71 research outputs found
Gemini Planet Imager Observational Calibrations II: Detector Performance and Calibration
The Gemini Planet Imager is a newly commissioned facility instrument designed
to measure the near-infrared spectra of young extrasolar planets in the solar
neighborhood and obtain imaging polarimetry of circumstellar disks. GPI's
science instrument is an integral field spectrograph that utilizes a HAWAII-2RG
detector with a SIDECAR ASIC readout system. This paper describes the detector
characterization and calibrations performed by the GPI Data Reduction Pipeline
to compensate for effects including bad/hot/cold pixels, persistence,
non-linearity, vibration induced microphonics and correlated read noise.Comment: 11 pages, 6 figures. Proceedings of the SPIE, 9147-28
Radial thresholding to mitigate Laser-Guide-Star aberrations on Centre-of-Gravity-based Shack-Hartmann wavefront sensors
Sodium Laser Guide Stars (LGSs) are elongated sources due to the thickness
and the finite distance of the sodium layer. The fluctuations of the sodium
layer altitude and atom density profile induce errors on centroid measurements
of elongated spots, and generate spurious optical aberrations in closed--loop
adaptive optics (AO) systems. According to an analytical model and experimental
results obtained with the University of Victoria LGS bench demonstrator, one of
the main origins of these aberrations, referred to as LGS aberrations, is not
the Centre-of-Gravity (CoG) algorithm itself, but the thresholding applied on
the pixels of the image prior to computing the spot centroids. A new
thresholding method, termed ``radial thresholding'', is presented here,
cancelling out most of the LGS aberrations without altering the centroid
measurement accuracy.Comment: 8 pages, 9 figures, accepted for publication in MNRA
Feasibility of an Infrared Parallax Program Using the Fan Mountain Tinsley Reflector
Despite the continuing importance of ground-based parallax measurements, few
active programs remain. Because new members of the solar neighborhood tend
towards later spectral types, infrared parallax programs are particularly
desirable. Consequently, the astrometric quality of the new infrared camera,
FanCam, developed by the Virginia Astronomical Instrumentation Laboratory
(VAIL) for the 31-in (0.8-m) Tinsley reflector at Fan Mountain Observatory was
assessed using 68 J-band exposures of an open cluster, NGC 2420, over a range
of hour angles during 2005. Positions of 16 astrometric evaluation stars were
measured and the repeatability of those positions was evaluated using the mean
error in a single observation of unit weight. Overall, a precision of 1.3 +/-
0.7 microns in x (RA) and 1.3 +/- 0.8 microns in y (Dec) was attained, which
corresponds to 0.04" +/- 0.02" in each axis. Although greater precision is
expected from CCDs in the visual and near-infrared, this instrument can achieve
precision similar to that of the ESO NTT infrared parallax program. Therefore,
measuring parallaxes in the infrared would be feasible using this equipment. If
initiated, such a program could provide essential distances for brown dwarfs
and very low mass stars that would contribute significantly to the solar
neighborhood census.Comment: accepted by New Astronomy, minor revisions per refere
Evaluating the GeoSnap 13-m Cut-Off HgCdTe Detector for mid-IR ground-based astronomy
New mid-infrared HgCdTe (MCT) detector arrays developed in collaboration with
Teledyne Imaging Sensors (TIS) have paved the way for improved 10-m
sensors for space- and ground-based observatories. Building on the successful
development of longwave HAWAII-2RGs for space missions such as NEO Surveyor, we
characterize the first 13-m GeoSnap detector manufactured to overcome the
challenges of high background rates inherent in ground-based mid-IR astronomy.
This test device merges the longwave HgCdTe photosensitive material with
Teledyne's 2048x2048 GeoSnap-18 (18-m pixel) focal plane module, which is
equipped with a capacitive transimpedance amplifier (CTIA) readout circuit
paired with an onboard 14-bit analog-to-digital converter (ADC). The final
assembly yields a mid-IR detector with high QE, fast readout (>85 Hz), large
well depth (>1.2 million electrons), and linear readout.
Longwave GeoSnap arrays would ideally be deployed on existing ground-based
telescopes as well as the next generation of extremely large telescopes. While
employing advanced adaptive optics (AO) along with state-of-the-art diffraction
suppression techniques, instruments utilizing these detectors could attain
background- and diffraction-limited imaging at inner working angles <10
, providing improved contrast-limited performance compared to JWST
MIRI while operating at comparable wavelengths. We describe the performance
characteristics of the 13-m GeoSnap array operating between 38-45K,
including quantum efficiency, well depth, linearity, gain, dark current, and
frequency-dependent (1/f) noise profile.Comment: 17 pages, 17 figures. Accepted for publication in special addition of
Astronomische Nachrichten / Astronomical Notes as a contribution to SDW202
The near-infrared size-luminosity relations for Herbig Ae/Be disks
We report the results of a sensitive K-band survey of Herbig Ae/Be disk sizes
using the 85-m baseline Keck Interferometer. Targets were chosen to span the
maximum range of stellar properties to probe the disk size dependence on
luminosity and effective temperature. For most targets, the measured
near-infrared sizes (ranging from 0.2 to 4 AU) support a simple disk model
possessing a central optically-thin (dust-free) cavity, ringed by hot dust
emitting at the expected sublimation temperatures (T_sub~1000-1500K).
