179 research outputs found
A Calibrated Measurement of the Near-IR Continuum Sky Brightness Using Magellan/FIRE
We characterize the near-IR sky background from 308 observations with the
FIRE spectrograph at Magellan. A subset of 105 observations selected to
minimize lunar and thermal effects gives a continuous, median spectrum from
0.83 to 2.5 microns which we present in electronic form. The data are used to
characterize the broadband continuum emission between atmospheric OH features
and correlate its properties with observing conditions such as lunar angle and
time of night. We find that the moon contributes significantly to the
inter-line continuum in the Y and J bands whereas the observed H band continuum
is dominated by the blended Lorentzian wings of multiple OH line profiles even
at R=6000. Lunar effects may be mitigated in Y and J through careful scheduling
of observations, but the most ambitious near-IR programs will benefit from
allocation during dark observing time if those observations are not limited by
read noise. In Y and J our measured continuum exceeds space-based average
estimates of the Zodiacal light, but it is not readily identified with known
terrestrial foregrounds. If further measurements confirm such a fundamental
background, it would impact requirements for OH-suppressed instruments
operating in this regime.Comment: 25 pages, 11 figures, accepted to PAS
Precision of a Low-Cost InGaAs Detector for Near Infrared Photometry
We have designed, constructed, and tested an InGaAs near-infrared camera to
explore whether low-cost detectors can make small (<1 m) telescopes capable of
precise (<1 mmag) infrared photometry of relatively bright targets. The camera
is constructed around the 640x512 pixel APS640C sensor built by FLIR
Electro-Optical Components. We designed custom analog-to-digital electronics
for maximum stability and minimum noise. The InGaAs dark current halves with
every 7 deg C of cooling, and we reduce it to 840 e-/s/pixel (with a
pixel-to-pixel variation of +/-200 e-/s/pixel) by cooling the array to -20 deg
C. Beyond this point, glow from the readout dominates. The single-sample read
noise of 149 e- is reduced to 54 e- through up-the-ramp sampling. Laboratory
testing with a star field generated by a lenslet array shows that 2-star
differential photometry is possible to a precision of 631 +/-205 ppm (0.68
mmag) hr^-0.5 at a flux of 2.4E4 e-/s. Employing three comparison stars and
de-correlating reference signals further improves the precision to 483 +/-161
ppm (0.52 mmag) hr^-0.5. Photometric observations of HD80606 and HD80607 (J=7.7
and 7.8) in the Y band shows that differential photometry to a precision of 415
ppm (0.45 mmag) hr^-0.5 is achieved with an effective telescope aperture of
0.25 m. Next-generation InGaAs detectors should indeed enable Poisson-limited
photometry of brighter dwarfs with particular advantage for late-M and L types.
In addition, one might acquire near-infrared photometry simultaneously with
optical photometry or radial velocity measurements to maximize the return of
exoplanet searches with small telescopes.Comment: Accepted to PAS
Near-Infrared InGaAs Detectors for Background-limited Imaging and Photometry
Originally designed for night-vision equipment, InGaAs detectors are
beginning to achieve background-limited performance in broadband imaging from
the ground. The lower cost of these detectors can enable multi-band
instruments, arrays of small telescopes, and large focal planes that would be
uneconomical with high-performance HgCdTe detectors. We developed a camera to
operate the FLIR AP1121 sensor using deep thermoelectric cooling and
up-the-ramp sampling to minimize noise. We measured a dark current of 163-
s pix, a read noise of 87- up-the-ramp, and a well depth of
80k-. Laboratory photometric testing achieved a stability of 230 ppm
hr, which would be required for detecting exoplanet transits. InGaAs
detectors are also applicable to other branches of near-infrared time-domain
astronomy, ranging from brown dwarf weather to gravitational wave follow-up.Comment: Submitted to Proc. SPIE, Astronomical Telescopes + Instrumentation
(2014
Fiber link design for the NASA-NSF extreme precision Doppler spectrograph concept "WISDOM"
We describe the design of the fiber-optic coupling and light transfer system of the WISDOM (WIYN Spectrograph for DOppler Monitoring) instrument. As a next-generation Precision Radial Velocity (PRV) spectrometer, WISDOM incorporates lessons learned from HARPS about thermal, pressure, and gravity control, but also takes new measures to stabilize the spectrograph illumination, a subject that has been overlooked until recently. While fiber optic links provide more even illumination than a conventional slit, careful engineering of the interface is required to realize their full potential. Conventional round fiber core geometries have been used successfully in conjunction with optical double scramblers, but such systems still retain a memory of the input illumination that is visible in systems seeking sub-m/s PRV precision. Noncircular fibers, along with advanced optical scramblers, and careful optimization of the