Focus Meeting 12 - Abstracts
CALSPEC: HST Spectrophotometric Flux Standards from 0.115 to 30 µm
Bohlin, Ralph
The STIS and NICMOS low dispersion spectometers on the Hubble Space Telescope (HST) have flux calibrations tracable to the three unreddened, pure-hydrogen WDs, G191B2B, GD153, and GD71, which are in the effective temperature range 33590–59000 K. STIS covers the 0.115–1 µm range with 5 gratings at a resolution of R~500, while NICMOS has three grisms covering 0.8–2.5 µm at R~100. The fluxes, i.e. spectral energy distributions (SEDs), of these primary standard stars are established using the Rauch Tu ¨bingen NLTE model atmosphere code to determine the relative flux vs. wavelength. Alternate NLTE Tlusty models for the same temperature and gravity differ in their relative flux distributions and are used to define the systematic uncertainties. The absolute normalization of the three models is from a weighted average of the absolute flux of Vega at 5556 °A and the MSX absolute flux measures of Sirius at 8–21 µm. The result of this reconciliation of the visible and mid-IR implies a flux for Vega at 5556 °A of 3.44 erg cm-2 s-1 °A-1 ±0.5%.Once calibrated using the three standard candles, the STIS and NICMOS spectrographs on HST measure the absolute flux of secondary stars from 0.115–2.5 µm. The SEDs of several dozen secondaries and the three primary stars reside in the CALSPEC1 database along with the covariance error matrix of uncertainties. Challenges to achieving the goal of 1% precision in the measured CALSPEC fluxes include non-linearities, changing sensitivity with time, and the high premium on HST time. However, synthetic photometry from the CALSPEC stars agrees with precision Landolt photometry to ~1% (10 mmag) in the B, V, R, and I bands. Model stellar atmospheres that fit these measured SEDs to ~1% extend the wavelength range to the JWST limit of 30 µm.1 www.stsci.edu/hst/observatory/crds/calspec.html
Atomic data for stellar spectroscopy
Heiter, Urike
High-precision spectroscopy of large stellar samples plays a crucial role for several topical issues in astrophysics. Examples include studying the chemical evolution of the Milky Way Galaxy, tracing the origin of chemical elements, and characterizing planetary host stars. Data are accumulating from instruments that obtain high-quality spectra of stars in the ultraviolet, optical and infrared wavelength regions on a routine basis. The interpretation of these spectra is often based on synthetic stellar spectra, either calculated on the fly or taken from a spectral library. One of the most important ingredients of these spectra is a set of high-quality transition data for numerous species, in particular neutral and singly ionized atoms. We rely heavily on the continuous activities of laboratory astrophysics groups that produce and improve the relevant experimental and theoretical atomic data. I will give an overview of current efforts to compile, assess, and distribute the best available data in a standard way, for example in the context of the Gaia-ESO Public Spectroscopic Survey. This work is facilitated by databases and electronic infrastructures such as the NIST Atomic Spectra Database, the VALD database, or the Virtual Atomic and Molecular Data Centre.
Discovery of stars with the perfect black body spectrum and their use for calibrations
Fukugita, Masataka
We discovered 17 stars that exhibit the spectrum ofnearly perfect black-body radiation to the accuracy of 1\% level.We found that these stars are DB white dwarfswith temperature 7000K to 12000K, too low to develophelium absorption features, as confirmed withexplicitly constructing atmospheremodels with predominantly helium. These stars can beused to examine the accuracy of the AB magnitudesystem across various wavelength passbands. We foundthat the SDSS magnitude system provides an accuratephotometric standard from the u to the z band, whenthe original zero points are adopted without adding anyoffset constants, which would in turn make the systemtilted.
