First-Order Grating G230MB

Description

The G230MB grating mode uses an intermediate resolution grating originally designed for use with the NUV-MAMA which has been redirected for use with the CCD to provide R~6000 spectroscopy in the NUV which takes advantage of the CCD’s higher throughput longward of λ=2500 Å. The G230MB grating mode has a spectral range from 1640-3190 Å.

As a single exposure with this grating covers only 150 Å, the grating must be scanned with a series of exposures taken at 11 distinct settings to cover the full spectral range of the grating.

Recommended Uses

The G230MB grating is designed for programs that require the highest available sensitivity in the NUV from ~2500 to 3100Å or more generally to allow observations of sources too bright for the MAMAs in NUV.

Special Considerations

In making the plots for the G230MB grating mode, we assumed that the CCD is subject to a change in the effective quantum yield resulting in the creation of multiple electron-hole pairs per photon for λ<3400 Å.

Also, because of the high sensitivity of the CCD to red light, observations of red stars with G230MB are more likely be affected by scatter than those using the MAMA G230M mode.

 

Grating	Spectral Range		Average Dispersion (Å per Pixel)	Plate Scale (arcsec / pixel)	Tilts	Central Wavelengths
	Complete	Per Tilt
G230MB	1640-3190	156	0.15	0.05078	Prime	1713, 1854,1995, 2135, 2276, 2416, 2557, 2697, 2836, 2976, 3115
					Secondary	2794

Figure 13.24: Wavelength Ranges for the G230MB Grating Settings

G230MB Sensitivities

Table 13.8: G230MB Throughputs & Sensitivities for a Point Source

λ	Sensitivity	% Throughput
1700	6.5E11	0.1
1800	1.7E12	0.3
1900	2.8E12	0.4
2000	4.2E12	0.6
2100	6.2E12	0.9
2200	8.8E12	1.2
2300	1.2E13	1.5
2400	1.3E13	1.6
2500	1.4E13	1.7
2600	1.5E13	1.7
2700	1.6E13	1.7
2800	1.8E13	1.9
2900	2.1E13	2.1
3000	2.6E13	2.5
3100	3.1E13	2.9

Figure 13.25: G230MB 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.

G230MB 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 13.26: Diffuse Source Signal-to-Noise as a Function of STMAG for G230MB

Figure 13.27: Point Source Signal-to-Noise as a Function of STMAG for G230MB