12.10 Coronagraphic Spectroscopy

STIS offers the capability to perform both spectroscopic observations with occulting bars located in the long slits and imaging observations with occulting bars in the coronagraphic mask 50CORON. Coronagraphic imaging is described in detail in Section 12.11. Note that all barred spectroscopy is available-but-unsupported (except for 52X0.2F1 which is only available for first-order gratings), see Appendix A.2 for more information.

Barred Spectroscopy

The 52X0.2F1 aperture is the same physical slit as 52X0.2 with the target centered behind BAR5 (0.5 arcseconds occulting bar), instead of at the center of the 52X0.2 long slit. When this aperture is specified the slit wheel is rotated slightly to bring the bar into the center of the detector's field of view. (Figure 12.5) This results in a tilt of 1.30° with respect to AXIS2 (i.e., the spatial axis perpendicular to the dispersion). Also, the full 52 arcseconds length of the long-slit field of view is decreased by about 20% for CCD spectroscopy.

This occulting bar is well suited to scientific programs conducting spectroscopy of faint extended material around a bright central source. Typical examples of such a program include QSO host galaxies, dynamics of jets in young stellar objects, spectroscopic characterization of circumstellar disk material, and spectroscopy of resolved binaries and companions. Observers performing barred spectroscopy should be sure to consult Section 13.7.3. Spectroscopic observations with the 0.5 arcsecond occulting bar, BAR5, are limited to first-order spectroscopic modes.

Aperture 52X0.2F1 refers to the occulting bar itself, while 52X0.2F1-R represents a reference position offset from the bar. The reference aperture was designed to allow the observer to peakup on the target prior to moving it behind the occulting bar.  But because peakups are not recommended for apertures larger than 0.1 arcsecond in width, the 52X0.2F1-R is no longer used. The occulting bar is even wider (0.5 arcsecond), so a simple ACQ exposure should suffice to center the target in the 52X0.2F1 aperture.

Table 12.5 below shows the complete set of aperture names and their application for the 0.5 arcsecond fiducial bar on the 52x0.2 slit. An example of an acquisition into the 52X0.2F1 aperture is provided in Chapter 8.

Table 12.5: Aperture Names and Applications on 52X0.2 Slit.

There are two important ramifications you should be aware of when using the 52X0.2F1 aperture: (1) ORIENT specification and (2) fringing correction.

Specifying ORIENT with the 52X0.2F1 Aperture

The ORIENT special requirement parameter is a specified position angle of the orientation reference vector U3, which is offset by 45° from the STIS detector spatial axis (AXIS2), in the direction North through East. Given a position angle on the sky of θ and taking into account the additional 1.30° offset for the 52X0.2F1 aperture (see Figure 12.5):

ORIENT = θ + 46.30° 
or
ORIENT = θ + 226.30°.

For example, if we want to place the STIS 52X0.2F1 aperture along the jet of M87 (the example in Figure 11.10 for a standard slit), we would do the following. Given a position angle of 110° on the sky:

ORIENT = 110° + 46.30° = 156.3°
or
ORIENT = 110° + 226.30° = 336.3°.

Correcting Fringing for the 52X0.2F1 Aperture

The STIS CCD shows fringing for wavelengths longward of 7000 Å with the G750L and G750M gratings, limiting the signal-to-noise that can be obtained in these modes. The fringing is caused by interference of multiple reflections between the two surfaces of the CCD. (See Chapter 7 for details.) The recommended way to correct for fringing is by using contemporaneous fringe flats in conjunction with your science observations. Contemporaneous flat-field exposures need to be budgeted in the Phase I time request and specified more fully during the Phase II proposal generation process with APT. Section 8.5.2, entitled "Target_Name=CCDFLAT", of the HST Phase II Instructions provides a description of the parameter settings necessary to obtain contemporaneous flat-field exposures. It is the observer’s responsibility to include contemporaneous flat fields in their Phase II proposals.

For spectra of point sources, contemporaneous fringe flats are normally taken using a short slit which is concentric with the long slit used for the science exposures. When point source fringe flats are taken at or near a standard aperture position near the center of the detector, the best alignment between science image and fringe flat will usually be for fringe flats taken using the 0.3X0.09 aperture. In the special case of the 52X0.2F1 aperture, however, we recommend obtaining long slit fringe flats to fully sample the PSF, since the spatial coverage of the short slit is only a few CCD pixels larger than that of the occulting bar of the 52X0.2F1 slit. A short slit fringe flat does give a somewhat better fringe correction for point sources than a long slit fringe flat. Consequently, if you are also taking an unocculted image of your star with the 52X0.2 aperture, we recommend obtaining a short slit flat as well as a long slit flat to rectify that image.

Optimization of the instrument configuration for the acquisition of fringe flats, the subsequent data reduction, and fringing analysis are described in more detail in a series of STIS Instrument Science Reports and in the STIS Data Handbook. STIS ISR 1997-15 (Revision A) provides a general overview of fringing. STIS ISR 1997-16 discusses fringing in spectra of extended sources. STIS ISR 1998-19 (Revision A) discusses fringing associated with point sources, fringing analysis, and details related to the 52X0.2F1 aperture. STIS ISR 1998-29, STIS ISR 2021-01, and the stistools defringe documentation provide detailed descriptions of the STIS tasks (normspflat, mkfringeflat, defringe, and prepspec) in the stistools package that use contemporaneous flat-field images to remove infrared (IR) fringing from STIS G750L and G750M spectra. Section 3.5.5 of the STIS Data Handbook provides guidance and updates to the instructions for removing near-infrared (NIR) fringing presented in STIS ISR 1998-29 and STIS ISR 2021-01.