spectrograph optical system itself are therefore necessary to study Earth-sized planets. For WISDOM, we have developed such a state-of-the-art fiber link concept. Its design is driven primarily by PRV requirements, but it also manages to preserve high overall throughput. Light from the telescope is coupled into a set of six, 32 μm diameter octagonal core fibers, as high resolution is achieved via pupil slicing. The low-OH, step index, fused silica, FBPI-type fibers are custom designed for their numerical aperture that matches the convergence of the feeding beam and thus minimizes focal ratio degradation at the output. Given the demanding environment at the telescope the fiber end tips are mounted in a custom fused silica holder, providing a perfect thermal match. We used a novel process, chemically assisted photo etching, to manufacture this glass fiber holder. A single ball-lens scrambler is inserted into the 25m long fibers. Employing an anti-reflection (AR) coated, high index, cubic-zirconia ball lens the alignment of the scrambler components are straightforward, as the fiber end tips (also AR coated) by design touch the ball lens and thus eliminate spacing tolerances. A clever and simple opto-mechanical design and assembly process assures micron-level self-alignment, yielding a ~87% throughput and a scrambling gain of >20,000. To mitigate modal noise the individual fibers then subsequently combined into a pair of rectangular fibers, providing a much larger modal area thanks to the 34x106 micron diameter. To minimize slit height, and thus better utilize detector area, the octagonal cores are brought very close together in this transition. The two outer fibers are side polished at one side, into a D-shaped cladding, while the central fiber has a dual side polish. These tapered, side-flattening operations are executed with precise alignment to the octagonal core. Thus the cores of the 3 fibers are brought together and aligned within few microns of each other before spliced onto the rectangular fiber. Overall throughput kept high and FRD at bay by careful management of fiber mounting, vacuum feed-through, application of efficient AR coatings, and implementation of thermal breaks that allow for independent expansion of the fibers and the protective tubing
Pupil slicer design for the NASA-NSF extreme precision Doppler spectrograph concept WISDOM
The WIYN Spectrograph for Doppler Monitoring (WISDOM) was a concept responding to NASA's solicitation for an extreme precision radial velocity instrument for the 3.5 meter WIYN telescope on Kitt Peak in Arizona. In order to meet the spectral resolution requirement of R = 110,000 while maintaining good throughput and a manageable beam diameter, the front end design of the instrument employed a pupil slicing technique wherein a collimated beam is sliced and fed to six separate fibers. This paper presents the optical and mechanical design of the pupil slicer subassembly, a unique method of dealing with thermally induced defocus error, and the methods and results of aligning a prototype
The Incidence of Low-Metallicity Lyman-Limit Systems at z~3.5: Implications for the Cold-Flow Hypothesis of Baryonic Accretion
Cold accretion is a primary growth mechanism of simulated galaxies, yet
observational evidence of "cold flows" at redshifts where they should be most
efficient (-4) is scarce. In simulations, cold streams manifest as
Lyman-limit absorption systems (LLSs) with low heavy-element abundances similar
to those of the diffuse IGM. Here we report on an abundance survey of 17 H
I-selected LLSs at -4.4 which exhibit no metal absorption in SDSS
spectra. Using medium-resolution spectra obtained at Magellan, we derive
ionization-corrected metallicities (or limits) with a Markov-Chain Monte Carlo
sampling that accounts for the large uncertainty in measurements
typical of LLSs. The metal-poor LLS sample overlaps with the IGM in metallicity
and is best described by a model where are drawn from the
IGM chemical abundance distribution. These represent roughly half of all LLSs
at these redshifts, suggesting that 28-40 of the general LLS population at
could trace unprocessed gas. An ancillary sample of ten LLSs without
any a priori metal-line selection is best fit with of
metallicities drawn from the IGM. We compare these results with regions of a
moving-mesh simulation; the simulation finds only half as many baryons in
IGM-metallicity LLSs, and most of these lie beyond the virial radius of the
nearest galaxy halo. A statistically significant fraction of all LLSs have low
metallicity and therefore represent candidates for accreting gas; large-volume
simulations can establish what fraction of these candidates actually lie near
galaxies and the observational prospects for detecting the presumed hosts in
emission.Comment: 19 pages, 17 figures; Submitted to ApJ; Corrected figure 16
Discovery of excess O I absorption towards the z = 6.42 QSO SDSS J1148+5251
We present a search for O I in the spectra of nine 4.9 < z_qso < 6.4 QSOs
taken with Keck/HIRES. We detect six systems with N(O I) > 10^13.7 cm^{-2} in
the redshift intervals where O I 1302 falls redward of the Ly-alpha forest.
