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 nine 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-infrared (NIR).

Special Considerations

There is a partial ghost spectrum present in the λ= 5734 setting which is ~1.2% of the prime spectrum intensity and is both inverted with respect to the prime spectrum and offset by ~70 pixels. It arises 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 Section 7.3).


Grating

Spectral Range

Average
Dispersion
(Å per Pixel)

Plate Scale
(arcsec/pixel)

Tilts

Central Wavelengths

Complete

Per Tilt

G750M

5450–10140

572

0.56

0.05078

Prime

5734, 6252, 6768, 7283, 7795, 8311, 8825, 9336, 9851


 

Secondary

6094, 6581, 8561, 9286, 9806

Figure 13.6: Wavelength Range for the G750M Grating Setting.


G750M Sensitivities

Table 13.4: G750M Sensitivities & Throughputs for a Point Source.

λ

Sensitivity

% Throughput

   5500

6.6E14

  9.5

   6000

9.0E14

12.0

   6500

1.1E15

13.4

   7000

1.2E15

13.3

   7500

1.1E15

11.5

   8000

8.5E14

  8.5

   8500

7.2E14

  6.7

   9000

7.6E14

  6.7

   9500

6.4E14

  5.4

10000

3.0E14

  2.4

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 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 13.8: 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.