Strategies for flux calibration in massive spectroscopic surveys
Allende Prieto, Carlos
Optical large-scale medium-resolution spectroscopic surveys such as SDSS, LAMOST, DESI, WEAVE or 4MOST are subject to constraints that limit the choice of flux calibrators, and the attained precision. The use of optical fibers, a large but limited field of view, the tiling strategies and tight schedules, are all factors that call for a careful evaluation of the flux calibration procedures. These and other issues such as the current state of the art for model atmospheres and model spectra, or the current knowledge available for potential calibration targets, must be considered and will be discussed in this presentation.
LSST's Prototype Collimated Beam Projector
Coughlin, Michael
One of the main components of the LSST's calibration hardware system is a ``collimated beam projector,'' which is designed to measure a telescope's instrumental response function versus wavelength. This response function, along with atmospheric transmission, are the key components of photometric calibration. The collimated beam projector projects monochromatic light, monitored with a NIST-calibrated photodiode, through a mask and a collimating optic onto the telescope. In this presentation, we describe results from this prototype system at the CTIO 0.9 meter telescope. We discuss the measurement of the telescope's instrumental response function, including the optical system and camera, to 5% precision. As a cross-check of our system's accuracy, we compare the measurements of the filters to nominal values from the vendor.
A network of faint DA White Dwarfs as Spectrophotometric Standards
Saha, Abhijit
DA white dwarfs with aurface temperatures higher than 20,000K are believed to have pure hydrogen radiative atmospheres, which are the least complicated to model. Spectroscopic analysis of their Balmer lines can be used to derive their surface temperature and gravity, which in turn predict the spectral energy distribution (SED) of the radiation emerging from their surface. Three such realtively bright stars, near enough to have no compliactions from interstellar reddening, are currently used as absolute flux calibrators for HST. I will describe an experiment undertaken with my colleagues which combines such spectroscopic characterization from the ground of two dozen fainter (V > 17) DA white dwarfs spread across the sky with panchromatic photometry obtained using HST (avoiding effects from the terrestrial atmosphere that have plagued past standardization work) to validate these model predictions. These fainter stars, which are at larger distances, are affected by interstellar reddening: the comparison of measured colors against model predictions must allow for this (by solving for it) within the constraints placed by the interstellar reddening law. We find sub-percent agreement of photometry with model predictions for ~90% of the objects. These objects, which were selected to be within the dynamic range of extant and future large area surveys are thus established as sub-percent SED standards. We are currently extending our calibartion to the southern skies. When completed, the distribution in the sky of this network of standards will be such that at any instance, from any site on Earth at least two of them are visible at airmass < 2.
The SkyMapper Southern Survey and its calibration
Wolf, Christian
I will present the progress of the SkyMapper Southern Survey with a focus on photometric calibration issues, highlighting all the challenges we have confronted so far. The SkyMapper Southern Survey covers over 20,000 deg2 of Southern sky in six optical passbands. It spans a dynamic range of 13 mag by combining its Shallow Survey of shorter exposures with its deeper Main Survey, both covering the whole hemisphere. We also have standard fields with HST spectrophotometric standards in them, and cross-matched our catalogue with external catalogues such as PS1 and Gaia.
The ASTRA Spectrophotometric Telescope
Adelman, Saul
The first scientific observations from the ASTRA Spectrophotometer and its 0.5-m automated telescope should be available this summer. ASTRA should observe stars whose declination range is at least +85° to -28°. The instrument will obtain flux calibrated spectra at classification dispersion. From these fluxes studies of the continuum spectra can be performed and the strongest lines can be measured. A small CCD camera finds and centers the target and then guides on the zeroth order spectrum. The spectrophotometer uses both a grating and a cross-dispersing prism to produce spectra from both orders simultaneously. In an hour exposure time the system should obtain S/N = 200 observations of stars as faint as 9.5 mag. Vega will require about 25 seconds for the exposures and readout of both orders. The telescope will find its next target in less than a minute. The square 30 arc second sky fields for each order do not overlap. The resolution is 7 Å in second and 14 Å in first order. The wavelength range is approximately ??3300-9000. We initially will use about 10 minutes/hour to observe Vega and secondary standard candidates. Our scientific CCD is electronically cooled to -50° C with a water recirculation system heat sink. The same 4° C recycling water system provides thermal stabilization of the instrument. The ASTRA telescope is rocked to expose the image from the top to the bottom of the entrance aperture. Three LINUX servers will have databases of ASTRA observations. ASTRA can observe standard stars at a regular rate throughout the night, any accessible target at a given time, variable stars, slow moving targets, and targets of opportunity. ASTRA will produce considerable high-quality data: of order 15000 standard stars and 40000 target objects per year. The four P. I.’s will operate the system. Due to the large volume of data expected each year, they will work in collaboration with other astronomers to make the best scientific usage of ASTRA.
