4.4 Objective Prism Spectroscopy

Beginning in Cycle 30, the use of the STIS PRISM will be an available-but-unsupported mode due to the growing uncertainty of its absolute flux calibration (see Section 13.3/PRISM). Observers interested in using this optical element who require absolute flux calibration will need to plan to observe their own flux standard star.

The STIS PRISM is used with the STIS NUV-MAMA and provides spectra from 1150 to 3620 Å at resolving powers of up to ~2500 in the FUV declining to ~10 at optical wavelengths. In Figure 4.11 we compare a direct NUV-MAMA image of the star cluster NGC 604 with a PRISM exposure of the same field. 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. Note that the ACS also has the capability of performing slitless UV spectroscopy.

As can be seen in Figure 4.11, an NUV objective-prism spectrum does not cover the full range of the detector in the dispersion direction. In many cases, there will be a significant number of counts only over an extent of ~200 pixels due to the large dispersion and low sensitivity at short wavelengths. This characteristic can be an advantage over a grating such as the G230L in crowded fields, since it reduces the possibility of overlap between different sources.

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 4.12 for each setting. The dispersion as a function of wavelength is shown in Figure 4.13 for each setting.

Figure 4.11: NUV-MAMA image (left) and objective-prism exposure (right) of NGC 604.

These images of NGC 604, a Scaled OB Association, were obtained under GO program 9096. The dispersion direction is nearly parallel to the x-axis.
Figure 4.12: Wavelength vs. Pixel Number along the Central Spectral Trace for the PRISM at Wavelength Settings 1200 and 2125 Å.

Figure 4.13: Dispersion as a Function of Wavelength for the PRISM at Wavelength Settings 1200 Å (solid) and 2125 Å (dot-dash).

The PRISM can be used with the clear MAMA aperture (25MAMA) or with either longpass UV filtered aperture (F25SRF2 or F25QTZ) to provide a 25 × 25 arcsecond field of view (see also F25SRF2 and F25QTZ). The longpass filter F25SRF2 blocks geocoronal Lyman-α 1216 Å and the F25QTZ longpass filter blocks both geocoronal Lyman-α and geocoronal O I 1302 Å triplet, 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 also obtain a direct image of the field when taking an objective prism spectrum, 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 Section 3.2.2, Slit and Grating Wheels). For a discussion of the observations needed to measure these shifts, see Section 12.1.

Note that slitless 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, slitless PRISM spectroscopic data will not be calibrated automatically by the STScI pipeline. Instead, users will have to reduce and analyze their data off-line.