Showing papers in "Publications of the Astronomical Society of the Pacific in 2007"
TL;DR: The Kilodegree Extremely Little Telescope (KELT) project as mentioned in this paper is a survey for planetary transits of bright stars, which consists of a small-aperture, wide-field automated telescope located at Winer Observatory near Sonoita, Arizona.
Abstract: The Kilodegree Extremely Little Telescope (KELT) project is a survey for planetary transits of bright stars. It consists of a small‐aperture, wide‐field automated telescope located at Winer Observatory near Sonoita, Arizona. The telescope surveys a set of 26° × 26° fields that together cover about 25% of the northern sky, and targets stars in the range of 8
439 citations
TL;DR: The development of an FX-style correlator for very long baseline interferometry (VLBI), implemented in software and intended to run in multiprocessor computing environments, such as large clusters of commodity machines (Beowulf clusters) or computers specifically designed for high-performance computing, such ASM shared-memory machines.
Abstract: We describe the development of an FX-style correlator for very long baseline interferometry (VLBI), implemented in software and intended to run in multiprocessor computing environments, such as large clusters of commodity machines (Beowulf clusters) or computers specifically designed for high-performance computing, such as multiprocessor shared-memory machines. We outline the scientific and practical benefits for VLBI correlation, these chiefly being due to the inherent flexibility of software and the fact that the highly parallel and scalable nature of the correlation task is well suited to a multiprocessor computing environment. We suggest scientific applications where such an approach to VLBI correlation is most suited and will give the best returns. We report detailed results from the Distributed FX (DiFX) software correlator running on the Swinburne supercomputer (a Beowulf cluster of ∼300 commodity processors), including measures of the performance of the system. For example, to correlate all Stokes products for a 10 antenna array with an aggregate bandwidth of 64 MHz per station, and using typical time and frequency resolution, currently requires an order of 100 desktop- class compute nodes. Due to the effect of Moore's law on commodity computing performance, the total number and cost of compute nodes required to meet a given correlation task continues to decrease rapidly with time. We show detailed comparisons between DiFX and two existing hardware-based correlators: the Australian Long Baseline Array S2 correlator and the NRAO Very Long Baseline Array correlator. In both cases, excellent agreement was found between the correlators. Finally, we describe plans for the future operation of DiFX on the Swinburne supercomputer for both astrophysical and geodetic science.
379 citations
TL;DR: In this paper, the stellar calibrator sample and the conversion from instrumental to physical units for the 24 μm channel of the MIPS were presented, and the primary calibrators are A stars.
Abstract: We present the stellar calibrator sample and the conversion from instrumental to physical units for the 24 μm channel of the Multiband Imaging Photometer for Spitzer (MIPS). The primary calibrators are A stars, and the calibration factor based on those stars is MJy sr^−1 (DN s^−1)^−1, with a nominal uncertainty of 2%. We discuss the data reduction procedures required to attain this accuracy; without these procedures, the calibration factor obtained using the automated pipeline at the Spitzer Science Center is lower. We extend this work to predict 24 μm flux densities for a sample of 238 stars that covers a larger range of flux densities and spectral types. We present a total of 348 measurements of 141 stars at 24 μm. This sample covers a factor of 460 in 24 μm flux density, from 8.6 mJy up to 4.0 Jy. We show that the calibration is linear over that range with respect to target flux and background level. The calibration is based on observations made using 3 s exposures; a preliminary analysis shows that the calibration factor may be 1% and 2% lower for 10 and 30 s exposures, respectively. We also demonstrate that the calibration is very stable: over the course of the mission, repeated measurements of our routine calibrator, HD 159330, show a rms scatter of only 0.4%. Finally, we show that the point-spread function (PSF) is well measured and allows us to calibrate extended sources accurately; Infrared Astronomy Satellite (IRAS) and MIPS measurements of a sample of nearby galaxies are identical within the uncertainties.
344 citations
TL;DR: In this paper, a method of reconstructing full spectral cubes, with two spatial and one spectral dimension, from rastered spectral mapping observations employing a single slit in a traditional slit spectrograph is described.
Abstract: Three‐dimensional spectroscopy of extended sources is typically performed with dedicated integral field spectrographs. We describe a method of reconstructing full spectral cubes, with two spatial and one spectral dimension, from rastered spectral mapping observations employing a single slit in a traditional slit spectrograph. When the background and image characteristics are stable, as is often achieved in space, the use of traditional long slits for integral field spectroscopy can substantially reduce instrument complexity over dedicated integral field designs, without loss of mapping efficiency—particularly compelling when a long‐slit mode for single unresolved source follow‐up is separately required. We detail a custom flux‐conserving cube reconstruction algorithm, discuss issues of extended‐source flux calibration, and describe CUBISM, a tool that implements these methods for spectral maps obtained with the Spitzer Space Telescope’s Infrared Spectrograph.
