75 research outputs found
Optical performance of the JWST MIRI flight model: characterization of the point spread function at high-resolution
The Mid Infra Red Instrument (MIRI) is one of the four instruments onboard
the James Webb Space Telescope (JWST), providing imaging, coronagraphy and
spectroscopy over the 5-28 microns band. To verify the optical performance of
the instrument, extensive tests were performed at CEA on the flight model (FM)
of the Mid-InfraRed IMager (MIRIM) at cryogenic temperatures and in the
infrared. This paper reports on the point spread function (PSF) measurements at
5.6 microns, the shortest operating wavelength for imaging. At 5.6 microns the
PSF is not Nyquist-sampled, so we use am original technique that combines a
microscanning measurement strategy with a deconvolution algorithm to obtain an
over-resolved MIRIM PSF. The microscanning consists in a sub-pixel scan of a
point source on the focal plane. A data inversion method is used to reconstruct
PSF images that are over-resolved by a factor of 7 compared to the native
resolution of MIRI. We show that the FWHM of the high-resolution PSFs were
5-10% wider than that obtained with Zemax simulations. The main cause was
identified as an out-of-specification tilt of the M4 mirror. After correction,
two additional test campaigns were carried out, and we show that the shape of
the PSF is conform to expectations. The FWHM of the PSFs are 0.18-0.20 arcsec,
in agreement with simulations. 56.1-59.2% of the total encircled energy
(normalized to a 5 arcsec radius) is contained within the first dark Airy ring,
over the whole field of view. At longer wavelengths (7.7-25.5 microns), this
percentage is 57-68%. MIRIM is thus compliant with the optical quality
requirements. This characterization of the MIRIM PSF, as well as the
deconvolution method presented here, are of particular importance, not only for
the verification of the optical quality and the MIRI calibration, but also for
scientific applications.Comment: 13 pages, submitted to SPIE Proceedings vol. 7731, Space Telescopes
and Instrumentation 2010: Optical, Infrared, and Millimeter Wav
The Mid-Infrared Instrument for the James Webb Space Telescope, V: Predicted Performance of the MIRI Coronagraphs
The imaging channel on the Mid-Infrared Instrument (MIRI) is equipped with
four coronagraphs that provide high contrast imaging capabilities for studying
faint point sources and extended emission that would otherwise be overwhelmed
by a bright point-source in its vicinity. Such bright sources might include
stars that are orbited by exoplanets and circumstellar material, mass-loss
envelopes around post-main-sequence stars, the near-nuclear environments in
active galaxies, and the host galaxies of distant quasars. This paper describes
the coronagraphic observing modes of MIRI, as well as performance estimates
based on measurements of the MIRI flight model during cryo-vacuum testing. A
brief outline of coronagraphic operations is also provided. Finally, simulated
MIRI coronagraphic observations of a few astronomical targets are presented for
illustration
Dust in the inner regions of debris disks around A stars
We present infrared interferometric observations of the inner regions of two
A-star debris disks, beta Leo and zeta Lep, using the FLUOR instrument at the
CHARA interferometer on both short (30 m) and long (>200 m) baselines. For the
target stars, the short baseline visibilities are lower than expected for the
stellar photosphere alone, while those of a check star, delta Leo, are not. We
interpret this visibility offset of a few percent as a near-infrared excess
arising from dust grains which, due to the instrumental field of view, must be
located within several AU of the central star. For beta Leo, the near-infrared
excess producing grains are spatially distinct from the dust which produces the
previously known mid-infrared excess. For zeta Lep, the near-infrared excess
may be spatially associated with the mid-infrared excess producing material. We
present simple geometric models which are consistent with the near and
mid-infrared excess and show that for both objects, the near-infrared producing
material is most consistent with a thin ring of dust near the sublimation
radius with typical grain sizes smaller than the nominal radiation pressure
blowout radius. Finally, we discuss possible origins of the near-infrared
emitting dust in the context of debris disk evolution models.Comment: 20 pages, 2 figures, to appear in the Astrophysical Journa
The Mid-Infrared Instrument for the James Webb Space Telescope, III: MIRIM, The MIRI Imager
In this article, we describe the MIRI Imager module (MIRIM), which provides
broad-band imaging in the 5 - 27 microns wavelength range for the James Webb
Space Telescope. The imager has a 0"11 pixel scale and a total unobstructed
view of 74"x113". The remainder of its nominal 113"x113" field is occupied by
the coronagraphs and the low resolution spectrometer. We present the instrument
optical and mechanical design. We show that the test data, as measured during
the test campaigns undertaken at CEA-Saclay, at the Rutherford Appleton
Laboratory, and at the NASA Goddard Space Flight Center, indicate that the
instrument complies with its design requirements and goals. We also discuss the
operational requirements (multiple dithers and exposures) needed for optimal
scientific utilization of the MIRIM.Comment: 29 pages, 9 figure
How dark the sky: the JWST backgrounds
We describe the sources of stray light and thermal background that affect
JWST observations; report actual backgrounds as measured from commissioning and
early science observations; compare those background levels to pre-launch
predictions; estimate the impact of the backgrounds on science performance; and
explore how the backgrounds probe the achieved configuration of the deployed
observatory. We find the observatory is limited by the irreducible
astrophysical backgrounds, rather than scattered stray light and thermal
self-emission, for all wavelengths micron, thus meeting the
level 1 requirement. This result was not assured given the open architecture
and thermal challenges of JWST, and is the result of meticulous attention to
stray light and thermal issues in the design, construction, integration, and
test phases. From background considerations alone, JWST will require less
integration time in the near-infrared compared to a system that just met the
stray light requirements; as such, JWST will be even more powerful than
expected for deep imaging at 1--5 micron. In the mid-infrared, the measured
thermal backgrounds closely match pre-launch predictions. The background near
10 micron is slightly higher than predicted before launch, but the impact on
observations is mitigated by the excellent throughput of MIRI, such that
instrument sensitivity will be as good as expected pre-launch. These measured
background levels are fully compatible with JWST's science goals and the Cycle
1 science program currently underway.Comment: Submitted to the "JWST Overview" special issue of PAS
JWST MIRI flight performance: The Medium-Resolution Spectrometer
The Medium-Resolution Spectrometer (MRS) provides one of the four operating
modes of the Mid-Infrared Instrument (MIRI) on board the James Webb Space
Telescope (JWST). The MRS is an integral field spectrometer, measuring the
spatial and spectral distributions of light across the 5-28 wavelength
range with a spectral resolving power between 3700-1300. We present the MRS's
optical, spectral, and spectro-photometric performance, as achieved in flight,
and we report on the effects that limit the instrument's ultimate sensitivity.