Standardization in the UV with Astrosat and its issues related to star cluster studies
Shah, Priya
The Ultra-Violet Imaging Telescope (UVIT) is one of the payloads in Astrosat, the first Indian Space Observatory. The UVIT instrument has two 375 mm telescopes: one for the far-ultraviolet (FUV) channel (1300–1800 °A), and the other for the near-ultraviolet (NUV) channel (2000–3000 °A) and the visible (VIS) channel (3200–5500 °A). In this paper, I shall discuss the issues with standardization in the UV with reference to Astrosat Observations (Cycle A04) made my me and a collaborator of young stars in the halo of the Milky Way. I shall discuss the problems faced in data-analysis and how these in turn lead to serious issues dealing with the color-magnitude diagarms, membership and age of the young embedded clusters I studied.
StarDICE: An artificial star for absolute broadband flux calibration
Betoule, Marc
StarDICE is an absolute flux calibration experiment hosted by the Observatoire de Haute Provence (OHP), whose purpose is to confront the broaband calibration of the CALPSEC spectral library to the instrumental flux scale established by the NIST. It consists in the use of a stable, convenient and NIST-calibrated polychromatic light-source called DICE to measure the broadband response to point-like illumination of a short focal length robotic telescope. The dedicated telescope is then used to follow the evolution of the broadband fluxes of spectrophotometric standard stars as a function of airmass over a very large number of nights. The experiment is currently in the last stage of its test phase whose goal is to evaluate the accuracy of the calibration transfer accross the atmosphere.In this contribution, we will review the experiment, present early results from the test phase and discuss prospects for the next phase.
The Stagger-grid: synthetic spectra and broad-band photometry with 3D model stellar atmospheres across the Hertzsprung-Russell diagram
Chiavassa, Andrea
Stars are not smooth. Their photosphere is covered by a granulation patterns associated with heat transport by convection. These structures have different size, depth, and temporal variations across stars in the Hertzsprung-Russell diagram. Crucially, convection shapes the atmospheric structure and the emergent flux we observe. Our 3D radiative hydrodynamical simulations of stellar convection with the STAGGER code are now able to model these complex phenomena across most of the Hertzsprung-Russell diagram. We present a library of high-resolution stellar synthetic fluxes obtained from these 3D simulations (Chiavassa et al. 2018). Our spectra show small but significant differences with respect to the emergent flux predicted by 1D hydrostatic models: we discuss these effects in terms of synthetic colors, the implication of convective shifts for precise radial velocity measurements, and the broader implication of our models for calibration works.
EXOFIND: RV fitting with stellar activity and reliable Bayesian model comparison
Rukdee, Surangkhana
Upcoming cm-precision radial velocity (RV) surveys aim to detect Earth mass planets orbiting low-mass cool stars. Stellar activity can however easily corrupt the small RV signal. Additionally, most current RV fitting methods cannot reliably decide whether the data support 0,1,2 or 3 planets, as the Extremely Precise Radial Velocities (EPRV) Evidence Challenge recently uncovered. Our EXOFIND Python package aims to solve these two problems: 1) We characterize a red noise Gaussian process from stellar activity indicators and apply it to the RV shifts while propagating uncertainties. 2) For rigorous model comparison (0,1,2,3 planets), we use nested and importance sampling. With real and simulated data, we verify that our approach is practical to run and avoids false positives. EXOFIND will be released as open source software to the community.