282 citations
Carnegie Institution for Science1, University of California, Berkeley2, University of Chicago3, Fermilab4, Max Planck Society5, Ohio State University6, University of Arizona7, University of Chile8, Texas A&M University9, Stanford University10, California Institute of Technology11, University of Notre Dame12, University of Cape Town13, University of Washington14, New Mexico State University15, Space Telescope Science Institute16, University of Pennsylvania17, University of Tokyo18
TL;DR: In this article, photometry and optical spectroscopy of the Type Ia supernova (SN) 2005hk was performed and the spectra revealed that SN 2005 hk was nearly identical in its observed properties to SN 2002cx, which has been called the most peculiar known Type Iaa supernova and exhibited high-ionization SN 1991T-like premaximum spectra, yet low peak luminosities like that of SN 1991bg.
Abstract: We present extensive \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc}
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ormalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $u^{\prime }g^{\prime }r^{\prime }i^{\prime }BVRIYJHK_{s}$ \end{document} photometry and optical spectroscopy of the Type Ia supernova (SN) 2005hk. These data reveal that SN 2005hk was nearly identical in its observed properties to SN 2002cx, which has been called “the most peculiar known Type Ia supernova.” Both supernovae exhibited high‐ionization SN 1991T–like premaximum spectra, yet low peak luminosities like that of SN 1991bg. The spectra reveal th...
249 citations
TL;DR: In this paper, the absolute calibration and characterization of the MIPS 70 μm coarse and fine-scale imaging modes are presented based on over 2.5 years of observations, and the point-spread function (PSF) fitting photometry is found to give more accurate flux densities than aperture photometry.
Abstract: The absolute calibration and characterization of the Multiband Imaging Photometer for Spitzer (MIPS) 70 μm coarse‐and fine‐scale imaging modes are presented based on over 2.5 yr of observations. Accurate photometry (especially for faint sources) requires two simple processing steps beyond the standard data reduction to remove long‐term detector transients. Point‐spread function (PSF) fitting photometry is found to give more accurate flux densities than aperture photometry. Based on the PSF fitting photometry, the calibration factor shows no strong trend with flux density, background, spectral type, exposure time, or time since anneals. The coarse‐scale calibration sample includes observations of stars with flux densities from 22 mJy to 17 Jy, on backgrounds from 4 to 26 MJy sr^(−1), and with spectral types from B to M. The coarse‐scale calibration is 702 ± 35 MJy sr^(−1) MIPS70^(−1) (5% uncertainty) and is based on measurements of 66 stars. The instrumental units of the MIPS 70 μm coarse‐ and fine‐scale imaging modes are called MIPS70 and MIPS70F, respectively. The photometric repeatability is calculated to be 4.5% from two stars measured during every MIPS campaign and includes variations on all timescales probed. The preliminary fine‐scale calibration factor is 2894 ± 294 MJy sr^(−1) MIPS70F^(−1) (10% uncertainty) based on 10 stars. The uncertainties in the coarse‐ and fine‐scale calibration factors are dominated by the 4.5% photometric repeatability and the small sample size, respectively. The 5 σ, 500 s sensitivity of the coarse‐scale observations is 6–8 mJy. This work shows that the MIPS 70 μm array produces accurate, well‐calibrated photometry and validates the MIPS 70 μm operating strategy, especially the use of frequent stimulator flashes to track the changing responsivities of the Ge:Ga detectors.
225 citations
TL;DR: In this article, the TFIT software package is used to measure galaxy photometry using prior information from high-resolution observations, which is similar in principle but different in detail from previous procedures for crowded field photometry.
Abstract: We describe the TFIT software package to measure galaxy photometry using prior information from high‐resolution observations. Our basic methodology is similar in principle but different in detail from previous procedures for crowded field photometry. We use the spatial positions and morphologies of objects in an image with higher angular resolution to construct object templates, which are then fitted to a lower resolution image, solving for the object fluxes as free parameters. Using extensive experiments on both simulated and real data, we show that this template‐fitting method measures accurate object photometry to the limiting sensitivity of the image. In this limit, our method derives robust flux upper limits for objects fainter than the limiting image surface brightness. We describe the challenges encountered in applying this technique to real data, and methods to cope with some of them.
210 citations
TL;DR: The SCUBA-2 polarimeter (POL-2) polarimeter was used in this paper to obtain the first statistically significant set of polarization maps in the submillimeter.
