arXiv subjects

Eduardo Sepulveda

Publications and source records attributed to Eduardo Sepulveda.

8 recordsLinked to original sources

arXiv2026

StarDICE IV: correcting visible photometry from atmospheric gray extinction using thermal infrared observations

Ground-based surveys such as the Vera C. Rubin Observatory's Legacy Survey of Space and Time require photometric calibration that is both long-term stable and spatially uniform at the sub-per cent level, even during non-photometric conditions. Achieving this precision motivates new approaches to characterize atmospheric transmission, particularly to mitigate grey extinction from clouds. The StarDICE experiment aims to establish a metrology chain linking laboratory standards to astrophysical fluxes with 1 mmag accuracy in the $\textit{griz}$ bands, a goal for which controlling variable atmospheric effects is essential. We present a method that corrects photometric measurements using simultaneous radiometric information from an infrared thermal camera. The grey-extinction model is fit on an image-by-image basis using thermal radiance excess and the difference between synthetic and instrumental fluxes of calibration stars, without requiring assumptions about the spatial structure of extinction. The method relies on a forward model that incorporates environmental monitoring, radiative-transfer simulations, and Gaia DR3 stellar catalogues. Using data from a remote observing system that repeatedly monitored two fields under diverse atmospheric conditions, we show that the corrections reduce residuals between corrected and reference magnitudes and produce extinction maps with 2-arcmin resolution and ~0.01 mag accuracy. Using this technique, we can recover data acquired under non-photometric conditions with a precision comparable to data obtained under photometric conditions. For the most affected exposures, the mean absolute error improves from 0.64 to 0.11 mag, and temporal extinction variations can be reduced to 0.025 mag per source. We discuss the implications of this technique for future surveys and outline directions for further refinement.

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arXiv2025

A transmission hologram for slitless spectrophotometry on a convergent telescope beam. Optimisation and characterization

This article details the optimisation and the characterisation of the hologram described in a companion paper published in 2021, which showed the superiority of a holographic optical element over a periodic grating as a disperser installed in the path of a converging beam on an on-axis detector (unbent spectrograph) for slitless spectroscopy. In this article, we describe in detail the development and optimisation of the final optical holographic element installed on the spectrograph of the auxiliary telescope (AuxTel) at the Rubin-LSST observatory. After recalling the general principle of a hologram used as a dispersing and focusing element, we describe the technical resources - optical bench and sky measurements - and modeling tools that enabled us to determine the optimum production parameters for the AuxTel hologram after 4 prototyping phases. We also describe the on-sky verifications and measurements carried out with various telescopes. Thanks to these various techniques, we have succeeded in obtaining a diffraction efficiency in the first order close to the maximum theoretically possible with our thin-type hologram. This hologram has been in place on AuxTel's spectrograph since February 2021, and has since given full satisfaction, coupled with analysis software adapted to slitless spectroscopy.

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arXiv2024

StarDICE III: Characterization of the photometric instrument with a Collimated Beam Projector

The measurement of type Ia supernovae magnitudes provides cosmological distances, which can be used to constrain dark energy parameters. Large photometric surveys require a substantial improvement in the calibration precision of their photometry to reduce systematic uncertainties in cosmological constraints. The StarDICE experiment is designed to establish accurate broadband flux references for these surveys, aiming for sub-percent precision in magnitude measurements. This requires a precise measurement of the filter bandpasses of both the StarDICE and survey instruments with sub-nanometer accuracy. To that end, we have developed the Collimated Beam Projector (CBP), an optical device capable of calibrating the throughput of an astronomical telescope and of its filters. The CBP is built from a tunable laser source and a reversed telescope to emit a parallel monochromatic light beam that is continuously monitored in flux and wavelength. The CBP output light flux is measured using a large area photodiode, previously calibrated relative to a NIST photodiode. We derive the StarDICE telescope throughput and filter transmissions from the CBP measurements, anchoring it to the absolute calibration provided by the NIST. After analyzing the systematic uncertainties, we achieved sub-nanometer accuracy in determining filter central wavelengths, measured each filter transmission with a precision of 0.5% per 1nm bin, and detected out-of-band leakages at 0.01%. Furthermore, we have synthesized the equivalent transmission for full pupil illumination from four sample positions in the StarDICE telescope mirror, with an accuracy of approximately 0.2nm for central wavelengths and 7mmag for broadband fluxes. We demonstrated our ability to characterize a telescope throughput down to the mmag, and paved the way for future developments, such as a portable CBP version for in-situ transmission monitoring.

