Halo ratio from ground based all-sky imaging

© Author(s) 2019.The halo ratio (HR) is a quantitative measure characterizing the occurrence of the 22 halo peak associated with cirrus. We propose to obtain it from an approximation to the scattering phase function (SPF) derived from allsky imaging. Ground-based fisheye cameras are used to retrieve the SPF by implementing the necessary image transformations and corrections. These consist of geometric camera characterization by utilizing positions of known stars in a camera image, transforming the images from the zenithcentred to the light-source-centred system of coordinates and correcting for the air mass and for vignetting, the latter using independent measurements from a sun photometer. The SPF is then determined by averaging the image brightness over the azimuth angle and the HR by calculating the ratio of the SPF at two scattering angles in the vicinity of the 22° halo peak. In variance from previous suggestions we select these angles to be 20 and 23°, on the basis of our observations. HR time series have been obtained under various cloud conditions, including halo cirrus, non-halo cirrus and scattered cumuli. While the HR measured in this way is found to be sensitive to the halo status of cirrus, showing values typically > 1 under halo-producing clouds, similar HR values, mostly artefacts associated with bright cloud edges, can also be occasionally observed under scattered cumuli. Given that the HR is an ice cloud characteristic, a separate cirrus detection algorithm is necessary to screen out non-ice clouds before deriving reliable HR statistics. Here we propose utilizing sky brightness temperature from infrared radiometry: Both its absolute value and the magnitude of fluctuations obtained through detrended fluctuation analysis. The brightness temperature data permit the detection of cirrus in most but not all instances.Peer reviewe

XMM-Newton observation of the relaxed cluster A478: gas and dark matter distribution from 0.01 R_200 to 0.5 R_200

We present an \xmm mosaic observation of the hot ($kT\sim6.5$ keV) and nearby ($z=0.0881$) relaxed cluster of galaxies A478. We derive precise gas density, gas temperature, gas mass and total mass profiles up to 12\arcmin (about half of the virial radius $R_{200}$). The gas density profile is highly peaked towards the center and the surface brightness profile is well fitted by a sum of three $\beta$--models. The derived gas density profile is in excellent agreement, both in shape and in normalization, with the published Chandra density profile (measured within 5\arcmin of the center). Projection and PSF effects on the temperature profile determination are thoroughly investigated. The derived radial temperature structure is as expected for a cluster hosting a cooling core, with a strong negative gradient at the cluster center. The temperature rises from $\sim2$ keV up to a plateau of $\sim6.5$ keV beyond 2' (i.e. $r>208\rm{kpc}=0.1 R_{200}$, $R_{200}=2.08$ Mpc being the virial radius). From the temperature profile and the density profile and under the hypothesis of hydrostatic equilibrium, we derived the total mass profile of A478 down to 0.01 and up to 0.5 the virial radius. We tested different dark matter models against the observed mass profile. The Navarro, Frenk & White (\cite{navarro97}) model is significantly preferred to other models. It leads to a total mass of $M_{200}=1.1\times 10^{15}$ M$_\odot$ for a concentration parameter of $c=4.2\pm0.4$. The gas mass fraction slightly increases with radius. The gas mass fraction at a density contrast of $\delta=2500$ is \fgas=0.13\pm0.02, consistent with previous results on similar hot and massive clusters. We confirm the excess of absorption in the direction of A478.[abridged]Comment: 15 pages, 11 figures, accepted for publication in A&A, corrected typo

Short time-scale optical variability of the dwarf Seyfert nucleus in NGC 4395

We present optical spectroscopic observations of the least-luminous known Seyfert 1 galaxy, NGC 4395, which was monitored every half-hour over the course of 3 nights. The continuum emission varied by ~35 per cent over the course of 3 nights, and we find marginal evidence for greater variability in the blue continuum than the red. A number of diagnostic checks were performed on the data in order to constrain any systematic or aperture effects. No correlations were found that adequately explained the observed variability, hence we conclude that we have observed real intrinsic variability of the nuclear source. No simultaneous variability was measured in the broad H-beta line, although given the difficulty in deblending the broad and narrow components it is difficult to comment on the significance of this result. The observed short time-scale continuum variability is consistent with NGC 4395 having an intermediate-mass (~10^5 solar masses) central supermassive black hole, rather than a very low accretion rate. Comparison with the Seyfert 1 galaxy NGC 5548 shows that the observed variability seems to scale with black hole mass in roughly the manner expected in accretion models. However the absolute time-scale of variability differs by several orders of magnitude from that expected in simple accretion disc models in both cases.Comment: 16 pages, 14 figures, 5 tables, accepted for publication in MNRA

Neural Lens Modeling

Recent methods for 3D reconstruction and rendering increasingly benefit from end-to-end optimization of the entire image formation process. However, this approach is currently limited: effects of the optical hardware stack and in particular lenses are hard to model in a unified way. This limits the quality that can be achieved for camera calibration and the fidelity of the results of 3D reconstruction. In this paper, we propose NeuroLens, a neural lens model for distortion and vignetting that can be used for point projection and ray casting and can be optimized through both operations. This means that it can (optionally) be used to perform pre-capture calibration using classical calibration targets, and can later be used to perform calibration or refinement during 3D reconstruction, e.g., while optimizing a radiance field. To evaluate the performance of our proposed model, we create a comprehensive dataset assembled from the Lensfun database with a multitude of lenses. Using this and other real-world datasets, we show that the quality of our proposed lens model outperforms standard packages as well as recent approaches while being much easier to use and extend. The model generalizes across many lens types and is trivial to integrate into existing 3D reconstruction and rendering systems.Comment: To be presented at CVPR 2023, Project webpage: https://neural-lens.github.i

Automated Spectral Reduction Pipelines

The Liverpool Telescope automated spectral data reduction pipelines perform both removal of instrumental signatures and provide wavelength calibrated data products promptly after observation. Unique science drivers for each of three instruments led to novel hardware solutions which required reassessment of some of the conventional CCD reduction recipes. For example, we describe the derivation of bias and dark corrections on detectors with neither overscan or shutter. In the context of spectroscopy we compare the quality of at fielding resulting from different algorithmic combinations of dispersed and non-dispersed sky and lamp flats in the case of spectra suffering from 2D spatial distortions. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only