Abstract: This paper describes a James Clerk Maxwell Telescope (JCMT) legacy survey that has been awarded roughly 500 hr of observing time to be carried out from 2007 to 2009. In this survey, we will map with SCUBA-2 (Submillimetre Common-User Bolometer Array 2) almost all of the well-known low-mass and intermediate-mass star-forming regions within 0.5 kpc that are accessible from the JCMT. Most of these locations are associated with the Gould Belt. From these observations, we will produce a flux-limited snapshot of star formation near the Sun, providing a legacy of images, as well as point-source and extended-source catalogs, over almost 700 deg(2) of sky. The resulting images will yield the first catalog of prestellar and protostellar sources selected by submillimeter continuum emission, and should increase the number of known sources by more than an order of magnitude. We will also obtain with the array receiver HARP (Heterodyne Array Receiver Program) CO maps, in three CO isotopologues, of a large typical sample of prestellar and protostellar sources. We will then map the brightest hundred sources with the SCUBA-2 polarimeter (POL-2), producing the first statistically significant set of polarization maps in the submillimeter. The images and source catalogs will be a powerful reference set for astronomers, providing a detailed legacy archive for future telescopes, including ALMA, Herschel, and JWST.
188 citations
TL;DR: In this paper, the stellar parameters for the M dwarf GJ 436, which hosts a Neptune-mass planet, were determined at low and high resolution, examining the agreement between model and observed optical spectra.
Abstract: We determine stellar parameters for the M dwarf GJ 436, which hosts a Neptune-mass planet. We employ primarily spectral modeling at low and high resolution, examining the agreement between model and observed optical spectra of five comparison stars of type M0-M3. The modeling of high-resolution optical spectra suffers from uncertainties in TiO transitions, affecting the predicted strengths of both atomic and molecular lines in M dwarfs. The determination of Teff, gravity, and metallicity from optical spectra remains at ~10%. As molecules provide opacity both in lines and as an effective continuum, determining molecular transition parameters remains a challenge facing models such as the PHOENIX series, best verified with high resolution and spectrophotometric spectra. Our analysis of GJ 436 yields an effective temperature of Teff = 3350 ± 300 K and a mass of 0.44 M⊙. New Doppler measurements of GJ 436 with a precision of 3 m s–1 taken during 6 years improve the Keplerian model of the planet, giving it a minimum mass M sin i = 0.0713 MJup = 22.6 M⊕, period P = 2.6439 days, and eccentricity e = 0.16 ± 0.02. The noncircular orbit contrasts with the tidally circularized orbits of all close-in exoplanets, implying either ongoing pumping of eccentricity by a more distant companion, or a higher Q value for this low-mass planet. The velocities indeed reveal a long-term trend, indicating a possible distant companion.
158 citations
TL;DR: The methodology of the HET queue is reviewed, along with its strengths and weaknesses, to show how the flexibility of the queue and the simultaneous availability of three instruments have allowed optimum use to be made of variable weather conditions.
Abstract: This paper presents a summary of the first 10 years of operating the Hobby‐Eberly Telescope (HET) in queue mode. The scheduling can be quite complex but has worked effectively for obtaining the most science possible with this uniquely designed telescope. The queue must handle dozens of separate scientific programs, the involvement of a number of institutions with individual Telescope Allocation Committees, as well as engineering and instrument commissioning. We have continuously revised our queue operations as we have learned from experience. The flexibility of the queue and the simultaneous availability of three instruments, along with a staff trained for all aspects of telescope and instrumentation operation, have allowed optimum use to be made of variable weather conditions and have proven to be especially effective at accommodating targets of opportunity and engineering tasks. In this paper, we review the methodology of the HET queue, along with its strengths and weaknesses.
155 citations
TL;DR: In this paper, the authors describe the absolute calibration of the MIPS 160 μm channel and derive a calibration factor of 41.7 MJy sr^(−1) MIPS160 + 1 (MIPS160 being the instrumental units).
Abstract: We describe the absolute calibration of the Multiband Imaging Photometer for Spitzer (MIPS) 160 μm channel. After the on‐orbit discovery of a near‐IR ghost image that dominates the signal for sources hotter than about 2000 K, we adopted a strategy utilizing asteroids to transfer the absolute calibrations of the MIPS 24 and 70 μm channels to the 160 μm channel. Near‐simultaneous observations at all three wavelengths are taken, and photometry at the two shorter wavelengths is fit using the standard thermal model. The 160 μm flux density is predicted from those fits and compared with the observed 160 μm signal to derive the conversion from instrumental units to surface brightness. The calibration factor we derive is 41.7 MJy sr^(−1) MIPS160^(−1) (MIPS160 being the instrumental units). The scatter in the individual measurements of the calibration factor, as well as an assessment of the external uncertainties inherent in the calibration, lead us to adopt an uncertainty of 5.0 MJy sr^(−1) MIPS160^(−1) (12%) for the absolute uncertainty on the 160 μm flux density of a particular source as determined from a single measurement. For sources brighter than about 2 Jy, nonlinearity in the response of the 160 μm detectors produces an underestimate of the flux density: for objects as bright as 4 Jy, measured flux densities are likely to be ≃20% too low. This calibration has been checked against that of the ISO (using ULIRGs) and IRAS (using IRAS‐derived diameters), and is consistent with those at the 5% level.
