STIS offers the capability to perform 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.
The 52X0.2F1 aperture is the same physical slit as
52X0.2 with the target centered on the smaller of the two occulting bars (i.e., 0.5 arcseconds), 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. This results in a tilt of 1.30
o 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, and spectroscopy of resolved binaries. Observers performing barred spectroscopy should be sure to consult
Section 13.7.3. Spectroscopic observations with the 0.5 arcsecond occulting bar 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.
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Slit (width=0.″2, length=52 ″). Fiducial=0. ″5
Reference point off-fiducial
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The ORIENT special requirement parameter is a specified position angle of the orientation reference vector U3, which is offset by 45
o 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
o offset for the
52X0.2F1 aperture (see
Figure 12.5):
ORIENT = θ + 46.30
o or θ + 226.30
o
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
o on the sky:
ORIENT = 110
o + 46.30
o = 156.3
o
ORIENT = 110
o + 226.30
o = 336.3
o
The STIS CCD shows fringing for wavelengths longward of 7000 Angstroms 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 13.11.2, 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 provides a detailed description of the STIS tasks (
normspflat,
mkfringeflat,
defringe, and
prepspec) in the
stsdas.hst_calib.stis 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.
