Abstract
The Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) contains the only currently operating coronagraph in space that is not trained on the Sun. In an era of extreme-adaptive-optics-fed coronagraphs, and with the possibility of future space-based coronagraphs, we re-evaluate the contrast performance of the STIS CCD camera. The 50CORON aperture consists of a series of occulting wedges and bars, including the recently commissioned BAR5 occulter. We discuss the latest procedures in obtaining high-contrast imaging of circumstellar disks and faint point sources with STIS. For the first time, we develop a noise model for the coronagraph, including systematic noise due to speckles, which can be used to predict the performance of future coronagraphic observations. Further, we present results from a recent calibration program that demonstrates better than 10 − 6 point-source contrast at 0.6″, ranging to 3 × 10 − 5 point-source contrast at 0.25″. These results are obtained by a combination of subpixel grid dithers, multiple spacecraft orientations, and postprocessing techniques. Some of these same techniques will be employed by future space-based coronagraphic missions. We discuss the unique aspects of STIS coronagraphy relative to ground-based adaptive-optics-fed coronagraphs.
Highlights
Space Telescope Imaging Spectrograph (STIS) is a second generation instrument for Hubble Space Telescope (HST) that was installed in early 1997 (Ref. 1) during Servicing Mission Two
We determine what sets its limits on contrast performance at small inner working angles; in addition, we can assess its implications for what might be possible with second-generation highcontrast imagers in space, e.g., the coronagraphic instrument (CGI) proposed for the Wide Field Infrared Survey Telescope (WFIRST) mission.[6,7]
We review here the sources of noise that are present in coronagraphic observations, including noise terms that are generally not accounted for within the traditional signal-to-noise ratio (SNR) calculations that are performed for the direct imaging of sources
Summary
Space Telescope Imaging Spectrograph (STIS) is a second generation instrument for Hubble Space Telescope (HST) that was installed in early 1997 (Ref. 1) during Servicing Mission Two. We determine what sets its limits on contrast performance at small inner working angles; in addition, we can assess its implications for what might be possible with second-generation highcontrast imagers in space, e.g., the coronagraphic instrument (CGI) proposed for the Wide Field Infrared Survey Telescope (WFIRST) mission.[6,7] In Sec. 2, we talk about general strategies to obtain high contrast with STIS, building upon previous results;[8,9,10] therein we include the development of a coronagraphic noise model, which can be used to predict the performance of a given observing strategy; together with an analysis of the impact that charge transfer inefficiency has on high-contrast imaging with CCDs. In Sec. 3, we present a calibration program recently obtained with BAR5 to demonstrate high-contrast capabilities with STIS that rival current ground-based near-IR-optimized coronagraphs, which are equipped with extreme adaptive optics (AO) systems, as well as a discussion of STIS’s sensitivity to point sources and circumstellar disks.
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