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 (UV) (see Figure 4.1
To illustrate the broad wavelength coverage provided by STIS, and the relative throughputs achievable across STIS’ 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, System Throughput of STIS’ Grating Modes
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. 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 Chapter 13
. 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%.
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.
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.
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 secondary settings, as calibrations for the latter will be inferred from those taken at prime settings. A few frequently used secondary 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
In the near-ultraviolet (NUV), where the CCD has comparable sensitivity to the NUV-MAMA
, you may want to consider using the G230LB
gratings with the CCD instead of the G230L
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.