Radial Velocity Measurements with m/s Precision in the Visible and Near-IR
Quirrenbach, Andreas
Precise radial velocity measurements require careful attention to detail not only in the construction of the spectrograph, but also in the calibration and data reduction procedures. While laser frequency combs provide highly stable wide-band wavelength references, the combination of hollow-cathode lamps with Fabry-Pérot etalons is a cost-effective alternative. To ensure stable coupling of starlight as well as calibration light into the spectrograph, the light-scrambling properties and modal behavior of the fiber feeds must be well understood. Unexpected problems lurk in seemingly innocent active components such as steering and switching mirrors in the front end and calibration unit, as higher-order errors due to coupling between misalignments and fiber modes cannot be neglected.Detectors are never perfect, and near-infrared arrays present a set of challenges that is not encountered in CCDs, associated e.g. with cosmetic defects, electronic ghosts, and – most insidiously – memory effects. The latter place strong constraints on the calibration strategy, as exposing the detectors to bright lines of calibration sources shortly before taking science data must be avoided.Equal care is required in the data reduction, from processing of the raw data to extraction of the spectra, barycentric correction (requiring precise knowledge of the photon-weighted midpoint of the exposure), and computation of the radial velocity with a suitable cross-correlation technique. Compared to spectrographs working mostly in the blue, red-sensitive instruments are much more sensitive to subtleties in the treatment of telluric contamination, as non-optimal approaches can lead to systematic periodic effects on the several m/s level.We will discuss the end-to-end process of obtaining highly precise radial velocities over a wide wavelength range based on our experience with CARMENES, the first spectrograph that has been optimized for exoplanet searches in the red and near-infrared (0.52-1.72µm).
Prospects for a UV High-Resolution Theoretical Stellar Spectral Library for Old Stellar Systems
Peterson, Ruth
Many Fe I levels have unknown energies, wreaking havoc on attempts to model the UV in solar-type stars. Stellar UV spectra have alleviated this somewhat: Peterson & Kurucz 2015 (ApJS, 216, 1) and Peterson, Kurucz, & Ayres 2017 (ApJS, 229, 23) have now published identifications and line lists for 124 previously unknown Fe I levels, and gf-values good to ~0.1-0.2 dex for sufficiently strong and unblended newly-identified Fe I lines.However, our theoretical calculations still seriously underestimate the UV fluxes of early-G stars in regions where many unknown Fe I lines remain, notably blueward of 2700A (PK15, Fig. 1). This prevents the determinations of reliable elemental abundances for several atomic elements that have lines in the UV exclusively. Our work also fails to reproduce the UV flux distributions of nearby stars and distant galaxies, unless the Kurucz predicted line list is invoked, with errors of 10's of A in line positions.Here we outline steps we are taking in the ongoing HST programs GO-14161 and GO-15179 to remedy this situation. When complete, these programs will provide additional echelle UV spectra that will help to identify the strong unknown lines remaining in the UV, and will also provide observational template echelle UV spectra for stars at the turnoff of old stellar systems of solar and near-solar metallicity. Our ultimate goal is to generate from these, and from the Fe I levels discovered from them, a reliable grid of theoretical stellar UV spectra at a resolution of 30,000 or better. The anticipated grid should include stars ranging from the hot blue horizontal branch stars and blue stragglers of old, metal-poor, relatively nearby systems, to the turnoff F- and early G-type stars of systems of solar metallicity and higher in remote, red-and-dead galaxies at low to intermediate redshifts.
