32,801 research outputs found
The Multi-Object, Fiber-Fed Spectrographs for SDSS and the Baryon Oscillation Spectroscopic Survey
We present the design and performance of the multi-object fiber spectrographs
for the Sloan Digital Sky Survey (SDSS) and their upgrade for the Baryon
Oscillation Spectroscopic Survey (BOSS). Originally commissioned in Fall 1999
on the 2.5-m aperture Sloan Telescope at Apache Point Observatory, the
spectrographs produced more than 1.5 million spectra for the SDSS and SDSS-II
surveys, enabling a wide variety of Galactic and extra-galactic science
including the first observation of baryon acoustic oscillations in 2005. The
spectrographs were upgraded in 2009 and are currently in use for BOSS, the
flagship survey of the third-generation SDSS-III project. BOSS will measure
redshifts of 1.35 million massive galaxies to redshift 0.7 and Lyman-alpha
absorption of 160,000 high redshift quasars over 10,000 square degrees of sky,
making percent level measurements of the absolute cosmic distance scale of the
Universe and placing tight constraints on the equation of state of dark energy.
The twin multi-object fiber spectrographs utilize a simple optical layout
with reflective collimators, gratings, all-refractive cameras, and
state-of-the-art CCD detectors to produce hundreds of spectra simultaneously in
two channels over a bandpass covering the near ultraviolet to the near
infrared, with a resolving power R = \lambda/FWHM ~ 2000. Building on proven
heritage, the spectrographs were upgraded for BOSS with volume-phase
holographic gratings and modern CCD detectors, improving the peak throughput by
nearly a factor of two, extending the bandpass to cover 360 < \lambda < 1000
nm, and increasing the number of fibers from 640 to 1000 per exposure. In this
paper we describe the original SDSS spectrograph design and the upgrades
implemented for BOSS, and document the predicted and measured performances.Comment: 43 pages, 42 figures, revised according to referee report and
accepted by AJ. Provides background for the instrument responsible for SDSS
and BOSS spectra. 4th in a series of survey technical papers released in
Summer 2012, including arXiv:1207.7137 (DR9), arXiv:1207.7326 (Spectral
Classification), and arXiv:1208.0022 (BOSS Overview
Spectro-Perfectionism: An Algorithmic Framework for Photon Noise-Limited Extraction of Optical Fiber Spectroscopy
We describe a new algorithm for the "perfect" extraction of one-dimensional
spectra from two-dimensional (2D) digital images of optical fiber
spectrographs, based on accurate 2D forward modeling of the raw pixel data. The
algorithm is correct for arbitrarily complicated 2D point-spread functions
(PSFs), as compared to the traditional optimal extraction algorithm, which is
only correct for a limited class of separable PSFs. The algorithm results in
statistically independent extracted samples in the 1D spectrum, and preserves
the full native resolution of the 2D spectrograph without degradation. Both the
statistical errors and the 1D resolution of the extracted spectrum are
accurately determined, allowing a correct chi-squared comparison of any model
spectrum with the data. Using a model PSF similar to that found in the red
channel of the Sloan Digital Sky Survey spectrograph, we compare the
performance of our algorithm to that of cross-section based optimal extraction,
and also demonstrate that our method allows coaddition and foreground
estimation to be carried out as an integral part of the extraction step. This
work demonstrates the feasibility of current- and next-generation multi-fiber
spectrographs for faint galaxy surveys even in the presence of strong night-sky
foregrounds. We describe the handling of subtleties arising from fiber-to-fiber
crosstalk, discuss some of the likely challenges in deploying our method to the
analysis of a full-scale survey, and note that our algorithm could be
generalized into an optimal method for the rectification and combination of
astronomical imaging data.Comment: 9 pages, 4 figures, emulateapj; minor corrections and clarifications;
to be published in the PAS
Metrology Camera System of Prime Focus Spectrograph for Subaru Telescope
The Prime Focus Spectrograph (PFS) is a new optical/near-infrared multi-fiber
spectrograph designed for the prime focus of the 8.2m Subaru telescope. The
metrology camera system of PFS serves as the optical encoder of the COBRA fiber
motors for the configuring of fibers. The 380mm diameter aperture metrology
camera will locate at the Cassegrain focus of Subaru telescope to cover the
whole focal plane with one 50M pixel Canon CMOS sensor. The metrology camera is
designed to provide the fiber position information within 5{\mu}m error over
the 45cm focal plane. The positions of all fibers can be obtained within 1s
after the exposure is finished. This enables the overall fiber configuration to
be less than 2 minutes.Comment: 10 pages, 12 figures, SPIE Astronomical Telescopes and
Instrumentation 201
Astronomical Spectroscopy
Spectroscopy is one of the most important tools that an astronomer has for
studying the universe. This chapter begins by discussing the basics, including
the different types of optical spectrographs, with extension to the ultraviolet
and the near-infrared. Emphasis is given to the fundamentals of how
spectrographs are used, and the trade-offs involved in designing an
observational experiment. It then covers observing and reduction techniques,
noting that some of the standard practices of flat-fielding often actually
degrade the quality of the data rather than improve it. Although the focus is
on point sources, spatially resolved spectroscopy of extended sources is also
briefly discussed. Discussion of differential extinction, the impact of
crowding, multi-object techniques, optimal extractions, flat-fielding
considerations, and determining radial velocities and velocity dispersions
provide the spectroscopist with the fundamentals needed to obtain the best
data. Finally the chapter combines the previous material by providing some
examples of real-life observing experiences with several typical instruments.Comment: An abridged version of a chapter to appear in Planets, Stars and
Stellar Systems, to be published in 2011 by Springer. Slightly revise
p3d: a general data-reduction tool for fiber-fed integral-field spectrographs
The reduction of integral-field spectrograph (IFS) data is demanding work.
