Space Telescope Imaging Spectrograph Instrument Handbook for Cycle 17

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 re-directed for use with the STIS/CCD to enable R~700 spectroscopy in the near-UV which takes advantage of the CCD's higher throughput and dynamic range longward of =2500 Å.

Recommended Uses

The G230LB grating mode is designed for programs needing the highest available sensitivity in the near-UV from ~2500 to 3100 Å or more generally to allow observation of sources too bright for the MAMA in the near UV.

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 <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	0.05	Prime	2375

Figure 13.18: Wavelength Range for the G230LB Grating Setting


 

G230LB Sensitivities

Table 13.7: G230LB Sensitivities & Throughputs for a Point Source

	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 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 13.20: 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