Los Alamos National Laboratory1, University of Arizona2, Max Planck Society3, University of California, Santa Cruz4, Princeton University5, Carnegie Institution for Science6, Massachusetts Institute of Technology7, University of Amsterdam8, University of Oxford9, University of Tokyo10, University of Virginia11, Australian National University12, California Institute of Technology13
TL;DR: In this paper, the authors review the current set of progenitor scenarios for long-duration gamma-ray bursts and the observational constraints on these scenarios and find that single stars cannot be the only progenitors for longduration GRBs.
Abstract: Although there is strong support for the collapsar engine as the power source of long-duration gamma-ray bursts (GRBs), we still do not definitively know the progenitor of these explosions. Here we review the current set of progenitor scenarios for long-duration GRBs and the observational constraints on these scenarios. Examining these models, we find that single stars cannot be the only progenitor for long-duration GRBs. Several binary progenitors can match the solid observational constraints and also have the potential to match the trends that we are currently seeing in the observations. Type Ib/c supernovae are also likely to be produced primarily in binaries; we discuss the relationship between the progenitors of these explosions and those of the long-duration GRBs.
TL;DR: In this article, detailed abundances of a variety of elements between the halo and all dwarf galaxies studied to date, including both dwarf spheroidals and irregulars, were compared.
Abstract: We review Galactic halo formation theories and supporting evidence, in particular, kinematics and detailed chemical abundances of stars in some relevant globular clusters as well as Local Group dwarf galaxies. Outer halo red HB clusters tend to have large eccentricities and inhabit the area of the Lee diagram populated by dwarf spheroidal stars, favoring an extragalactic origin. Old globular clusters show the full range of eccentricities, while younger ones seem to have preferentially high eccentricities, again hinting at their extragalactic origin. However, the three outer halo second parameter clusters with well-determined orbits indicate they come from three independent systems. We compare detailed abundances of a variety of elements between the halo and all dwarf galaxies studied to date, including both dwarf spheroidals and irregulars. The salient feature is that halo abundances are essentially unique. In particular, the general α vs. [Fe/H] pattern of 12 of the 13 galaxies studied are similar to each other and very different from the Milky Way. Sgr appears to be the only possible exception. At the metal-poor end the extragalactic sample is only slightly deficient compared to the halo but begins to diverge by [Fe/H] ~ –2 and the difference is particularly striking for stars with [Fe/H] ~ –1. Only Sgr, the most massive dSph, has some stars similar in α-abundance to Galactic stars at intermediate metallicities, even the most extreme low-α subset most likely to have been accreted. It appears very unlikely that a significant fraction of the metal-rich halo could have come from disrupted dSphs of low mass. However, at least some of the metal-poor halo may have come from typical dSphs, and a portion of the intermediate metallicity and metal-rich halo may have come from very massive systems like Sgr. This argues against the standard hierarchical galaxy formation scenario and the Searle-Zinn paradigm for the formation of the Galactic halo via accretion of "fragments" composed of stars like those we see in typical present-day dSphs. The chemical differences between the dwarfs and the halo are due to a combination of a low star formation efficiency and a high galactic wind efficiency in the former. AGB stars are also more important in the chemical evolution of the dwarfs. The formation problem may be solved if the majority of halo stars formed within a few, very massive satellites accreted very early. However, any such satellites must either be accreted much earlier than postulated, before the onset of SNe Ia, or star formation must be prevented to occur in them until only shortly before they are accreted. The intrinsic scatter in many elements, particularly the α-elements, indicates that the halo was also mixed on a surprisingly short timescale, a further problem for hierarchical formation theories.
TL;DR: In this article, the authors describe the time and position-dependent point spread function (PSF) variation of the wide field channel (WFC) of the Advanced Camera for Surveys (ACS) with the principal component analysis (PCA) technique.
