9
2800	5.1E12	0.9
2900	4.5E12	0.8
3000	3.4E12	0.6
3100	1.2E12	0.2

Figure 13.35: G230M 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 (cross dispersion) plate scale in units of arcseconds per pixel and by the width of the desired slit in units of arcseconds.

G230M 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 13.36: Diffuse Source Signal-to-Noise as a Function of STMAG for G230M


 
Figure 13.37: Point Source Signal-to-Noise as a Function of STMAG for G230M


 

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