378 research outputs found
A Tunable Echelle Imager
We describe and evaluate a new instrument design called a Tunable Echelle
Imager (TEI). In this instrument, the output from an imaging Fabry-Perot
interferometer is cross-dispersed by a grism in one direction and dispersed by
an echelle grating in the perpendicular direction. This forms a mosaic of
different narrow-band images of the same field on a detector. It offers a
distinct wavelength multiplex advantage over a traditional imaging Fabry-Perot
device.
Potential applications of the TEI include spectrophotometric imaging and
OH-suppressed imaging by rejection.Comment: 11 pages, 12 figures, accepted by PAS
Circularly Symmetric Apodization via Starshaped Masks
Recently, we introduced a class of shaped pupil masks, called spiderweb
masks, that produce point spread functions having annular dark zones. With such
masks, a single image can be used to probe a star for extrasolar planets. In
this paper, we introduce a new class of shaped pupil masks that also provide
annular dark zones. We call these masks starshaped masks. Given any circularly
symmetric apodization function, we show how to construct a corresponding
starshaped mask that has the same point-spread function (out to any given outer
working distance) as obtained by the apodization.Comment: Paper also at:
http://www.orfe.princeton.edu/~rvdb/tex/starshape/ms.pdf Updated to clarify
misleading statements regarding total throughput for apodizations and their
corresponding starshaped mask
The Performance and Scientific Rationale for an IR Imaging Fourier Transform Spectrograph on a Large Space Telescope
We describe a concept for an imaging spectrograph for a large orbiting
observatory such as NASA's proposed Next Generation Space Telescope (NGST)
based on an imaging Fourier transform spectrograph (IFTS). An IFTS has several
important advantages which make it an ideal instrument to pursue the scientific
objectives of NGST. We review the operation of an IFTS and make a quantitative
evaluation of the signal-to-noise performance of such an instrument in the
context of NGST. We consider the relationship between pixel size, spectral
resolution, and diameter of the beamsplitter for imaging and non-imaging
Fourier transform spectrographs and give the condition required to maintain
spectral modulation efficiency over the entire field of view. We give examples
of scientific programs that could be performed with this facility.Comment: 20 pages, 7 Postscript figures. PASP in pres
Theoretical Limits on Extrasolar Terrestrial Planet Detection with Coronagraphs
Many high contrast coronagraph designs have recently been proposed. In this
paper, their suitability for direct imaging of extrasolar terrestrial planets
is reviewed. We also develop a linear-algebra based model of coronagraphy that
can both explain the behavior of existing coronagraphs and quantify the
coronagraphic performance limit imposed by fundamental physics. We find that
the maximum theoretical throughput of a coronagraph is equal to one minus the
non-aberrated non-coronagraphic PSF of the telescope. We describe how a
coronagraph reaching this fundamental limit may be designed, and how much
improvement over the best existing coronagraph design is still possible. Both
the analytical model and numerical simulations of existing designs also show
that this theoretical limit rapidly degrades as the source size is increased:
the ``highest performance'' coronagraphs, those with the highest throughput and
smallest Inner Working Angle (IWA), are the most sensitive to stellar angular
diameter. This unfortunately rules out the possibility of using a small IWA
(lambda/d) coronagraph for a terrestrial planet imaging mission.
Finally, a detailed numerical simulation which accurately accounts for
stellar angular size, zodiacal and exozodiacal light is used to quantify the
efficiency of coronagraph designs for direct imaging of extrasolar terrestrial
planets in a possible real observing program. We find that in the photon noise
limited regime, a 4m telescope with a theoretically optimal coronagraph is able
to detect Earth-like planets around 50 stars with 1hr exposure time per target
(assuming 25% throughput and exozodi levels similar to our solar system). We
also show that at least 2 existing coronagraph design can approach this level
of performance in the ideal monochromatic case considered in this study.Comment: Accepted for publication to ApJ Sup
A Tunable Lyot Filter at Prime Focus: a Method for Tracing Supercluster Scales at z ~ 1
Tunable narrow-band, emission-line surveys have begun to show the ease with
which star forming galaxies can be identified in restricted redshift intervals
to z ~ 5 with a 4m class telescope. These surveys have been carried out with
imaging systems at the Cassegrain or Nasmyth focus and are therefore restricted
to fields smaller than 10 arcmin. We now show that tunable narrowband imaging
is possible over a 30 arcmin field with a high-performance Lyot filter placed
directly in front of a CCD mosaic at the prime focus. Our design is intended
for the f/3.3 prime focus of the AAT 3.9m, although similar devices can be
envisaged for the Subaru 8m (f/2), Palomar 5m (f/3.4), VISTA 4m (f/6), Mayall
4m (f/2.6) or CFHT 3.6m (f/4). A modified Wynne doublet ensures sub-arcsecond
performance over the field. In combination with the new Wide-Field Imaging 8K x
8K mosaic (WFI) at the AAT, the overall throughput (35%) of the system to
unpolarised light is expected to be comparable to the TAURUS Tunable Filter
(TTF). Unlike the TTF, the field is fully monochromatic and the instrumental
profile has much better wing suppression. For targetted surveys of
emission-line sources at z ~ 1, a low-resolution (R ~ 150 at 550nm) Lyot filter
on a 4m telescope is expected to be comparable or superior to current
instruments on 8-10m class telescopes. We demonstrate that the 30 arcmin field
is well matched to superclusters at these redshifts such that large-scale
structure should be directly observable.Comment: Astrophysical Journal, accepted. 53 pages, 16 figures, aaste
Methane detection scheme based upon the changing optical constants of a zinc oxide/platinum matrix created by a redox reaction and their effect upon surface plasmons
We detect changes in the optical properties of a metal oxide semiconductor (MOS), ZnO, in a multi-thin-film matrix with platinum in the presence of the hydrocarbon gas methane. A limit of detection of 2% by volume with concentrations from 0 to 10% and maximum resolution of 0.15% with concentrations ranging from 30% to 80% at room temperature are demonstrated along with a selective chemical response to methane over carbon dioxide and the other alkane gases. The device yields the equivalent maximum bulk refractive index spectral sensitivity of 1.8 Ă 105 nm/RIU. This is the first time that the optical properties of MOS have been monitored to detect the presence of a specific gas. This single observation is a significant result, as MOS have a potentially large number of target gases, thus offering a new paradigm for gas sensing using MOSs
- âŠ