Abstract: . We describe the time- and position-dependent point-spread function (PSF) variation of the wide-field channel (WFC) of the Advanced Camera for Surveys (ACS) with the principal component analysis (PCA) technique. The time-dependent change is caused by the temporal variation of the HST focus, whereas the position-dependent PSF variation in ACS WFC at a given focus is mainly the result of changes in aberrations and charge diffusion across the detector, which appear as position-dependent changes in the elongation of the astigmatic core and blurring of the PSF, respectively. Using >400 > 400 archival images of star cluster fields, we construct an ACS PSF library covering diverse environments of the HST observations (e.g., focus values). We find that interpolation of a small number (20 20 ) of principal components or eigen-PSFs per exposure can robustly reproduce the observed variation of the ellipticity and size of the PSF. Our primary interest in this investigation is the application of this PSF libr...
TL;DR: In this paper, it was shown that transiting planets with eccentric orbits are more likely to transit than circularly orbiting planets with the same semimajor axis by a factor of 1.
Abstract: It is shown herein that planets with eccentric orbits are more likely to transit than circularly orbiting planets with the same semimajor axis by a factor of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc}
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ormalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $( 1-e^{2}) ^{-1}$ \end{document} . If the orbital parameters of discovered transiting planets are known, as from follow‐up radial velocity observations, then the transit‐detected planet population is easily debiased of this effect. The duration of a planet’s transit depends on its eccentricity and longitude of periast...
TL;DR: The entire FUSE data set has recently been reprocessed with CalFUSE version 3.2, the latest version of this software, and the format of the resulting calibrated data files are described.
Abstract: Since its launch in 1999, the Far Ultraviolet Spectroscopic Explorer (FUSE) has made over 4900 observations of some 2500 individual targets. The data are reduced by the principal investigator team at the Johns Hopkins University and archived at the Multimission Archive at STScI (MAST). The data reduction software package, called CalFUSE, has evolved considerably over the lifetime of the mission. The entire FUSE data set has recently been reprocessed with CalFUSE version 3.2, the latest version of this software. This paper describes CalFUSE version 3.2, the instrument calibrations on which it is based, and the format of the resulting calibrated data files.
TL;DR: In this paper, a new polarimeter installed on the high-resolution fiber-fed echelle spectrograph (called BOES) of the 1.8 m telescope at the Bohyunsan Optical Astronomy Observatory, Korea is presented.
Abstract: We introduce a new polarimeter installed on the high-resolution fiber-fed echelle spectrograph (called BOES) of the 1.8 m telescope at the Bohyunsan Optical Astronomy Observatory, Korea. The instrument is intended to measure stellar magnetic fields with high-resolution ( ) spectropolarimetric observations R ∼ 60,000 of intrinsic polarization in spectral lines. In this paper we describe the spectropolarimeter and present test observations of the longitudinal magnetic fields in some well-studied F-B main-sequence magnetic stars (m ! v ). The results demonstrate that the instrument is able to detect the fields of these stars with high precision, 8.8 mag with typical accuracies ranging from about 2 to a few tens of gauss.
TL;DR: In this paper, the authors present a model of how noise scales with the number of multiple nondestructive reads sampling up the ramp of the James Webb Space Telescope (JWST).
Abstract: We describe how the James Webb Space Telescope (JWST) Near-Infrared Spectrograph's (NIRSpec) detectors will be read out, and present a model of how noise scales with the number of multiple nondestructive reads sampling up the ramp. We believe that this noise model, which is validated using real and simulated test data, is applicable to most astronomical near-infrared instruments. We describe some nonideal behaviors that have been observed in engineering-grade NIRSpec detectors, and demonstrate that they are unlikely to affect NIRSpec sensitivity, operations, or calibration. These include a HAWAII-2RG reset anomaly and random telegraph noise (RTN). Using real test data, we show that the reset anomaly is (1) very nearly noiseless and (2) can be easily calibrated out. Likewise, we show that large-amplitude RTN affects only a small and fixed population of pixels. It can therefore be tracked using standard pixel operability maps.
TL;DR: In this paper, an echelle spectrograph for an automatic telescope at Tennessee State University is used to monitor radial velocities and line profiles of cool giant and supergiant stars.
Abstract: This paper discusses data reduction for an echelle spectrograph we have developed for an automatic telescope at Tennessee State University and are using to monitor radial velocities and line profiles of cool giant and supergiant stars. Although our approach to data reduction is rather conventional, we discuss flat-fielding and extraction of velocities in ways that should be of general interest, establish a transformation to the IAU radial velocity system (+0.35 ± 0.09 km s–1), and determine the external precision for measured velocities (0.10-0.11 km s–1). Also, we present results of the first 2-3 years of monitoring radial velocities in about 120 cool giants and compare those results with the level of variability found with photometry. These new data confirm the widely held understanding that K and M giants are all radial velocity variables at the level of 0.1 km s–1.
TL;DR: In this paper, the authors present a characterization of the night-sky at the Calar Alto observatory for the time period between 2004 and 2007, using optical spectrophotometric data, photometric calibrated images taken in moonless observing periods, together with the observing conditions regularly monitored at the observatory.