The MRS flight performance has been quantified using observations of stars,
planetary nebulae, and planets in our Solar System. The precision and accuracy
of this calibration was checked against celestial calibrators with well-known
flux levels and spectral features. We find that the MRS geometric calibration
has a distortion solution accuracy relative to the commanded position of 8 mas
at 5 and 23 mas at 28 . The wavelength calibration is accurate
to within 9 km/sec at 5 and 27 km/sec at 28 . The uncertainty in
the absolute spectro-photometric calibration accuracy was estimated at 5.6 +-
0.7 %. The MIRI calibration pipeline is able to suppress the amplitude of
spectral fringes to below 1.5 % for both extended and point sources across the
entire wavelength range. The MRS point spread function (PSF) is 60 % broader
than the diffraction limit along its long axis at 5 and is 15 % broader
at 28 . The MRS flight performance is found to be better than prelaunch
expectations. The MRS is one of the most subscribed observing modes of JWST and
is yielding many high-profile publications. It is currently humanity's most
powerful instrument for measuring the mid-infrared spectra of celestial sources
and is expected to continue as such for many years to come.Comment: 16 pages, 21 figure
JWST/MIRI Data Reduction and Products
The Mid-Infrared Instrument (MIRI) is one of four science instruments to be flown aboard the James Webb Space Telescope (JWST). MIRI operates from 5 to 28.5 microns and provides a suite of versatile capabilities including imaging, low-resolution spectroscopy (LRS), medium-resolution spectroscopy (MRS) via an integral field unit, and coronagraphy. The MIRI pipeline consists of three stages: 1) Raw to Slope Images, 2) Calibrated Slope Images, and 3) Multiple Exposures Combined. The pipeline is designed to provide well-calibrated, high level data products that maximize the scientific return from the instrument
Calibrating the James Webb Space telescope filters as star formation rate indicators
We have calibrated the 6.5m James Webb Space Telescope (JWST) mid-infrared (MIR) filters as star formation rate
(SFR) indicators, using JWST photometry synthesized from Spitzer spectra of 49 low-redshift galaxies, which cover a
wider luminosity range than most previous studies. We use Balmer-decrement-corrected Hα luminosity and
synthesized MIR photometry to empirically calibrate the Spitzer, WISE, and JWST filters as SFR indicators. Our
Spitzer and WISE calibrations are in good agreement with recent calibrations from the literature. While MIR
luminosity may be directly proportional to SFR for high-luminosity galaxies, we find a power-law relationship
between MIR luminosity and SFR for low-luminosity galaxies. We find that for galaxies with
an Hα luminosity of 1040 erg s-1 (corresponding to an SFR of~0.055 M yr-1), the corresponding JWST MIR ν Lν
luminosity is between 1040.50 and 1041.00 erg s-1. Power-law fits of JWST luminosity as a function of Hα luminosity
have indices between 1.17 and 1.32. We find that the scatter in the JWST filter calibrations decreases with increasing
wavelength from 0.39 to 0.20 dex, although F1000W is an exception where the scatter is just 0.24 dex
Status of the mid-IR ELT imager and spectrograph (METIS)
The Mid-Infrared ELT Imager and Spectrograph (METIS) is one of three first light instruments on the ELT. It will provide high-contrast imaging and medium resolution, slit-spectroscopy from 3 - 19um, as well as high resolution (R ∼ 100,000) integral field spectroscopy from 2.9-5.3μm. All modes observe at the diffraction limit of the ELT, by means of adaptive optics, yielding angular resolutions of a few tens of milliarcseconds. The range of METIS science is broad, from Solar System objects to active galactic nuclei (AGN). We will present an update on the main science drivers for METIS: circum-stellar disks and exoplanets. The METIS project is now in full steam, approaching its preliminary design review (PDR) in 2018. In this paper we will present the current status of its optical, mechanical and thermal design as well as operational aspects. We will also discuss the challenges of building an instrument for the ELT, and the required technologies. © 2018 SPIE
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