Precise measurements of galaxy spectral energy distributions
Brown, Michael
We present spectral energy distributions of individual galaxies and quasars spanning from 0.1 to 35 microns (and in some instances 0.1 to 300 microns). Spectral energy distributions are used for photometric redshifts, k-corrections, star formation rate calibrations, exposure time calculators, modelling object selection and validating photometry. The spectral energy distributions incorporate (but are not limited to) photometry and spectroscopy from FUSE, IUE, Hubble, SDSS, PanSTARRS, Skyampper, IRTF, GNIRS, XShooter, Akari, Spitzer and WISE. Accurate calibration and measurement of photometry and spectrophotometry was critical to the construction of our spectral energy distributions, and we identified and mitigated background over-subtraction, filter curve errors and photon coincidence losses (for extended sources). The WISE W4 effective wavelength error in particular illustrates how non-stellar sources can be used to identify and mitigate calibration errors. The templates have also been used to identify and mitigate wavelength dependent errors in GAMA survey photometry.
Passband reconstruction from photometry
Jordi, Carme
Based on an initial expectation from laboratory measurements or instrument simulations, photometric passbands are usually subject to refinements. These refinements use photometric observations of astronomical sources with known spectral energy distribution. This work investigates the methods for and limitations in determining passbands from photometric observations. A simple general formalism for passband determinations from photometric measurements is derived. The problem of passband determination is formulated in a basic functional analytic framework. For the solution of the resulting equations, functional principal component analysis is applied. We find that, given a set of calibration sources, the passband can be described with respect to the set of calibration sources as the sum of two functions, one which is uniquely determined by the set of calibration sources, and one which is entirely unconstrained. The results are applied to the passbands of HIPPARCOS, Tycho, and Gaia.
All sky photometric zero-points from stellar effective temperatures
Casagrande, Luca
I use SkyMapper DR1 to explore the quality of its uvgriz photometry, and zero-points across the sky. Physical flux transformations, and zero-points appropriate for this release are derived. I introduce a formalism to derive photometric zero-points across the sky by benchmarking against stars with well known effective temperatures, bypassing the need for absolute spectrophotometry.
Calibration Challenges for WFIRST
Casertano, Stefano
Large-scale survey programs represent a significant part of the scientific motivation for the WFIRST mission, the intended NASA successor to the Hubble and Webb observatories. The goals of the large-scale surveys include very high precision and fidelity determination of cosmological parameters, especially the parameters of the dark energy equation of state, as well as the determination of the frequency of exoplanets down to sub-Earth masses. Achieving these goals will require unprecedented precision in the calibration of both photometry and astrometry, as well as other instrumental parameters such as detector effects and the instantaneous Point Spread Function. We present a thorough, quantitative review of the calibration needs for the mission, as well as a comprehensive plan to achieve these goals with a combination of pre-launch characterization, laboratory testing, in-flight data, and external observations.
Gaia Photometric Catalogue: the calibration of the DR2 photometry
Evans, Dafydd
Gaia DR2 was released in April 2018 and contained a photometric catalogue of more than 1 billion sources. This release contains colour information in the form of integrated BP and RP photometry in addition to the latest G-band photometry. The level of accuracy can be as low as 2 mmag with some residual systematics at the 10 mmag level. This addition of colour information greatly enhances the value of the photometric data for the scientific community. A high level overview of the photometric processing, with a focus on the improvements with respect to Gaia DR1, will be given. The definition of the Gaia photometric system, a crucial part of the calibration of the photometry, will also be explained. Finally, some of the photometric improvements expected for the DR3 data release will be described.
