1,477 research outputs found
Phase Errors in Diffraction-Limited Imaging: Contrast Limits for Sparse Aperture Masking
Bispectrum phase, closure phase and their generalisation to kernel-phase are
all independent of pupil-plane phase errors to first-order. This property, when
used with Sparse Aperture Masking (SAM) behind adaptive optics, has been used
recently in high-contrast observations at or inside the formal diffraction
limit of large telescopes. Finding the limitations to these techniques requires
an understanding of spatial and temporal third-order phase effects, as well as
effects such as time-variable dispersion when coupled with the non-zero
bandwidths in real observations. In this paper, formulae describing many of
these errors are developed, so that a comparison can be made to fundamental
noise processes of photon- and background-noise. I show that the current
generation of aperture-masking observations of young solar-type stars, taken
carefully in excellent observing conditions, are consistent with being limited
by temporal phase noise and photon noise. This has relevance for plans to
combine pupil-remapping with spatial filtering. Finally, I describe calibration
strategies for kernel-phase, including the optimised calibrator weighting as
used for LkCa 15, and the restricted kernel-phase POISE technique that avoids
explicit dependence on calibrators.Comment: 11 pages, 5 figures, resubmitted to MNRAS after responding to
referee's comment
Effects of moderate abundance changes on the atmospheric structure and colours of Mira variables (Research Note)
Aims. We study the effects of moderate deviations from solar abundances upon
the atmospheric structure and colours of typical Mira variables. Methods. We
present two model series of dynamical opacity-sampling models of Mira variables
which have (1) 1 solar metallicity 3 and (2) "mild" S-type C/O abundance ratio
([C/O]=0.9) with typical Zr enhancement (solar +1.0). These series are compared
to a previously studied solar-abundance series which has similar fundamental
parameters (mass, luminosity, period, radius) that are close to those of o Cet.
Results. Both series show noticeable effects of abundance upon stratifications
and infrared colours but cycle-to-cycle differences mask these effects at most
pulsation phases, with the exception of a narrow-water-filter colour near
minimum phase.Comment: 4 pages, 3 figures, accepted for A&
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