12.11 Coronagraphic Imaging - 50CORON

STIS has a single coronagraphic mask aperture for direct imaging. The aperture (50CORON) contains two occulting bars 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 larger rectangular bar (BAR10) measures 10 by 3 arcseconds. The small occulting finger on the right (BAR5) was damaged during assembly of STIS and was not used until Cycle 25, when its performance was tested. Now, the STIS team recommends using BAR5 as a fully supported capability for coronagraphic science. See STIS ISR 2017-03 for more details. The entire coronagraphic aperture measures 50 × 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. The December 2013 STScI Analysis Newsletters (STAN) discusses additional details of this effect. 

The aperture cannot be combined with a filter and so, when used with the CCD, yields a bandpass of ~2000–10,300 Å. 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 0.6¹ 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 combination with the option of a coronagraphic mask, there is a limited amount of apodization via a Lyot stop which masks the outer perimeter of the re-imaged exit pupil. Consequently, diffraction from the secondary mirror assembly and the telescope spider is not apodized. The STIS coronagraphic imaging facility is well suited to imaging problems involving faint material surrounding a relatively bright source. Typical examples include circumstellar disks, such as β Pictoris, and the host galaxies of bright QSOs.

When imaging without the coronagraph, the CCD long wavelength halo from the central source is present as well as window reflection ghosts. In practice the coronagraph provides substantial additional suppression of the PSF wings, especially at wavelengths >8000 Å, where the halo of light scattered within the CCD itself dominates the far wings of the PSF. Observing with the coronagraphic aperture locations in concert with post-processing and the use of multiple spacecraft orientations can further improve contrast by orders of magnitude. Figure 12.7, found in the JATIS article by Debes, Ren, & Schneider (2019), shows STIS' contrast performance for point sources in the visible interior to 1 arcsecond using the BAR5 aperture location and classical azimuthal differential imaging (ADI) or Karhunen–Loève Image Projection (KLIP) post processing. At distances beyond 6", contrasts of 10^–9 are possible with STIS and other coronagraphic aperture locations. Due to STIS' broad bandpass, it still retains sensitivity to both stellar and substellar objects. For more information see the JATIS article by Debes, Ren, & Schneider (2019), "Pushing the limits of the coronagraphic occulters on Hubble Space Telescope/Space Telescope Imaging Spectrograph".

Due to the very broad bandpass of the unfiltered STIS CCD, the STIS coronagraphic PSF shape is very strongly dependent on the target's spectral energy distribution. When using the coronagraph to look for a point source or a localized structure that is strongly asymmetric (such as an edge-on disk), the best approach is to observe the target star at a minimum of two and preferably three different roll angles, and then compare the images to separate the stellar PSF from the real circumstellar structure. When looking for more diffuse or symmetric material, it will be necessary to use a separate comparison star. Here it is important to match the colors of the target and comparison star as closely as possible. We suggest that all the broadband UVBRI color differences be less then 0.08 magnitudes. In either case, it is also essential to compare stars at the same location on the coronagraphic mask.

Breathing and focus differences will also significantly affect the quality of such a subtraction, but the observer has only limited control over these parameters. The best alignment of STIS PSF images tends to occur when comparing images taken in the same part of adjacent orbits. When observing the same star at multiple roll angles, it is therefore often useful to do a sequence of adjacent one-orbit visits, each at a different roll angle. As large departures from the nominal roll angle can also affect the PSF shape, it may be helpful to keep the roll changes as small as is consistent with the structure to be imaged. When observing a separate comparison star, it is best if possible to observe a star in the same part of the sky, during an adjacent orbit, and at the same angle relative to the nominal spacecraft roll, as the observations of the prime target, but remember that picking a comparison star with a good color match must be the first priority.

An alternative strategy would be to take observations separated by several days, but constrained so that each observation is done at the nominal spacecraft roll. When very large roll changes or several orbit-long visits are required, this might give better results than doing the observations in adjacent orbits, but there is very little operational experience using this approach.

Attempting to observe multiple coronagraphic targets or the same target at different roll angles in a single orbit is not recommended. The overheads required to do separate visits in a single orbit are very large, and the PSF alignment between different parts of the same orbit are usually inferior to that obtained between the same part of adjacent orbits.

Coronagraphic images of stars of various colors have been obtained as parts of calibration programs 7151, 7088, 8419, 8842, and 8844 and are available from the archive. These images may be useful in providing comparison objects or in estimating exposure times. However, for the best PSF subtraction, we still recommend that each coronagraphic program include its own tailored PSF observations.

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" occulter (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. The BAR10 rounded-corner aperture is currently available-but-unsupported. Even though there are no current plans to incorporate this aperture into APT, it can be implemented using POS-TARGs from other aperture positions. BAR5 is now a currently-supported mode and is incorporated into APT. A summary of the initial results of this program, as well as detailed suggestions on how to implement observations using these new aperture positions, is available within STIS ISR 2017-03 ("Enabling Narrow(est) IWA Coronagraphy with STIS BAR5 and BAR10 Occulters") and additional information on BAR5 may be found here.

Users wishing to achieve a particular contrast are encouraged to read Sections 2.3-2.5 of Debes, Ren, & Schneider 2019 (JATIS article) in order to predict the possible contrast performance for a star of a given magnitude. See the STIS Coronagraphic Visualization Tool available on the Data Analysis and Software Tools page for help with planning and preparing STIS coronagraphic observations.

Table 12.6: Apertures for Coronagraphic Mask.

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¹ The 0.6 arcsecond location is on wedge A only and was added in Cycle 12.

² The BAR5 location was first supported in Cycle 24.