First-Order Grating G750M

Description

Like G750L, the G750M grating is used with the CCD and has a spectral range from 5500-10,000 Å. With a resolving power R~5000, a single exposure with this grating covers only 570 Å, and the grating must be scanned, with a series of exposures taken at 9 distinct settings to cover the full range of the grating.

Recommended Uses

This grating is designed for relatively high spectral resolution work centered on selected wavelength regions of the optical to near-IR.

Special Considerations

There is a partial ghost spectrum present in the _c=5734 setting which is ~1.2% of, is inverted with respect to, is offset by ~ 70 pixels from, the prime spectrum, arising from back reflections between the CCD and the order sorter.

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

The secondary central wavelengths at 6581 & 8561 Å have had direct sensitivity and wavelength calibrations performed during Cycle 7 to support the large number of observations using these positions

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Grating	Spectral Range		Average Dispersion (Å per Pixel)	Plate Scale (arcsec / pixel)	Tilts	Central Wavelengths
	Complete	Per Tilt
G750M	5450-10140	570	0.56	0.05	Prime	5734, 6252, 6768, 7283, 7795, 8311, 8825, 9336, 9851
					Secondary	6094, 6581, 8561, 9286, 9806

Figure 13.6: Wavelength Ranges for the G750M Grating Settings

G750M Sensitivities

Table 13.4: G750M Sensitivities & Throughputs for a Point Source

	Sensitivity	% Throughput
5500	6.6E14	9.6
6000	9.0E14	12.0
6500	1.1E15	13.4
7000	1.2E15	13.3
7500	1.1E15	11.5
8000	8.3E14	8.3
8500	7.2E14	6.7
9000	7.5E14	6.7
9500	6.4E14	5.4
10000	3.0E14	2.8
10500	0.0E0	0.0

Figure 13.7: G750M 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.

G750M 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 13.8: Diffuse Source Signal-to-Noise as a Function of STMAG for G750M

Figure 13.9: Point Source Signal-to-Noise as a Function of STMAG for G750M