understanding systematics in exoplanet observations
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Understanding systematics in exoplanet observations
Pieter Deroo (1), Mark Swain (1)) & Tom Greene (2) 1: NASA/Jet Propulsion Laboratory, California Institute of Technology; 2: NASA/Ames
National Aeronautics and Space Administration
Abstract: The most successful exoplanet characterization method to date – combined light photometry – relies on extremely accurate spectrophotometric differential precision. The required dynamic range has pushed exoplanet observations into previously unexplored instrument systematics. Mitigating these small but very significant effects is core to exoplanet science and the methodology how to remove the instrument noise is frequently a source of controversy. Here, we model the systematic noise floor induced by the most significant instrument systematic effects. We focus on near-IR spectroscopic characterization of exoplanet atmospheres using HgCdTe detectors. We establish the: 1. Raw systematics noise floor 2. Decorrelated/post-processed systematics noise floor and this for a range in instrument parameters (right-hand side). Three specific instruments are considered as well (bottom): HST/NICMOS which allowed for the first spectroscopic comparative exoplanetology; JWST/NIRSpec &FINESSE, which are future instruments that will characterize exoplanet atmospheres from the optical to the near-IR (~ 0.5 – 5 µm). FINESSE is a 76 cm aperture Explorer mission with well-sampled spectra at R ~ 1000.
Methodology: A Monte Carlo simulation in which common instrument errors (pointing & focus errors) are used as forcing functions for photometric variation induced by:
1. Position dependent gain variations and uncertainties in the focal plane array 2. Slit-losses
Various illumina,on pa.erns – PSFs with different width and posi,on (le:) – are propagated onto a detector with a specific inter and intra pixel gain varia,on (middle) and the oversampled PSF is read out at the different detector pixels (right). The FPA is Monte Carlo simulated; the ensemble of MC simula,ons together with a sta,s,cal distribu,on in poin,ngs and PSF sizes (FWHM) is used to establish the systema,c noise floor.
Instrument design vs spectrophotometric differential precision:
Raw Capability Decorrelated
Assuming: state-of-the-art HgCdTe inter and intra pixel response; Gaussian instrument errors. Overall systematics noise floor: combine the different partials appropriately The approximate location for NIRSpec & FINESSE @ 2 µm is annotated. The error distribution for those instruments can significantly differ from the assumed Gaussian. Our instrument specific results are given below.
Instrument specific systematics
Raw instrument noise partials @ 1.7 µm
HST/NICMOS JWST/NIRSpec FINESSE
Poin,ng induced gain changes 480 ppm/orbit 364 ppm/10,000s 17 ppm/orbit
Systema,c PSF varia,ons 932 ppm/orbit 3434 ppm/2wks 60 ppm/orbit
Poin,ng induced slit-‐loss NA 1 ppm/10,000s 22 ppm/orbit
HST/NICMOS JWST/NIRSpec FINESSE
Poin,ng induced gain changes 21ppm/orbit 75 ppm/10,000s 0.3 ppm/orbit
Systema,c PSF varia,ons 145ppm/orbit 49 ppm/2wks 4 ppm/orbit
Poin,ng induced slit-‐loss NA 1 ppm/10,000s 2 ppm/orbit
Decorrelated instrument noise partials @ 1.7 µm
HST/NICMOS JWST/NIRSpec FINESSE
RAW 1048 ppm/eclipse 1052 ppm/eclipse 67 ppm/eclipse
DECORRELATED 146 ppm/eclipse 217 ppm/eclipse 5 ppm/eclipse
Overall systematics noise floor per eclipse [8 hr] @ 1.7 µm
Systematics and exoplanet spectroscopy:
Simulated spectra (top panel) for the Super-‐Earth GJ 1214b and the warm Neptune GJ 436b as observed with FINESSE (middle panel) and JWST/NIRSpec (bo.om panel) taking into account all sources of noise, including systema,c noise. The noise for FINESSE is dominated by photon noise; for NIRSpec, systema,c noise dominates spectra at the short wavelength range. To illustrate the fact that NIRSpec requires mul,ple instrument secngs to cover the en,re wavelength range (and thus mul,ple eclipses), we have shaded the spectral region according to the minimum number of instrument secngs required.
Conclusions: Our simulations quantify the expected systematic noise floor for exoplanet spectroscopic observations and the improvements using the current-best decorrelation/post-processing [right, bottom]. Our simulations for HST/NICMOS are in agreement with the observed pre- and post-processed values, validating the simulations [bottom]. Simple trades in the instrument design can significantly optimize it for exoplanet spectrosopy [right]. As such, the well-sampled spectra of a modest aperture mission like FINESSE could have better limiting signal-to-noise than the poorly sampled JWST/NIRSpec at short wavelengths [bottom].
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FINESSE
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