Furthermore, we find a tight correlation of disk size with source luminosity R
propto L^(1/2) for Ae and late Be systems (valid over more than 2 decades in
luminosity), confirming earlier suggestions based on lower-quality data.
Interestingly, the inferred dust-free inner cavities of the highest luminosity
sources (Herbig B0-B3 stars) are under-sized compared to predictions of the
optically-thin cavity model, likely due to optically-thick gas within the inner
AU.Comment: Accepted by Astrophysical Journal; 24 pages, 4 figures, 4 table
Keck Interferometer observations of classical and weak line T Tauri stars
We present observations of the T Tauri stars BP Tau, DG Tau, DI Tau, GM Aur,
LkCa 15, RW Aur and V830 Tau, using long baseline infrared interferometry at K
band (2.2 microns) from the Keck Interferometer. The target sources have a
range of mass accretion rates and excess near-infrared emission. The
interferometer is most sensitive to extended emission on characteristic size
scales of 1 to 5 millarcseconds. All sources show evidence for resolved K band
emission on these scales, although a few of the sources are marginally
consistent with being unresolved. We calculate the infrared excess based on
fitting stellar photosphere models to the optical photometry and estimate the
physical size of the emission region using simple geometric models for the
sources with a significant infrared excess. Assuming that the K band resolved
emission traces the inner edge of the dust disk, we compare the measured
characteristic sizes to predicted dust sublimation radii and find that the
models require a range of dust sublimation temperatures and possibly optical
depths within the inner rim to match the measured radii.Comment: 19 pages, 2 figures, to appear in the Astrophysical Journa
JWST Near-Infrared Detector Degradation: Finding the Problem, Fixing the Problem, and Moving Forward
The James Webb Space Telescope (JWST) is the successor to the Hubble Space Telescope. JWST will be an infrared optimized telescope, with an approximately 6.5 m diameter primary mirror, that is located at the Sun-Earth L2 Lagrange point. Three of JWST's four science instruments use Teledyne HgCdTe HAWAII-2RG (H2RG) near infrared detector arrays. During 2010, the JWST Project noticed that a few of its 5 micron cutoff H2RG detectors were degrading during room temperature storage, and NASA chartered a "Detector Degradation Failure Review Board" (DD-FRB) to investigate. The DD-FRB determined that the root cause was a design flaw that allowed indium to interdiffuse with the gold contacts and migrate into the HgCdTe detector layer. Fortunately, Teledyne already had an improved design that eliminated this degradation mechanism. During early 2012, the improved H2RG design was qualified for flight and JWST began making additional H2RGs. In this article we present the two public DD-FRB "Executiye Summaries" that: (1) determined the root cause of the detector degradation and (2) defined tests to determine whether the existing detectors are qualified for flight. We supplement these with a brief introduction to H2RG detector arrays, and a discussion of how the JWST Project is using cryogenic storage to retard the degradation rate of the existing flight spare H2RGs
Interferometer Observations of Subparsec-scale Infrared Emission in the Nucleus of NGC 4151
We report novel, high-angular resolution interferometric measurements that
imply the near-infrared nuclear emission in NGC 4151 is unexpectedly compact.
We have observed the nucleus of NGC 4151 at 2.2 microns using the two 10-meter
Keck telescopes as an interferometer and find a marginally resolved source ~0.1
pc in diameter. Our measurements rule out models in which a majority of the K
band nuclear emission is produced on scales larger than this size. The
interpretation of our measurement most consistent with other observations is
that the emission mainly originates directly in the central accretion disk.
This implies that AGN unification models invoking hot, optically thick dust may
not be applicable to NGC 4151.Comment: 9 pages, 1 figure, accepted for publication in the Astrophysical
Journal, Letter
Observations of DG Tauri with the Keck Interferometer
We present the first science results from the Keck Interferometer, a
direct-detection infrared interferometer utilizing the two 10-meter Keck
telescopes. The instrument and system components are briefly described. We then
present observations of the T Tauri object DG Tau, which is resolved by the
interferometer. The resolved component has a radius of 0.12 to 0.24 AU,
depending on the assumed stellar and extended component fluxes and the model
geometry used. Possible origins and implications of the resolved emission are
discussed.Comment: 10 pages, 2 figures, to appear in ApJ Letter
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