Objective-Prism Spectroscopy

The STIS PRISM is used with the STIS/NUV-MAMA and provides spectra from 1150 to 3620 Å at resolving powers of ~500 in the ultraviolet declining to ~10 in the optical. A highly idealized schematic of a STIS objective-prism observation of a star cluster is shown in Figure 4.10. This example illustrates the power of the prism mode to simultaneously provide spectra covering a wide wavelength range of many objects in a single field of view.

The PRISM can be used at two wavelength settings, 1200 and 2125 Å. These are approximately the wavelengths that will lie at the center of AXIS1 on the detector for the two settings. The relationship between wavelength and pixel number along the central spectral trace is shown in Figure 4.11 for each setting. The dispersion as a function of wavelength is shown in Figure 4.12 for each setting.

Figure 4.10: Schematic Example of a PRISM Image of a Star Cluster

Figure 4.11: Wavelength vs. Pixel Number along the Central Spectral Trace for the PRISM at wavelength settings 1200 and 2125 Å.

Figure 4.12: Dispersion as a function of Wavelength for the PRISM at wavelength settings 1200 and 2125 Å. The lines for the 1200 setting (solid) and 2125 setting (dot-dash) nearly overlap.

The prism can be used with the clear MAMA aperture (25MAMA) or with either longpass ultraviolet filtered aperture (F25SRF2 or F25QTZ) to provide a 25 x 25 arcsecond field of view (see also F25SRF2-NUV-MAMA, Longpass and F25ND5-NUV-MAMA). The longpass filter F25SRF2 blocks geocoronal Lyman- 1216 Å and the F25QTZ longpass filter blocks both geocoronal Lyman- and geocoronal O I 1302 Å, significantly reducing the background from these lines (which is otherwise spread throughout the image) at the price of losing the short-wavelength range of the spectrum. In addition, the neutral-density filters ( Table 5.1) are supported for PRISM spectroscopy, as are the 52X0.05, 52X0.1, 52X0.2, 52X0.5, and 52X2 long slits.

Observers will generally want to obtain a direct image of the field they are taking a prism image of, so they can later determine the centering of the objects in their prism data. Because the PRISM and the mirrors used for imaging are both in the Mode Selection Mechanism, zero-point shifts will occur between the PRISM and imaging data (see Slit and Grating Wheels). For a discussion of the observations needed to measure these shifts, see Slitless First-Order Spectroscopy.

Note that PRISM spectroscopy produces images in which, a priori, the wavelength at a given pixel is not known, and source-dependent overlap of spectra can occur. For these reasons, PRISM spectroscopic data will not be calibrated automatically by the STScI pipeline. Instead, users will have to reduce and analyze their data off-line.