Abstract: We present a characterization of the main properties of the night-sky at the Calar Alto observatory for the time period between 2004 and 2007. We use optical spectrophotometric data, photometric calibrated images taken in moonless observing periods, together with the observing conditions regularly monitored at the observatory, such as atmospheric extinction and seeing. We derive, for the first time, the typical moonless night-sky optical spectrum for the observatory. The spectrum shows a strong contamination by different pollution lines, in particular from Mercury lines, which contribution to the sky-brightness in the different bands is of the order of �0.09 mag, �0.16 mag and �0.10 mag in B, V and R respectively. Regarding the strength of the Sodium pollution line in comparison with the airglow emission, the observatory does not fulfill the IAU recomendations of a dark site. The zenith-corrected values of the moonless night-sky surface brightness are 22.39, 22.86, 22.01, 21.36 and 19.25 mag arcsec −2 in U, B, V , R and I, which indicates that Calar Alto is a particularly dark site for optical observations up to the I-band. The fraction of astronomical useful nights at the observatory
TL;DR: Mimir as mentioned in this paper is a near-infrared instrument for the 1.8 m Perkins telescope on Anderson Mesa outside Flagstaff, Arizona, which has been operating for three years, performing wide-field and narrow-field imaging, long-slit spectroscopy, and imaging polarimetry.
Abstract: Mimir, a new facility-class near-infrared instrument for the 1.8 m Perkins telescope on Anderson Mesa outside Flagstaff, Arizona, was commissioned and has been operating for three years. Mimir is multifunction, performing wide-field (F=5) and narrow-field (F=17) imaging, long-slit spectroscopy, and imaging polarimetry. The F=5 mode images at 0.59" per pixel onto the 1024 × 1024 pixel ALADDIN III InSb array detector, giving a 10 0 × 10 0 field of view. In the F=17 mode, the plate scale is 0.18" per pixel. Optically, Mimir is a refractive reimager for the F=17:5 Perkins beam. A six-lens collimator produces an achromatic 25 mm pupil, which is imaged by a five-lens camera (F=5), a four-lens camera (F=17), or a two-lens pupil viewer onto the detector. Three filter wheels precede the pupil, one follows the pupil. The wheels contain a rotating half-wave plate, broadband filters, narrowband filters, grisms, long-pass filters, a wire grid, and thermal IR blockers. The first telescope focus is within Mimir, where a slit and decker unit, consisting of two linear motion cars, selects one of 13 slit scenes. The slit and decker cars, the four filter wheels, the half-wave plate rotation, and the camera selector are all driven by stepper motors within the cold vacuum space. Cooling is provided by a CTI 1050 two-stage, closed-cycle helium refrigerator, keeping the optics, filters, and internal surfaces between 65 and 75 K and the detector at 33.5 K. Switching between Mimir's different modes takes only a few seconds, making it a versatile tool for conducting a wide range of investigations and for quickly reacting to changing observing conditions. Mimir on the Perkins telescope achieves imaging sensitivities 2-4 mag deeper than 2MASS, moderate resolution (R ∼ 700) JHK spectra of virtually any 2MASS source, high- precision wide-field imaging polarimetry, and L 0 and M 0 band imaging and spectroscopy.
TL;DR: The ability of the algorithm to discriminate RFI against the temporally and spectrally complex radio emission produced during solar radio bursts is demonstrated.
Abstract: A radio frequency interference (RFI) excision algorithm based on spectral kurtosis, a spectral variant of time‐domain kurtosis, is proposed and implemented in software. The algorithm works by providing a robust estimator for Gaussian noise that, when violated, indicates the presence of non‐Gaussian RFI. A theoretical formalism is used that unifies the well‐known time‐domain kurtosis estimator with past work related to spectral kurtosis, and leads naturally to a single expression encompassing both. The algorithm accumulates the first two powers of M power spectral density (PSD) estimates, obtained via Fourier transform, to form a spectral kurtosis (SK) estimator whose expected statistical variance is used to define an RFI detection threshold. The performance of the algorithm is theoretically evaluated for different time‐domain RFI characteristics and signal‐to‐noise ratios η. The theoretical performance of the algorithm for intermittent RFI (RFI present in R out of M PSD estimates) is evaluated and shown to depend greatly on the duty cycle, d = R/M. The algorithm is most effective for d = 1/(4 + η), but cannot distinguish RFI from Gaussian noise at any η when d = 0.5. The expected efficiency and robustness of the algorithm are tested using data from the newly designed FASR Subsystem Testbed radio interferometer operating at the Owens Valley Solar Array. The ability of the algorithm to discriminate RFI against the temporally and spectrally complex radio emission produced during solar radio bursts is demonstrated.
