Overview

There are 15 spectroscopic modes which are summarized in Table 4.1 below. They comprise low and intermediate-resolution first-order modes designed to be used with a complement of long slits over the entire wavelength range, and intermediate and high-resolution echelle modes which have been optimized for point-source observations through short echelle slits and are available only in the ultraviolet (see   Figure 4.1).

Table 4.1: STIS Spectroscopic Capabilities

	Spectral Range (Å)			Spectral Resolution
Grating	Complete	Per Tilt		Scale (Å per pixel)	Resolving Power (/2)	No. Prime Tilts1	Detector	Reference material page		Slits (apertures)2,3,4,5
CCD First-Order Spectroscopy
G750L	5240-10,270	5030		4.92	530-1040	1	CCD	278		52X0.05 52X0.1 52X0.2 52X0.5 52X2 52X0.2F1
G750M	5450-10,140	570		0.56	4870-9050	9	CCD	281
G430L	2900-5700	2800		2.73	530-1040	1	CCD	284
G430M	3020-5610	286		0.28	5390-10,020	10	CCD	287
G230LB	1680-3060	1380		1.35	620-1130	1	CCD	290
G230MB	1640-3190	155		0.15	5470-10,630	11	CCD	294
MAMA First-Order Spectroscopy
G230L	1570-3180	1610		1.58	500-1010	1	NUV-MAMA	298
G230M	1640-3100	90		0.09	9110-17,220	18	NUV-MAMA	301
G140L	1150-1730	610		0.60	960-1440	1	FUV-MAMA	304
G140M	1140-1740	55		0.05	11,400-17,400	12	FUV-MAMA	307
MAMA Echelle Spectroscopy
E230M	1570-3110	800		/60,000	30,000	2	NUV-MAMA	310		0.2X0.2,0.2X0.06
E230H	1620-3150	267		/228,000	114,000	6	NUV-MAMA	313		0.2X0.2,0.2X0.09
E140M	1150-1710	620		/91,700	45,800	1	FUV-MAMA	316		0.2X0.2,0.2X0.06
E140H	1150-1700	210		/228,000	114,000	3	FUV-MAMA	319		0.2X0.2,0.2X0.09
MAMA Prism Spectroscopy
PRISM	1150-3620	2470		0.2 - 72	2900 - 25	1	NUV-MAMA	322		52X0.05,52x0.1, 52X0.2, 52X0.5,52X2

1 Number of exposures at distinct tilts needed to cover spectral range of grating with 10% wavelength overlap between adjacent settings. 2 Naming convention gives dimensions of slit in arcseconds. For example, 52X0.1 indicates the slit is 52 arcsec long perpendicular to the dispersion direction and 0.1 arcsec wide in the dispersion direction. The F (e.g., in 52X0.2F1) indicates that it is the fiducial bar to be used for coronographic spectroscopy. F25MGII is supported with all NUV-MAMA gratings and the prism. 3 For the MAMA first-order modes, only ~ 25 arcseconds of the long slit projects on the detector. (See also Slits for First-Order Spectroscopy.). 4 Full-aperture clear (50CCD or 25MAMA), longpass-filtered (F25QTZ or F25SRF2 in UV), and neutral-density-filtered slitless spectroscopy are also supported with the appropriate first-order and echelle gratings, as well as the prism. 5 The 6X0.2 and 52X0.05 long slits are also supported for use with the echelle gratings, but with order overlap. The high S/N multislits 0.2X0.2FP(A-E) and 0.2X0.06FP(A-E) (see Chapter 12), as well as the very narrow 0.1X0.03 for maximum spectral resolution, are likewise supported with the echelles. For the echelles, the filtered slits 0.2X0.05ND and 0.3X0.05ND are also supported. The 0.1X0.09 and 0.1X0.2 slits are supported for E230H only.

Figure 4.1: Sample Uncalibrated Spectral Images (distortion is exaggerated)

Throughputs

To illustrate the broad wavelength coverage provided by STIS, and the relative throughputs achievable across STIS's wavelength regime, we show in Figure 4.2, the system throughput of the four low-resolution, first-order modes on a single plot (where the throughput is defined as the end-to-end effective area divided by the geometric area of a filled, unobstructed, 2.4 meter aperture). To allow you to judge the relative throughputs of different spectroscopic configurations, we plot in Figure 4.3 the efficiency of all grating modes for each of the four primary wavelength regimes on a common plot. These plots allow you to gauge the relative efficiencies of STIS in different configurations. Throughput changes in the first-order modes, determined from monitoring observations since STIS was installed, are discussed in Chapter 13.

