STIS Instrument Handbook for Cycle 24

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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.

 

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 Throughputs & Sensitivities 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.