TL;DR: In this article, a system for rapidly measuring the brightness of the night sky using a mosaic of CCD images obtained with a low-cost automated system is described, enabling the detailed characterization of natural sky conditions and light domes produced by cities.
Abstract: : We describe a system for rapidly measuring the brightness of the night sky using a mosaic of CCD images obtained with a low-cost automated system. The portable system produces millions of independent photometric measurements covering the entire sky, enabling the detailed characterization of natural sky conditions and light domes produced by cities. The measurements are calibrated using images of standard stars contained within the raw data, producing results closely tracking the Johnson V astronomical standard. The National Park Service has collected hundreds of data sets at numerous parks since 2001 and is using these data for the protection and monitoring of the night-sky visual resource. This system also allows comprehensive characterization of sky conditions at astronomical observatories. We explore photometric issues raised by the broadband measurement of the complex and variable night-sky spectrum, and potential indices of night-sky quality.
TL;DR: In this paper, the authors report the discovery of optical ghosts generated when using Volume Phase Holographic (VPH) gratings in spectrographs employing the Littrow configuration.
Abstract: We report the discovery of optical ghosts generated when using Volume Phase Holographic (VPH) gratings in spectrographs employing the Littrow configuration. The ghost is caused by light reflected off the detector surface, recollimated by the camera, recombined by, and reflected from, the grating, and reimaged by the camera onto the detector. This recombination can occur in two different ways. We observe this ghost in two spectrographs being developed by the University of Wisconsin-Madison: the Robert Stobie Spectrograph for the Southern African Large Telescope, and the Bench Spectrograph for the WIYN 3.5 m telescope. The typical ratio of the brightness of the ghost relative to the integrated flux of the spectrum is of order 10 4 , implying a recombination efficiency of the VPH gratings of order 10 3 or higher, consistent with the output of rigorous coupled wave analysis. Any spectrograph employing VPH gratings, including grisms, in Littrow configuration will suffer from this ghost, although the general effect is not intrinsic to VPH gratings themselves and has been observed in systems with conventional gratings in non-Littrow configurations. We explain the geometric configurations that can result in the ghost, as well as a more general prescription for predicting its position and brightness on the detector. We make recommendations for mitigating the ghost effects for spectrographs and gratings currently built. We further suggest design modifications for future VPH gratings to eliminate the problem entirely, including tilted fringes and/or prismatic substrates. We discuss the resulting implications for the spectrograph performance metrics.
TL;DR: In this paper, the physical processes that attenuate the passage of light through Earth's atmosphere are reviewed and compared, including photometry, spectroscopy, and LIDAR.
Abstract: Motivated by the recognition that variation in the optical transmission of the atmosphere is probably the main limitation to the precision of ground‐based CCD measurements of celestial fluxes, we review the physical processes that attenuate the passage of light through Earth’s atmosphere. The next generation of astronomical surveys, such as PanSTARRS and LSST, will greatly benefit from dedicated apparatus to obtain atmospheric transmission data that can be associated with each survey image. We review and compare various approaches to this measurement problem, including photometry, spectroscopy, and LIDAR. In conjunction with careful measurements of instrumental throughput, atmospheric transmission measurements should allow next‐generation imaging surveys to produce photometry of unprecedented precision. Our primary concerns are the real‐time determination of aerosol scattering and absorption by water along the line of sight, both of which can vary over the course of a night’s observations.
TL;DR: In this paper, the authors designed, constructed, and put into operation a very large area CCD camera that covers the field of view of the 1.2 m Schmidt Telescope at the Palomar Observatory.
Abstract: We have designed, constructed, and put into operation a very large area CCD camera that covers the field of view of the 1.2 m Samuel Oschin Schmidt Telescope at the Palomar Observatory. The camera consists of 112 CCDs arranged in a mosaic of four rows with 28 CCDs each. The CCDs are pixel Sarnoff 600 # 2400 thinned, back-illuminated devices with pixels. The camera covers an area of on the
TL;DR: In 2006, the Hubble Heritage Team obtained a large, four-filter (B, V, I, and Hα), 6 point mosaic data set of the prototypical starburst galaxy NGC 3034 (M82) with the Advanced Camera for Surveys on board the Hubble Space Telescope (HST). The resulting color composite Heritage image was released in 2006 April to celebrate HST's 16th anniversary.
