Space Telescope Imaging Spectrograph Instrument Handbook for Cycle 17

First-Order Grating G750L

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Description

The G750L grating is used with the CCD. It has high throughput but low resolving power (~500), and is designed for efficient, full spectral coverage. The grating has one prime tilt setting.

Recommended Uses

This grating is recommended for observations where high spectral resolution is not required, but efficient coverage in the red portion of the optical is desired.

Special Considerations

Fringing in the CCD compromises the realizable signal-to-noise longward of 7500 Å if contemporaneous fringe flats are not obtained (see Section 7.3).

Grating	Spectral Range		Average Dispersion (Å / Pixel)	Plate Scale (arcsec / pixel)	Tilts	Central Wavelengths
	Complete	Per Tilt
G750L	5240-10270	5030	4.92	0.05	Prime	7751

Figure 13.2: Wavelength Range for the G750L Grating Setting


 

G750L Sensitivities

Table 13.3: G750L Sensitivities & Throughputs for a Point Source

	Sensitivity	% Throughput
5500	3.4E15	5.5
6000	5.0E15	7.4
6500	5.8E15	8.0
7000	6.1E15	7.9
7500	5.7E15	6.8
8000	4.4E15	5.0
8500	3.7E15	3.9
9000	3.9E15	3.8
9500	3.3E15	3.1
10000	1.7E15	1.5
10500	8.5E13	0.1

Figure 13.3: G750L 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.

G750L 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 13.4: Diffuse Source Signal-to-Noise as a Function of STMAG for G750L. Fiducial = 7500 Å.


 
Figure 13.5: Point Source Signal-to-Noise as a Function of STMAG for G750L