3,234 research outputs found
Apodized phase mask coronagraphs for arbitrary apertures
Phase masks coronagraphs can be seen as linear systems that spatially
redistribute, in the pupil plane, the energy collected by the telescope. Most
of the on-axis light must ideally be rejected outside the aperture to be
blocked with a Lyot stop, while almost all off-axis light must go through it.
The unobstructed circular apertures of off-axis telescopes make this possible
but all major telescopes are however on-axis and the performance of these
coronagraphs is dramatically reduced by the central obstruction. Their
performance can be restored by using an additional optimally designed apodizer
that changes the amplitude in the first pupil plane so that the on-axis light
is rejected outside the obstructed aperture of the telescope. The numerical
optimization model is built by maximizing the apodizer's transmission while
setting constraints on the extremum values of the electric field that the Lyot
stop does not block. The coronagraphic image is compared to what a non-apodized
phase mask coronagraph provides and an analysis is made of the trade-offs that
exist between the apodizer transmission and the Lyot stop properties. The
existence of a solution and the mask transmission depend on the aperture and
the Lyot stop geometries, and on the constraints that are set on the on-axis
attenuation. The system throughput is a concave function of the Lyot stop
transmission. In the case of a VLT-like aperture, apodizers with a transmission
of 0.16 to 0.92 associated with a four-quadrant phase mask provide contrast as
low as a few 1e-10 at 1 lambda/D from the star. The system's maximum throughput
is 0.64, for an apodizer with an 0.88 transmission and a Lyot stop with a 0.69
transmission. Optimizing apodizers for a vortex phase mask requires computation
times much longer than in the previous case, and no result is presented for
this mask.Comment: 16 page
The Optimal Path of the Chinese Renminbi
This paper provides evidence on the consistency of the determination of the Chinese real effective exchange rate (REER) over time. Especially, we validate coin- tegration between the REER and a set of fundamentals using recent developments in model selection. Error correction model (ECM) path dependence in model se- lection is addressed by using the General-To-Specific (GETS) approach enabling us to obtain empirically constant and encompassing ECM. As inference in finite sam- ples is commonly of concern, statistics' distributional properties for cointegration tests are estimated by Monte Carlo simulations. The final specification of the model is compatible with the natural real exchange rate of Stein (1994). We study the implications of our findings in terms of foreign exchange policy.Exchange Rate, Equilibrium value, GETS, Global Imbalances
Apodized phase mask coronagraphs for arbitrary apertures. II. Comprehensive review of solutions for the vortex coronagraph
With a clear circular aperture, the vortex coronagraph perfectly cancels an
on-axis point source and offers a 0.9 or 1.75 lambda/D inner working angle for
topological charge 2 or 4, respectively. Current and near-future large
telescopes are on-axis, however, and the diffraction effects of the central
obscuration, and the secondary supports are strong enough to prevent the
detection of companions 1e-3 - 1e-5 as bright as, or fainter than, their host
star. Recent advances show that a ring apodizer can restore the performance of
this coronagraph by compensating for the diffraction effects of a circular
central obscuration in a 1D modeling of the pupil. We extend this work and
optimize apodizers for arbitrary apertures in 2D in order to tackle the
diffraction effects of the spiders and other noncircular artefacts in the
pupil. We use a numerical optimization scheme to compute hybrid coronagraph
designs that combine the advantages of the vortex coronagraph (small in IWA)
and of shaped pupils coronagraphs (robustness to central obscuration and pupil
asymmetric structures). We maximize the apodizer transmission, while
constraints are set on the extremum values of the electric field that is
computed in chosen regions of the Lyot plane through closed form expressions.
Optimal apodizers are computed for topological charges 2 and 4 vortex
coronagraphs and for telescope apertures with 10-30% central obscurations and
0-1% thick spiders. We characterize the impacts of the obscuration ratio and
the thickness of the spiders on the throughput and the IWA for the two
topological charges.Comment: 23 pages, 12 figures, 2 table
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