First-Order Grating G230LB

Description

The G230LB grating mode uses a low resolution grating originally designed for use with the STIS/NUV-MAMA, which has been repurposed for use with the STIS/CCD. It enables R ~ 700 spectroscopy in the near-ultraviolet (NUV) and takes advantage of the CCD's higher throughput and dynamic range longward of λ = 2500 Å.

Recommended Uses

The G230LB 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 G230LB 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 be aware that because of the high sensitivity of the CCD to red light, observations of red stars with G230LB are more likely to be affected by scatter than observations of red stars using the MAMA G230L mode.


Grating

Spectral Range

Average
Dispersion
(Å per Pixel)

Plate Scale
(arcsec/pixel)

Tilts

Central
Wavelengths

Complete

Per Tilt

G230LB

1680–3060

1380

1.35

0.05078

Prime

2375

Figure 13.18: Wavelength Range for the G2300LB Grating Setting.


G230LB Sensitivities


Table 13.7: G230LB Sensitivities & Throughputs for a Point Source.

λ

Sensitivity

% Throughput

1750

1.3E13

0.3

1850

2.5E13

0.5

1950

3.9E13

0.7

2050

6.3E13

1.0

2150

9.9E13

1.5

2250

1.3E14

2.0

2350

1.7E14

2.4

2450

1.9E14

2.6

2550

2.0E14

2.7

2650

2.1E14

2.7

2750

2.3E14

2.8

2850

2.6E14

3.1

2950

3.2E14

3.6

3050

3.6E14

4.0

Figure 13.19: G230LB 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.

G230LB 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 13.20: Diffuse Source Signal-to-Noise as a Function of STMAG for G230LB.


Figure 13.21: Point Source Signal-to-Noise as a Function of STMAG for G230LB.