7,282 research outputs found
Difference image photometry with bright variable backgrounds
Over the last two decades the Andromeda Galaxy (M31) has been something of a
test-bed for methods aimed at obtaining accurate time-domain relative
photometry within highly crowded fields. Difference imaging methods, originally
pioneered towards M31, have evolved into sophisticated methods, such as the
Optimal Image Subtraction (OIS) method of Alard & Lupton (1998), that today are
most widely used to survey variable stars, transients and microlensing events
in our own Galaxy. We show that modern difference image (DIA) algorithms such
as OIS, whilst spectacularly successful towards the Milky Way bulge, may
perform badly towards high surface brightness targets such as the M31 bulge.
Poor results can occur in the presence of common systematics which add spurious
flux contributions to images, such as internal reflections, scattered light or
fringing. Using data from the Angstrom Project microlensing survey of the M31
bulge, we show that very good results are usually obtainable by first
performing careful photometric alignment prior to using OIS to perform
point-spread function (PSF) matching. This separation of background matching
and PSF matching, a common feature of earlier M31 photometry techniques, allows
us to take full advantage of the powerful PSF matching flexibility offered by
OIS towards high surface brightness targets. We find that difference images
produced this way have noise distributions close to Gaussian, showing
significant improvement upon results achieved using OIS alone. We show that
with this correction light-curves of variable stars and transients can be
recovered to within ~10 arcseconds of the M31 nucleus. Our method is simple to
implement and is quick enough to be incorporated within real-time DIA
pipelines. (Abridged)Comment: 12 pages. Accepted for publication in MNRAS. Includes an expanded
discussion of DIA testing and results, including additional lightcurve
example
A GPU-accelerated real-time NLMeans algorithm for denoising color video sequences
Abstract. The NLMeans filter, originally proposed by Buades et al., is a very popular filter for the removal of white Gaussian noise, due to its simplicity and excellent performance. The strength of this filter lies in exploiting the repetitive character of structures in images. However, to fully take advantage of the repetitivity a computationally extensive search for similar candidate blocks is indispensable. In previous work, we presented a number of algorithmic acceleration techniques for the NLMeans filter for still grayscale images. In this paper, we go one step further and incorporate both temporal information and color information into the NLMeans algorithm, in order to restore video sequences. Starting from our algorithmic acceleration techniques, we investigate how the NLMeans algorithm can be easily mapped onto recent parallel computing architectures. In particular, we consider the graphical processing unit (GPU), which is available on most recent computers. Our developments lead to a high-quality denoising filter that can process DVD-resolution video sequences in real-time on a mid-range GPU
Weak Lensing of Intensity Mapping: the Cosmic Infrared Background
Gravitational lensing deflects the paths of cosmic infrared background (CIB)
photons, leaving a measurable imprint on CIB maps. The resulting statistical
anisotropy can be used to reconstruct the matter distribution out to the
redshifts of CIB sources. To this end, we generalize the CMB lensing quadratic
estimator to any weakly non-Gaussian source field, by deriving the optimal
lensing weights. We point out the additional noise and bias caused by the
non-Gaussianity and the `self-lensing' of the source field. We propose methods
to reduce, subtract or model these non-Gaussianities. We show that CIB lensing
should be detectable with Planck data, and detectable at high significance for
future CMB experiments like CCAT-Prime. The CIB thus constitutes a new source
image for lensing studies, providing constraints on the amplitude of structure
at intermediate redshifts between galaxies and the CMB. CIB lensing
measurements will also give valuable information on the star formation history
in the universe, constraining CIB halo models beyond the CIB power spectrum. By
laying out a detailed treatment of lens reconstruction from a weakly
non-Gaussian source field, this work constitutes a stepping stone towards lens
reconstruction from continuum or line intensity mapping data, such as the
Lyman-alpha emission, absorption, and the 21cm radiation.Comment: Accepted in Physical Review
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