Many repetitive operations are required in order to convert raw data into,
typically a large number of, spectra. This effort can be markedly simplified
through the use of a tool or pipeline, which is designed to complete many of
the repetitive operations without human interaction. Here we present our
semi-automatic data-reduction tool p3d that is designed to be used with
fiber-fed IFSs. Important components of p3d include a novel algorithm for
automatic finding and tracing of spectra on the detector, and two methods of
optimal spectrum extraction in addition to standard aperture extraction. p3d
also provides tools to combine several images, perform wavelength calibration
and flat field data. p3d is at the moment configured for four IFSs. In order to
evaluate its performance we have tested the different components of the tool.
For these tests we used both simulated and observational data. We demonstrate
that for three of the IFSs a correction for so-called cross-talk due to
overlapping spectra on the detector is required. Without such a correction
spectra will be inaccurate, in particular if there is a significant intensity
gradient across the object. Our tests showed that p3d is able to produce
accurate results. p3d is a highly general and freely available tool. It is
easily extended to include improved algorithms, new visualization tools and
support for additional instruments. The program code can be downloaded from the
p3d-project web site http://p3d.sourceforge.netComment: 18 pages, 15 figures, 3 tables, accepted for publication in A&
SparsePak: A Formatted Fiber Field-Unit for The WIYN Telescope Bench Spectrograph. II. On-Sky Performance
We present a performance analysis of SparsePak and the WIYN Bench
Spectrograph for precision studies of stellar and ionized gas kinematics of
external galaxies. We focus on spectrograph configurations with echelle and
low-order gratings yielding spectral resolutions of ~10000 between 500-900nm.
These configurations are of general relevance to the spectrograph performance.
Benchmarks include spectral resolution, sampling, vignetting, scattered light,
and an estimate of the system absolute throughput. Comparisons are made to
other, existing, fiber feeds on the WIYN Bench Spectrograph. Vignetting and
relative throughput are found to agree with a geometric model of the optical
system. An aperture-correction protocol for spectrophotometric standard-star
calibrations has been established using independent WIYN imaging data and the
unique capabilities of the SparsePak fiber array. The WIYN
point-spread-function is well-fit by a Moffat profile with a constant power-law
outer slope of index -4.4. We use SparsePak commissioning data to debunk a
long-standing myth concerning sky-subtraction with fibers: By properly treating
the multi-fiber data as a ``long-slit'' it is possible to achieve precision sky
subtraction with a signal-to-noise performance as good or better than
conventional long-slit spectroscopy. No beam-switching is required, and hence
the method is efficient. Finally, we give several examples of science
measurements which SparsePak now makes routine. These include H
velocity fields of low surface-brightness disks, gas and stellar
velocity-fields of nearly face-on disks, and stellar absorption-line profiles
of galaxy disks at spectral resolutions of ~24,000.Comment: To appear in ApJSupp (Feb 2005); 19 pages text; 7 tables; 27 figures
(embedded); high-resolution version at
http://www.astro.wisc.edu/~mab/publications/spkII_pre.pd
Near-Infrared-Spectroscopy with Extremely Large Telescopes: Integral-Field- versus Multi-Object-Instruments
Integral-field-spectroscopy and multi-object-spectroscopy provide the high
multiplex gain required for efficient use of the upcoming generation of
extremely large telescopes. We present instrument developments and designs for
both concepts, and how these designs can be applied to cryogenic near-infrared
instrumentation. Specifically, the fiber-based concept stands out the
possibility to expand it to any number of image points, and its modularity
predestines it to become the new concept for multi-field-spectroscopy. Which of
the three concepts --- integral-field-, multi-object-, or
multi-field-spectroscopy --- is best suited for the largest telescopes is
discussed considering the size of the objects and their density on the sky.Comment: 8 pages, 4 figures (converted to bitmap), to appear in the
proceedings of the Workshop on Extremely Large Telescopes, Sweden, June 1-2,
1999, uses spie.sty (V0.91) and spiebib.bst (V0.91
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