Calibration techniques for solar spectral irradiance (SSI) instruments
Snow, Martin
The Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado has a long history of making solar spectral irradiance (SSI) observations from space. We have flown spectrometers that measure the ultraviolet wavelength range starting with the Solar Mesosphere Explorer, launched in 1981. UV observations continued with the Solar-Stellar Irradiance Comparison Experiments on the Upper Atmosphere Research Satellite (1991-2005) and Solar Radiation and Climate Experiment (2003-present). LASP has also built the Extreme Ultraviolet and X-ray Irradiance Sensors instruments on the GOES-R series (2017-present). We began making observations in the visible and infra-red with the Solar Irradiance Monitor on SORCE (2003-present) and continue this data record with the Total and Spectral Irradiance Sensor (2017-present). CubeSat sized compact versions of both the SIM and SOLSTICE instruments have been developed and calibrated and await launches in 2018.All of these instruments undergo extensive ground calibration using SI-traceable standards. In the case of the UV instruments, we make extensive use of the Synchrotron Ultraviolet Radiation Facility at the National Institutes of Standards and Technology. For longer wavelengths, we have developed calibration facilities at LASP using NIST Spectral Irradiance and Radiance responsivity Calibration using Uniform Sources (SIRCUS) lasers and cryogenic radiometer. This talk will describe the calibration techniques that we use and the absolute uncertainties that we have achieved with each instrument and facility.
Photometric Calibration of the Dark Energy Survey
Smith, J. Allyn
The Dark Energy Survey (DES) is a 5000 sq deg grizY imaging survey of the Southern Galactic Cap being conducted with the 570-megapixel Dark Energy Camera (DECam) on the Blanco 4-m telescope at Cerro Tololo Interamerican Observatory (CTIO). In January of this year, DES publicly released its first 3 years of coadd data, and, in February, it completed its fifth and final full observing season of on-sky operations. By the end of the processing campaign for the first year data set, DES had already achieved better than 2% photometric calibrations over the survey area. By the end of the processing campaign of the first three years of data, DES had achieved sub-percent photometric calibrations over the full DES footprint, with the promise of better than 0.5% photometry for the full 5-year data set. Here, we discuss the survey strategy, the ancillary calibration hardware, and the software that allowed us to achieve this calibration performance.
Absolute Spectral Calibration of Vega and Sirius
Deustua, Susana
Absolute flux calibration of standard stars, traceable to the International System of Units (SI) standards, is essential for 21st century astrophysics. Dark energy investigations that rely on observations of Type Ia supernovae and precise photometric redshifts of weakly lensed galaxies require a minimum accuracy of 0.5 % in the absolute color calibration. Research that address fundamental stellar astrophysics also benefit. In the era of new, sensitive telescopes designed for deep, sky surveys, e.g. WFIRST, well-characterized, SI-traceable standard stars that are available over the whole sky AND over a broad wavelength range (e.g. from the UV to the IR), and that do not saturate are needed. Prior work to obtain absolute flux measurements of fundamental standards: Vega, Sirius and others, achieved total uncertainties of ~1% to 3%, depending on wavelength. I will describe early results from a plogram to determine the top-of-the-atmosphere absolute spectral irradiance of bright stars wherein we have developed a novel, fully SI-traceable laboratory calibration strategy that will enable achieving the demanding 0.5% requirement. This strategy has two key components: 1) An SI-traceable calibration of the entire instrument system, and 2) repeated spectroscopic measurements of the target star throughout the night.
FUV Spectrophotometry with the Cosmic Origins Spectrograph
De Rosa, Gisella
The Cosmic Origins Spectrograph (COS) was installed on the Hubble Space Telescope (HST) in May 2009. COS is designed to perform high-sensitivity spectroscopy in the far-ultraviolet (FUV) and near-ultraviolet (NUV) regimes with low-/medium resolution gratings. The COS FUV detector consists of two microchannel plates (MCPs). The spectrophotometric calibration of the FUV channel is complicated by several factors. First of all, the MCP’s efficiency at converting incoming photons into detectable events decreases with usage. This depletion of the detector’s gain (i.e. gain sag) results in unusable regions of the COS/FUV detector. The main culprit of gain sag is Lya geocoronal emission, even if all observations lead to gain sag. In order to mitigate the gain sag, a number of strategies have been employed over the instrument life time, which range from moving to different positions on the detectors, to managing the high voltage to extract a smaller amount of charge, to re-distributing the cenwave usage, to limiting the exposure of the affected detector to the geocoronal Lya. Moreover, the sensitivity of the COS FUV detector declines with time, with fluctuating rates that seem to correlate with the solar activity. Even taking all of this into account, the COS FUV sensitivity is still excellent, and with appropriate calibrations a local spectrophotometric precision of ~1.5% can be consistently reached over the whole wavelength range, as shown by an AGN reverberation mapping program performed in 2014 during Cycle 21 (PID 13330, De Rosa et al. 2015).