Four of these lie towards SDSS J1148+5251 (z_qso = 6.42). This imbalance is
unlikely to arise from variations in sensitivity among our data or from a
statistical fluctuation. The excess O I occurs over a redshift interval that
also contains transmission in Ly-alpha and Ly-beta. Therefore, if these O I
systems represent pockets of neutral gas, then they must occur within or near
regions of the IGM that are highly ionized. In contrast, no O I is detected
towards SDSS J1030+0524 (z_qso = 6.30), whose spectrum shows complete
absorption in Ly-alpha and Ly-beta over \Delta z ~ 0.2. Assuming no ionization
corrections, we measure mean abundance ratios = -0.04 +/- 0.06,
= -0.31 +/- 0.09, and = -0.34 +/- 0.07 (2 sigma), which are
consistent with enrichment dominated by Type II supernovae. The O/Si ratio
limits the fraction of silicon in these systems contributed by metal-free very
massive stars to < 30%, a result which is insensitive to ionization
corrections. The ionic comoving mass densities along the z_qso > 6.2
sightlines, including only the detected systems, are \Omega(O I) = (7.0 +/-
0.6) * 10^{-8}, \Omega(Si II) = (9.6 +/- 0.9) * 10^{-9}, and \Omega(C II) =
(1.5 +/- 0.2) * 10^{-8}.Comment: Submitted to ApJ, with changes to reflect referee's comment
The Distribution of Metallicity in the IGM at z~2.5: OVI and CIV Absorption in the Spectra of 7 QSOs
We present a direct measurement of the metallicity distribution function for
the high redshift intergalactic medium. We determine the shape of this function
using survival statistics, which account for both detections and non-detections
of OVI and CIV associated with HI absorption in quasar spectra. Our OVI sample
probes the metal content of ~50% of all baryons at z~2.5. We find a median
intergalactic abundance of [O,C/H]=-2.82; the differential abundance
distribution is approximately lognormal with mean ~-2.85 and
\sigma=0.75 dex. Some 60-70% the Lya forest lines are enriched to observable
levels ([O,C/H]>-3.5) while the remaining ~30% of the lines have even lower
abundances. Thus we have not detected a universal metallicity floor as has been
suggested for some Population III enrichment scenaria. In fact, we argue that
the bulk of the intergalactic metals formed later than the first stars that are
thought to have triggered reionization. We do not observe a significant trend
of decreasing metallicity toward the lower density IGM, at least within regions
that would be characterized as filaments in numerical simulations. However, an
[O/H] enhancement may be present at somewhat high densities. We estimate that
roughly half of all baryons at these redshifts have been enriched to
[O/H]>=-3.5. We develop a simple model for the metallicity evolution of the
IGM, to estimate the chemical yield of galaxies formed prior to z~2.5. We find
that the typical galaxy recycled 0.1-0.4% of its mass back into the IGM as
heavy elements in the first 3 Gyr after the Big Bang.Comment: 23 pages in emulateapj, 19 figures. Accepted to ApJ, pending review
of new changes. Revised comparison between our results and Schaye et al
(2003
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