STScI STIS Coron - NASA...STIS Coronagraphic Imaging ! Main function of STIS occulting wedge...
Transcript of STScI STIS Coron - NASA...STIS Coronagraphic Imaging ! Main function of STIS occulting wedge...
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STIS Coronagraphy
C.A. Grady Eureka Scientific and
Goddard Space Flight Center With G. Schneider (U. of Arizona) and the
HST GO 12228 team
STScI Calibration Workshop April 8, 2013
https://ntrs.nasa.gov/search.jsp?R=20130013630 2020-02-16T18:18:55+00:00Z
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HST Coronagraphs
� HST has supported 3 coronagraphs: NICMOS, ACS, and STIS
� Bulk of image properties are set by the OTA, but instrumental effects are also present
� None of the HST coronagraphs have been optimal - ACS spot size issues - NICMOS parts moving in dewar
- STIS incomplete apodization, and lack of peakdown software
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STIS Coronagraphic Imaging
� Main function of STIS occulting wedge structure is to prevent bleeding over region of interest in image
� Mainly used to image circumstellar disks – both protoplanetary and debris
� All disks have required removal of a PSF template to reveal the disk : the star typically accounts for 95-99% of the signal in the raw image
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The Bandpass - 50 CORON
The Mode
� Remaining working coronagraph on HST
� Simple Lyot coronagraph
� Broadest bandpass of the HST coronagraphs � Advantage: sensitivity to
faint nebulosity
� Disadvantage: huge color effects
Bandpass
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Shape of PSF is F(wavelength)
B3V A0V
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Shape of PSF is F(wavelength)
G0V M2V
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Matching the Shape of the PSF Improves Image Fidelity - NICMOS
HD 181327 NICMOS Discovery
Discovery NICMOS imagery
Schneider et al. 06
Radial streamers are difference in location of Dispersed speckles between the target and PSF
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Matching the Shape of the PSF Improves Image Fidelity -STIS
PSF star hr4413 – 2001 data PSF star HD 134970 – 2001 data
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Color-Matched and Contemporary PSF Further Improves Image Fidelity - STIS
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Composite of 6 rolls
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Analysis Quality Image can be deprojected and scaled by r2
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Far-field – sensitivity limited
ACS 2003 STIS 2011
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Alternate Approaches
� Use nebulosity-free regions of multi-roll image set with star itself as the PSF (ADI with few S/C rolls) � Works best for edge-on systems
or point source detection (Lowrance et al. 2005).
� May work if the PSF star is not as good a match to the science target (e.g. stellar activity for M stars)
� Can be used in an exploratory effort to detect disks (Krist et al. 2010), if can tolerate non-detection of disks with i<30-40 degrees.
Roll Differencing – AU Mic
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When the target star is a poor match to a reference object
� Options: have several reference objects and hope that one will be better, check what’s available in archival data. If have spectrum of target, explore which kinds of PSF target might be a reasonable fit to the target – use ETC for this.
� If extinction is an issue, select PSF object with has similar line-of-sight reddening - have recovered disks when the template is fainter than the target.
� If STIS coronagraphy continues to be popular may eventually have sufficient data to try LOCI or other techniques (Soummer et al. 2011) – these rely on a sufficient base of similar color imagery.
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HST Experience Relevant to JWST
� NICMOS observed targets which were redder in (J-H) or (H-K) than any MS star. STIS has observed young stars which are either more active or bluer due to accretion than MS comparison objects.
� For JWST, this situation will be routine as diskless stars will be blue compared to any system with circumstellar material near the star.
� 10-15° constraint on contemporary PSF templates is similar to field of regard issues with JWST.
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You CAN Teach an Old Coronagraph New Tricks!