4.1 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 that have been optimized for point-source observations through short echelle slits and are available only in the ultraviolet (UV; see Figure 4.1).

Table 4.1: STIS Spectroscopic Capabilities.

¹ See Section 13.6 for detailed estimates.
² Number of exposures at distinct tilts needed to cover spectral range of grating with 10% wavelength overlap between adjacent settings.
³ For a complete list of supported and available-but-unsupported apertures for each grating, see Table A.1.
⁴ Naming convention gives dimensions of slit in arcseconds. E.g., 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 a fiducial bar to be used for coronagraphic spectroscopy.
⁵ For MAMA first-order modes, only ~25 arcsec of a long slit's length projects on the detector. (See also Section 4.2.2.).
⁶ 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.
⁷ The following slits are also supported for all echelle gratings. The 6X0.2 and 52X0.05 long slits are intended for use with extended emission line objects; order overlap must be considered when using these slits. Also the high S/N multi-slits 0.2X0.2FP(A–E) and 0.2X0.06FP(A–E) (see Chapter 12), the very narrow 0.1X0.03 slit for maximum spectral resolution, and the 0.2X0.05ND, 0.3X0.05ND, and 31X0.05ND(A–C) neutral density slits.
⁸ The 0.1X0.09 and 0.1X0.2 slits are supported with E230H only. F25MGII is supported with all NUV-MAMA gratings and the PRISM.
⁹ The 0.2X0.2 aperture is also supported with all first-order gratings. It is available-but-unsupported with the PRISM.
¹⁰ The F25SRF2 aperture can be used with the PRISM to filter out (geocoronal) Lyman-α emission.
¹¹ Resolution of 200,000 or greater is possible when used with the 0.1X0.03 slit and special observing and data reduction techniques.

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

4.1.1 Throughputs

To illustrate the broad wavelength coverage provided by STIS and the relative throughputs achievable across STIS' wavelength regime,  Figure 4.2 shows 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 illustrate the relative throughputs of different spectroscopic configurations, Figure 4.3 plots the efficiency of all grating modes for each of the four primary wavelength regimes on a common plot. These plots allow a comparison of the relative efficiencies of STIS in different configurations. Note, however, that these curves give the throughput at the time that STIS was initially calibrated (approximately 1997.7). Throughput changes, determined from monitoring observations since STIS was installed, are discussed in Sections 7.2.5 and 7.4.3. The throughput curves shown for the echelle modes trace the peak of the echelle blaze function for each order; throughputs near the ends of each order are lower by ~20 to 40%.

4.1.2 Limiting Magnitudes

In Table 4.2 below, we give the V magnitude for an A0V star that gives a signal-to-noise ratio of 10 in the continuum (per spectral resolution element around the peak of the grating response), in a 1 hour exposure, 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. The adopted sensitivities are those estimated for August 2008.

Table 4.2: Limiting A0 V Star V Magnitudes.

4.1.3 Saturation

Both CCD and MAMA observations are subject to saturation at high total accumulated counts/pix. 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 Section 7.3.2 and Section 7.5.1, respectively.

4.1.4 MAMA Bright Object Limits

The MAMA detectors are subject to absolute bright object limits, above which targets cannot be observed.

We direct MAMA observers to the discussion presented in Section 7.7. For summary tables of bright object screening magnitudes for all spectroscopic modes, see Section 13.8. It is the observer’s responsibility to be sure that proposed observations do not exceed the MAMA bright object limits.

4.1.5 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. For the intermediate-resolution gratings, we expect the photometric and wavelength calibration accuracies to be higher for the prime settings than for most of the secondary settings, as calibrations for the latter are inferred from those taken at prime settings. Early in the operation of STIS, the photometric accuracies of the primary echelle settings were higher; however, for post-SM4 observations the photometric accuracies of the primary and secondary settings are comparable. The central wavelengths, and corresponding minimum and maximum wavelengths, are presented in the individual grating sections in Chapter 13.

4.1.6 Cross-Over Regions

In the near-ultraviolet (NUV), where the CCD has comparable sensitivity to the NUV-MAMA, it may be more advantageous to use the G230LB or G230MB gratings with the CCD instead of the G230L and G230M gratings with the MAMA. Advantages include improved throughput down to at least 2500 Å, a larger slit length, and no restrictions on bright objects (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 (see STIS ISR 2022-05). For a solar-type spectrum, CCD data at wavelengths shorter than 2100 Å are dominated by scattered light.

4.1.7 Neutral Density Slits for Spectroscopy

The use of the 31X0.05NDA, 31X0.05NDB, and 31X0.05NDC neutral density slits are supported with both the MAMA first order and echelle gratings. These slits are nominally 31 × 0.05 arcseconds in size, have throughput reduction factors of roughly 6, 14, and 33, respectively, and are implemented in both APT and the STIS ETC. At this time, their use with the PRISM remains "available-but-unsupported." Additional information can be found in 31X0.05ND(A-C) Apertures.