NUV Grating G285M

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Description

The G285M grating samples wavelengths between about 2500 and 3200 Å. The grating has 17 central-wavelength settings.

Special Considerations

G285M spectra consist of three 41 Å stripes separated by two 74 Å gaps. To acquire a complete spectrum requires the use of eight central-wavelength settings.

Because of declining throughput, NUV observations with the G285M grating are available but unsupported (See Section 5.11). Users interested in medium-resolution spectroscopic coverage of the wavelength region from 2500 to 3200 Å are encouraged to use STIS instead. The material in this section demonstrates the sensitivity of G285M in 2016, when it was much greater.


GratingResolving Power R = λ/ΔλDispersion (mÅ pixel−1)Spatial Resolution (milliarcsec pixel−1)Plate Scale (milliarcsec pixel−1)FP-POS Step (Å step−1)
Disp. AxisCross-Disp. Axis
G285M20,000–24,0004056 ± 124.324.42.1


Figure 13.23: Wavelength Ranges for the G285M Grating.

Dark lines represent wavelengths sampled by all four FP-POS positions.

G285M Point-Source Sensitivity

Table 13.10: G285M Point-Source Sensitivity for PSA.

Wavelength (Å)ThroughputSensitivity (counts pixel−1 sec−1 per erg cm−2 sec−1 Å−1)Effective Area (cm2)
24802.316e−03

5.3e+11

1.05e+02
25002.403e−03

5.6e+11

1.09e+02
25502.595e−03

6.1e+11

1.17e+02
26002.791e−03

6.7e+11

1.26e+02
26502.878e−03

7.0e+11

1.30e+02
27002.906e−03

7.2e+11

1.31e+02
27503.002e−03

7.6e+11

1.36e+02
28002.863e−03

7.4e+11

1.30e+02
28502.682e−03

7.1e+11

1.21e+02
29002.390e−03

6.4e+11

1.08e+02
29502.194e−03

6.0e+11

9.92e+01
30001.829e−03

5.1e+11

8.28e+01
30501.532e−03

4.3e+11

6.93e+01
31001.197e−03

3.4e+11

5.42e+01
31508.866e−04

2.6e+11

4.01e+01
32006.297e−04

1.9e+11

2.85e+01
32295.211e−04

1.6e+11

2.36e+01


Figure 13.24: G285M Point-Source Sensitivity for PSA and BOA.


G285M Signal-to-Noise Ratio

Figure 13.25: Point-Source Signal-to-Noise as a Function of STMAG for G285M.

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 cm−2 s−1 Å−1. Colors refer to exposure times in seconds. The edge of the shaded area corresponds to the bright-object screening limit. Use of the PSA is assumed.

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