Pushing the limits of the coronagraphic occulters on Hubble Space Telescope/Space Telescope Imaging Spectrograph

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-adaptiveoptics-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 adaptiveoptics-fed coronagraphs.

An Exo–Kuiper Belt with an Extended Halo around HD 191089 in Scattered Light

“…Assuming gray scattering for the dust in the ring seen with the NICMOS F110W filter and GPI H band, the NICMOS-NMF result will produce surface brightnesses of ∼2.2 and ∼1.0 mJy arcsec −2 , respectively. Therefore, even the disk is not removed by the reduction methods; it is below a detection threshold of 1σ (which corresponds to a total S/N of 5 for extended structure spanning ∼25 pixels; Debes et al 2019). In this way, its detectability is beyond the limit of the current methods with the current GPI H-band PSF library.…”

A Layered Debris Disk around M Star TWA 7 in Scattered Light

“…The pivot wavelength of STIS is 0.58 μm 12 (wavelength range: 0.2-1.1 μm) and the pixel size is 0 05072 (Riley 2018). In each visit, to obtain the best angular coverage, we use the mutually nearly perpendicular WEDGEA1.0 and the BAR5 occulters: the former offers an inner working angle (IWA) of 0 5, the latter 0 2 (Debes et al 2019). In each TWA 7 visit, there are 6 of 146.6 s readouts using BAR5, and 2 of 640.0 s readouts or 2 of 700.0 s readouts using WEDGEA1.0.…”

Using Data Imputation for Signal Separation in High-contrast Imaging

“…We first demonstrate the DI-sNMF method using RDI post-processing of a point source. We use the Hubble Space Telescope (HST)/STIS coronagraphic imaging observations of HD 38393 behind the BAR5 occulter (Schneider et al 2017;Debes et al 2019) for this purpose. HD 38393 was visited with 9 orbits during HST Cycle 23 (Proposal ID: 14426, PI: J. Debes), each with a distinct telescope orientation and a 3×3 sub-pixel dithering pattern.…”

“…We repeat the above procedure 3 times for a planet at different brightness levels-one that is as bright as the speckles (i.e., in the 3×3 pixel box, the total planet signal is equal to the total speckle signal), and two that are either 10 or 0.1 times of that. For the results obtained from all the reduction methods, we measure the brightness of the point source following Debes et al (2019): we first integrate over the 3×3 pixel box for the element-wise median of the 90 PSFsubtracted images, then subtract that value by 9 times the median of the surrounding pixel values (that are within 2 and 4 pixels from the point source center) to remove the background. We calculate a conservative uncertainty 4 by taking the square root of the quadratic sum of the element-wise standard deviation of the 90 PSF-subtracted images in the 3×3 pixel box.…”