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 repurposed for use with the CCD. It provides R ~ 6000 spectroscopy in the NUV and 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 mode is designed for programs needing the highest available sensitivity in the NUV from ~2500 to 3100 Å or more generally to allow observation of sources too bright for the MAMA in the 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 to be affected by scatter than observations of red stars 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 G2300MB Grating Setting.


G230MB Sensitivities


Table 13.8: G230MB Sensitivities & Throughputs 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.