The Unique Coronagraphic Capabilities of STIS: Direct Imaging

John Debes,, Marshall Perrin,, & Glenn Schneider,

Spatially resolving circumstellar disks and directly detecting exoplanets in close proximity to stars requires extreme suppression of starlight:  the light from such objects ranges from less than one part per billion up to a few hundred parts per million of the light from their host star.  Directly imaging faint objects close to bright stars requires the ability to subtract or suppress the diffracted light from the central star.  As a class, these techniques are called “high-contrast” observing.

Despite having been in space over 16 years so far, the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope still offers high-contrast imaging and spectroscopy not possible with any other current Hubble instrument—nor with any current or future ground-based adaptive-optics coronagraph. STIS’s coronagraph, or imaging mask, can suppress stray unwanted light from bright sources to reveal faint structures and discover sources at close angular proximity.

Since its repair during Servicing Mission 4, STIS has broken new ground in imaging scattered visible light from circumstellar dust in exoplanetary debris systems (Schneider et al. 2013), elucidating the orbit and nature of the enigmatic planetary candidate Fomalhaut b (Kalas et al. 2013; Currie et al. 2012), and discovering new, faint debris disks, which may host planetary systems (Krist et al. 2012).

Two capabilities of STIS are particularly noteworthy: imaging with exquisite contrast—about three hundred parts per million at apparent separations as small as 0.4” and about one part per million at 1”—and coronagraphic spectroscopy over visible wavelengths at similar contrasts.  In this article, we discuss the general strategies for coronagraphic imaging, and describe a Hubble program that has made use of these techniques.

High contrast at small separations

STIS was not primarily designed as a coronagraphic instrument. Its mask, which is rather simple, is a combination of image-plane wedges, which can be used to block the light from a central star (see Figure 1).  Typically, coronagraphs also actively suppress light from a star’s Airy disk, using a Lyot stop in the pupil plane. In addition, STIS coronagraphy relies on reference stars to subtract any residual diffracted light that passes the coronagraphic wedges.

Two of the supported, vertical, wedge positions in the STIS coronagraphic aperture are where the wedge is 1’’ (the WEDGEA1.0 position) or 0.6” wide (the WEDGEA0.6 position).  To obtain high contrast at small separations requires imaging the target at the WEDGEA0.6 position.  Because of the wedge structure, obtaining high contrast at 0.3” for all azimuthal angles around the occulted star requires imaging the target at six separate spacecraft rolls.  Hubble can typically support rolls up ±30° from a nominal orientation at any given time, primarily limited by an acceptable angle between the solar arrays and the Sun.