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arXiv2024

Slitless spectrophotometry with forward modelling: principles and application to atmospheric transmission measurement

In the next decade, many optical surveys will aim to tackle the question of dark energy nature, measuring its equation of state parameter at the permil level. This requires trusting the photometric calibration of the survey with a precision never reached so far, controlling many sources of systematic uncertainties. The measurement of the on-site atmospheric transmission for each exposure, or on average for each season or for the full survey, can help reach the permil precision for magnitudes. This work aims at proving the ability to use slitless spectroscopy for standard star spectrophotometry and its use to monitor on-site atmospheric transmission as needed, for example, by the Vera C. Rubin Observatory Legacy Survey of Space and Time supernova cosmology program. We fully deal with the case of a disperser in the filter wheel, which is the configuration chosen in the Rubin Auxiliary Telescope. The theoretical basis of slitless spectrophotometry is at the heart of our forward model approach to extract spectroscopic information from slitless data. We developed a publicly available software called Spectractor (https://github.com/LSSTDESC/Spectractor) that implements each ingredient of the model and finally performs a fit of a spectrogram model directly on image data to get the spectrum. We show on simulations that our model allows us to understand the structure of spectrophotometric exposures. We also demonstrate its use on real data, solving specific issues and illustrating how our procedure allows the improvement of the model describing the data. Finally, we discuss how this approach can be used to directly extract atmospheric transmission parameters from data and thus provide the base for on-site atmosphere monitoring. We show the efficiency of the procedure on simulations and test it on the limited data set available.

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arXiv2022

StarDICE I: sensor calibration bench and absolute photometric calibration of a Sony IMX411 sensor

The Hubble diagram of type-Ia supernovae (SNe-Ia) provides cosmological constraints on the nature of dark energy with an accuracy limited by the flux calibration of currently available spectrophotometric standards. The StarDICE experiment aims at establishing a 5-stage metrology chain from NIST photodiodes to stars, with a targeted accuracy of \SI{1}{mmag} in $griz$ colors. We present the first two stages, resulting in the calibration transfer from NIST photodiodes to a demonstration \SI{150}{Mpixel} CMOS sensor (Sony IMX411ALR as implemented in the QHY411M camera by QHYCCD). As a side-product, we provide full characterization of this camera. A fully automated spectrophotometric bench is built to perform the calibration transfer. The sensor readout electronics is studied using thousands of flat-field images from which we derive stability, high resolution photon transfer curves and estimates of the individual pixel gain. The sensor quantum efficiency is then measured relative to a NIST-calibrated photodiode. Flat-field scans at 16 different wavelengths are used to build maps of the sensor response. We demonstrate statistical uncertainty on quantum efficiency below \SI{0.001}{e^-/γ} between \SI{387}{nm} and \SI{950}{nm}. Systematic uncertainties in the bench optics are controlled at the level of \SI{1e-3}{e^-/γ}. Uncertainty in the overall normalization of the QE curve is 1\%. Regarding the camera we demonstrate stability in steady state conditions at the level of \SI{32.5}{ppm}. Homogeneity in the response is below \SI{1}{\percent} RMS across the entire sensor area. Quantum efficiency stays above \SI{50}{\percent} in most of the visible range, peaking well above \SI{80}{\percent} between \SI{440}{nm} and \SI{570}{nm}. Differential non-linearities at the level of \SI{1}{\percent} are detected. A simple 2-parameter model is proposed to mitigate the effect.

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arXiv2022

An Estimator for Lensing Potential from Galaxy Number Counts

We derive an estimator for the lensing potential from galaxy number counts which contains a linear and a quadratic term. We show that this estimator has a much larger signal-to-noise ratio than the corresponding estimator from intensity mapping. This is due to the additional lensing term in the number count angular power spectrum which is present already at linear order. We estimate the signal-to-noise ratio for future photometric surveys. Particularly at high redshifts, $z\gtrsim 1.5$, the signal to noise ratio can become of order 30. Therefore, the number counts in photometric surveys would be an excellent means to measure tomographic lensing spectra.

astro-ph.CO↗

arXiv2020

Tearing and related field distortions in deep-depletion CCDs

Tearing patterns affecting flat field frames in CCDs are a visually striking obstacle to performing Pixel Response Non-Uniformity corrections. These patterns can be explained by lateral field distortions, caused by the non-uniform distribution of holes in the channel stops between sensor columns. Over the course of LSST camera development, a number of practical fixes have been suggested to get rid of tearing. But applying these fixes to our 16-channel Teledyne-e2v sensors leaves at best a distortion pattern at the vertical edges of every segment. Our working hypothesis is that the origin of the tearing is the parallel clocking itself, which moves the holes that are present in the channel stops regions. The efficiency of these transfers depends strongly on the details of the clocking operations, resulting in the observed variety of distortion patterns. Removal of most of the distortion patterns can therefore be achieved by executing a purge operation, which flattens back the hole distribution in the channel stops, immediately before acquiring a frame. A more effective solution is to switch all clocking operations to use a bipolar voltage set.

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arXiv2019

The shape of the Photon Transfer Curve of CCD sensors

The Photon Transfer Curve (PTC) of a CCD depicts the variance of uniform images as a function of their average. It is now well established that the variance is not proportional to the average, as Poisson statistics would indicate, but rather flattens out at high flux. This "variance deficit", related to the brighter-fatter effect, feeds correlations between nearby pixels, that increase with flux, and decay with distance. We propose an analytical expression for the PTC shape, and for the dependence of correlations with intensity, and relate both to some more basic quantities related to the electrostatics of the sensor, that are commonly used to correct science images for the brighter-fatter effect. We derive electrostatic constraints from a large set of flat field images acquired with a CCD e2v 250, and eventually question the generally-admitted assumption that boundaries of CCD pixels shift by amounts proportional to the source charges. Our results show that the departure of flat field statistics from Poisson law is entirely compatible with charge redistribution during the drift in the sensor.

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