Las Campanas Stellar Library
Chilingarian, Igor
Stellar libraries are fundamental tools required to understand stellar populations in star clusters and galaxies as well as properties of individual stars. Comprehensive libraries exist in the optical domain, but the near-infrared (NIR) domain stays a couple of decades behind. Here we present the Las Campanas Stellar Library project which is the largest collection of signal-to-noise intermediate-resolution (R=6500) NIR spectra (0.83
Perfect Blackbody Spectra for Next Generation UV-Opt-IR Standard Star Network
Suzuki, Nao
In the era of precision cosmology, we are in need of advancing photometric standard star network system. Especially, Type Ia Cosmology is now being limited by systematic errors and one of the majour sources is the photometric standard stars and their models. Also, we are in need of bridging UV-Opt system (ABmagnitude) and IR system which most of the standars are still in Vega magnitude system. We would like to propose to make use of stars with nearly perfect blackbody spectra. We identified a dozen of blackbody stars from the Sloan Digital Sky Survey and they are consistent with balckbody from UV (GALEX), Optical (SDSS and Pan Starrs) through IR (WISE). With only two parameters (Temperature and Normalization), we can describe UV-Opt-IR magnitude and its error in subpercent level and avoid propagation of errors from the detectors. These are special type of white dwarfs and GAIA satellite provide us their distances which enables us to deduce their radius. We present its potential use in this presentation.
Towards a better scattered moonlight model and aerosol extinction determination at Cerro Paranal
Jones, Amy
Estimating the sky background is critical for ground-based astronomical research. In the optical, scattered moonlight dominates the sky background, when the moon is above the horizon. The most uncertain component of a scattered moonlight model is the aerosol scattering. The current, official sky background model for Cerro Paranal uses an extrapolated aerosol extinction curve. With a set of X-Shooter sky observations, we have tested the current model as well as determined the aerosol extinction from UV to NIR. These observations were taken of plain sky, during three different lunar phases, and at six different angular distances from the moon for each night/lunar phase. Using a set of models with varying aerosol distributions to compare with the observations, we found the most likely aerosol extinction curves, phase functions, and volume densities for the three nights of observations. While there were some degeneracies in the aerosol scattering properties, in general the current model had significantly less coarse particles compared to the favored volume densities from the X-Shooter data. This affects the phase function by being more peaked at small angular distances. Also the extinction curves flatten towards redder wavelengths and are overall less steep compared to the extrapolated curve used in the official model. Overall, the current model does reproduce the observations for average conditions decently well. For the three nights of sky observations, the aerosol distributions differed reflecting the changes in atmospheric conditions and aerosol content, which is expected. These changes among the three nights seemed to correlate with the atmospheric conditions recorded at the site. Using sky observations and the sky background model is a unique way to probe the aerosol content of the atmosphere.
Closing the Photometric Loop: Estimating top-of-atmosphere fluxes from ground based telescopes
Mondrik, Nicholas
For a ground based observatory, the observed flux of an astrophysical source is the product of its spectral energy distribution, the Earth's atmospheric transmission, and the instrumental response. Comparing space-based spectrophotometric observations and ground-based photometry of the same stars, one should be able to reconstruct the observed brightness of the source in a given band pass if the atmospheric and instrument transmission functions are measured. We describe some results of our work to 'close the photometric loop' for HST CALSPEC stars using data obtained with the CTIO 0.9 m telescope. The instrumental response is measured in situ with the Collimated Beam Projector, and spectroscopy of telluric standards provides an estimate of the atmospheric behavior.