Figure 4.2: System Throughput of STIS's Low-Resolution, First-Order Grating Modes

Figure 4.3: System Throughput of STIS's Grating Modes

Limiting Magnitudes

In Table 4.2 below, we give the A0 V type star V magnitude reached during a 1 hour integration which produces a signal-to-noise ratio of 10 in the continuum per spectral resolution element around the peak of the grating response, where we have integrated over the PSF in the direction perpendicular to the dispersion, and assumed the 52X0.2 slit for the first-order gratings and the 0.2X0.2 slit for the echelles.

Table 4.2: Limiting A0 V Star V Magnitudes

Grating	Wavelength (Å)	Magnitude
G750L	7000	20.9
G750M	7000	19.1
G430L	5500	20.9
G430M	5500	18.5
G230LB	3000	18.4
G230MB	3000	15.4
G230L	2600	18.4
G230M	2600	14.5
G140L	1350	16.7
G140M	1350	13.8
E230M	2700	13.5
E230H	2600	11.4
E140M	1400	11.4
E140H	1350	10.5
PRISM		20.1 @ = 2300 (slitless)

Saturation

Both CCD and MAMA observations are subject to saturation at high total accumulated counts per pixel. The CCD can be saturated due to the saturation of the detector itself or of the gain amplifier for CCDGAIN = 1. MAMA saturation can occur due to the 16-bit format of its memory buffer. The nature of the saturation for CCD and MAMA spectroscopic observations is described in CCD Saturation and MAMA Saturation-Overflowing the 16 Bit Buffer, respectively.

MAMA Bright-Object Limits

The MAMA detectors are subject to:

• Absolute bright-object limits, above which targets cannot be observed.
• Science limits, above which the MAMA response becomes non-linear and photometric information is lost.

We direct MAMA observers to the discussion presented in MAMA Bright-Object Limits. For summary tables of bright-object screening magnitudes for all spectroscopic modes, see MAMA Spectroscopic Bright-Object Limits. It is the observers' responsibility to be sure that proposed observations do not exceed the MAMA bright-object limits.

Scanned Gratings: Prime and Secondary (Tilt) Positions

For the intermediate-resolution gratings and echelles (except E140M), only a portion of the full spectral range of the grating falls on the detector in any one exposure, and the gratings must be scanned (tilted) with a separate exposure taken at each tilt position, in order to cover the full spectral range (see Figure 4.4 and Figure 4.5 below). Accordingly, for these scanned gratings, the user may select a single exposure at a given wavelength, or a series of exposures at different wavelengths to cover a larger wavelength range. The user must choose either prime or secondary settings. The prime settings cover the full spectral range with 10% wavelength overlap between observations taken at adjacent settings. The secondary settings cover selected absorption or emission lines and may be more convenient to use in some applications. We expect the photometric and wavelength calibration accuracies to be higher for the prime settings than for most of the intermediate settings, as calibrations for the latter will be inferred from those taken at prime settings. A few frequently used intermediate settings are being calibrated directly as noted in Chapter 13. The central wavelengths, and corresponding minimum and maximum wavelengths, are presented in the individual grating sections in Chapter 13.

Figure 4.4: Scanned First-Order Gratings

Figure 4.5: Scanned Echelle Gratings

Cross-Over Regions

In the near UV, where the CCD has comparable sensitivity to the NUV-MAMA, you may want to consider using the G230LB or G230MB gratings with the CCD instead of the more standard G230L and G230M gratings with the MAMA. You will get improved throughput down to at least 2500 Å, a larger slit length, and use of the CCD rather than the MAMA (see Figure 4.3 and Chapter 13). On the other hand, the CCD has read noise, cosmic-ray sensitivity, hot pixels, and charge transfer efficiency losses. Also, for red objects, scattered light can be more of a problem with the red-sensitive CCD than with the solar-insensitive NUV-MAMA. For a solar-type spectrum, CCD data at wavelengths shorter than 2100 Å are dominated by scattered light.