STIS has a single coronagraphic mask aperture for direct imaging. The aperture (50CORON) contains one occulting bar and two intersecting wedges and is shown in
Figure 12.6. This illustration of the coronagraphic aperture is derived from an on-orbit lamp flat. The approximate positions of the predefined aperture locations are marked. The wedges vary in width from 0.5 to 3.0 arcseconds over their 50 arcseconds length, while the rectangular bar measures 10 by 3 arcseconds. The small occulting finger on the right was damaged during the assembly of STIS and is not used. The entire coronagraphic aperture measures 50
в 50 arcseconds, slightly smaller than the size of the unobstructed CCD aperture. The parallel readout of the CCD is along the
AXIS2 direction, and heavily saturated images will bleed in this direction (vertically in this figure). Saturation near the top of the detector can result in serial transfer artifacts that produce tails in the AXIS1 direction; for additional details of this effect see the discussion in the first of the two
June 2013 STScI Analysis Newsletters (STAN).
The aperture cannot be combined with a filter and so, when used with the CCD, yields a bandpass of ~2000–10,300 е. See Section 5.2.2 for the spectral properties of the images obtained. A number of locations on the occulting masks have been specified, to correspond to widths of 2.75, 2.5, 2.0, 1.75, 1.0, and 0.6
1 arcseconds on each wedge. The mask is not available for use with the MAMA detectors due to concerns about bright object protection of the MAMAs.
In Figure 12.7 we provide a comparison of the PSF suppression provided by STIS coronagraphic imaging relative to WFPC2 imaging and the Optical Telescope Assembly scatter. It had been hoped that the optical performance of the STIS CCD clear aperture without the coronagraph would be comparable to that with the coronagraph, although, without the coronagraph, the CCD long wavelength halo from the central source and the window reflection ghosts are present. In practice the coronagraph does provide substantial additional suppression of the PSF wings, especially at wavelengths >8000 е, where the halo of light scattered within the CCD itself dominates the far wings of the PSF.
In planning any observing program with the 50CORON aperture, observers should carefully consider the required orientation of the target. The telescope’s V2 and V3 axes are at 45° to the STIS
AXIS1/AXIS2 coordinate system (see
Figure 11.1) and so diffraction spikes further reduce the unocculted field of view.
A series of apertures has been defined for the coronagraphic mask so that targets can be placed on the 3 arcseconds wide bar and 5 locations on each of the two wedges. These apertures are summarized in Table 12.6 below. We defined a special coronagraphic acquisition technique for placing stars at these predefined locations. This involves performing a bright-target acquisition with a filtered aperture, followed by a slew to the chosen location on the coronagraphic mask. An example of an acquisition into one of the bars on the
50CORON aperture is provided in
Section 8.5.6.
An outsourced Cycle 20 STIS calibration program, 12923 (PI: Gaspar), investigated new aperture locations near the corners of the coronagraphic bar (BAR10) as well as the ‘‘bent finger’’ wedge (BAR5). These new positions allow high contrast imaging at a minimum working angle of 0.15 arcseconds, with demonstrated performance to ~0.2 arcseconds -- roughly 3λ/D, and close to a factor of two better than WEDGEA0.6. These new apertures are currently available-but-unsupported. Even though currently there are no plans to incorporate these apertures into APT, they can be implemented using POS-TARGs from other aperture positions. A summary of the initial results of this program, as well as detailed suggestions on how to implement observations using these new aperture positions, are available at:
