PRISM

Description

The PRISM has two central wavelength settings optimized to cover the optical through the far-ultraviolet (FUV) tail down to 1150 Å. It is used with the NUV-MAMA detector.

Special Considerations

The full dispersion spreads over ~450 pixels with dispersion as shown in Figure 13.64; thus if you have sources covering the field of view in the dispersion direction, the red tail will be lost for some targets with this setting. For a plot of wavelength vs. pixel no., see Figure 4.12.

We note that the dispersion of the PRISM at wavelengths longer than ~2600 Å is very poorly known at this time.

Note also that the time-dependent sensitivity of PRISM does not follow that of the NUV gratings (STIS ISR 2005-01) and has not been measured since 2005. Its current sensitivity may be unknown by ~25% (see STIS ISR 2017-06).


Grating

Spectral Range

Average
Dispersion
(Å per Pixel)

Plate Scale
(arcsec/pixel)

Tilts

Central
Wavelength

Complete

Per Tilt

PRISM

1150–3620

2470

0.2−72

0.029

Prime

1200/2125

Figure 13.62: Wavelength Range for the PRISM Setting.


PRISM Sensitivities


Table 13.17: PRISM Sensitivities & Throughputs for a Point Source.

λ

Sensitivity

% Throughput

1250

1.3E13

0.6

1350

2.6E13

0.6

1450

4.1E13

0.5

1550

6.5E13

0.5

1650

1.0E14

0.6

1750

1.9E14

0.8

1850

3.2E14

1.0

1950

5.2E14

1.2

2050

8.3E14

1.5

2150

1.2E15

1.8

2250

1.6E15

1.9

2350

2.1E15

2.0

2450

2.6E15

2.0

2550

3.1E15

2.0

2650

3.5E15

2.0

2750

4.0E15

1.9

2850

4.0E15

1.7

2950

3.7E15

1.3

3050

2.5E15

0.8

3150

1.3E15

0.4

3250

6.3E14

0.

3350

3.1E14

0.1

Figure 13.63: PRISM 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.