Abstract: In 2006 March, the Hubble Heritage Team obtained a large, four‐filter (B, V, I, and Hα), 6 point mosaic data set of the prototypical starburst galaxy NGC 3034 (M82) with the Advanced Camera for Surveys on board the Hubble Space Telescope (HST). The resulting color composite Heritage image was released in 2006 April to celebrate HST’s 16th anniversary. Cycle 15 HST proposers were encouraged to submit General Observer and Archival Research proposals to complement and/or analyze this unique data set. Since our M82 mosaics represent a significant investment of expert processing beyond the standard archival products, we also released our drizzle‐combined FITS data as a High‐Level Science Product via the Multimission Archive at STScI in 2006 December. This paper documents the key aspects of the observing program and image processing: calibration, image registration and combination (drizzling), and the rejection of cosmic rays and detector artifacts.
TL;DR: In this paper, the authors show that the observation of the proper motion of Sgr A* vSgr A * = 0.4 ± 0.9 km s−1, and that future measurements of the periastron shifts for at least three stars may adequately determine the cluster parameters and the mass of the black hole.
Abstract: Several stars orbit around a black hole candidate of mass 3.7 × 106 M⊙ in the region of the Galactic center. Looking for general relativistic (GR) periastron shifts is limited by the existence of a stellar cluster around the black hole, which modifies the cluster orbits due to classical effects that might mask the GR effect. Only if one knows the cluster parameters (its mass and core radius) it is possible to unequivocally deduce the expected GR effects and then test them. In this paper, we show that the observation of the proper motion of Sgr A* vSgr A* = 0.4 ± 0.9 km s–1; Reid & Bruthaler) could help us to constrain the cluster parameters significantly, and that future measurements of the periastron shifts for at least three stars may adequately determine the cluster parameters and the mass of the black hole.
TL;DR: In this paper, a near-infrared spot projection system has been developed, and the effect of subpixel non-uniformity was studied, showing that for detectors with near 100% internal quantum efficiency, 1% photometry can be achieved with a point-spread function (PSF) size of about half a pixel.
Abstract: Wide‐field survey instruments are used to efficiently observe extended regions of the sky. To achieve a large field of view and to provide a high signal‐to‐noise ratio for faint sources, many modern instruments are undersampled. However, in undersampled detectors, sensitivity variations across individual pixels can severely impact science programs that require high photometric precision. To address this, a near‐infrared spot projection system has been developed. With this system, 1.7 μm cutoff detectors were characterized, and the effect of subpixel nonuniformity was studied. The measurements demonstrate that for detectors with near 100% internal quantum efficiency, 1% photometry can be achieved with a point‐spread function (PSF) size of about half a pixel. For detectors with large subpixel nonuniformity, photometric errors become negligible only if the PSF size is more than about two pixels.
TL;DR: In this article, a line-by-line radiative transfer model for the Earth's atmosphere was proposed to estimate the spectrum of the Earth in the near-IR spectral region.
Abstract: Astronomical spectra taken with ground-based telescopes in the near-IR spectral region are affected by strong absorptions due to molecules in the Earth's atmosphere, particularly CO2 and H2O. These features need to be removed in order to reveal the true spectrum of the object being observed. The traditional technique for doing this is to observe a standard star with a relatively featureless spectrum (a smooth spectrum standard) and divide the observed spectrum of the object by that of the standard. This technique has a number of practical difficulties but is also fundamentally flawed. Even with a perfectly smooth spectrum standard observed at an identical air mass to the object, this technique does not, in general, reproduce the spectrum of the object that would have been observed in the absence of the Earth's atmosphere. The problem arises because of the presence of high-resolution structure in the molecular absorption features, generally on a finer scale than the resolution of the observed spectrum. The transmission of the Earth's atmosphere for any spectral bin then depends on the unresolved spectral structure of the light being transmitted and cannot be represented by a unique value derived from the standard star, as the traditional technique assumes. We use high-resolution line-by-line radiative transfer models to quantify these effects for two cases: observations of a solar-type star, and observations of the planet Mars. For a solar-type star, application of the traditional technique causes errors of a few percent in the vicinity of strong atmospheric absorption features. The case of the planet Mars is an extreme case, since Mars has the same CO2 absorption features as the Earth's atmosphere. Our simulations show that applying the traditional astronomical technique to ground-based spectra of the planet Mars leads to systematic errors of up to ~50% in the vicinity of the strong CO2 absorption features. High-quality IR spectroscopy with ground-based telescopes requires an improved technique to handle the absorption in the Earth's atmosphere. We outline a possible approach based on the use of radiative transfer models for the Earth's atmosphere. We note that in general it is not possible to correct observed spectra for atmospheric absorption. However, a forward-modeling approach can be used in which a model spectrum for the object is generated, atmospheric transmission effects added, and the result compared with the observed spectrum. We present a demonstration of the ability of a model to accurately represent the Earth atmosphere transmission in the J band.