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THE STANDARD STAR NEWSLETTER

$\star$

An electronic publication of the Working Group on Standard Stars

(IAU Commissions 25, 29, 30, 45)

width 0.5pt 1.2cm No. 41 width 0.5pt depth 0.5pt editor: Richard O. Gray
October 2006 grayro@appstate.edu width .3pt depth .3pt width 0pt

CONTENTS:

Editorial p. 1
Note from the Working Group Chair, Chris Corbally p. 2
Meeting Report: WGSS p. 2
Obituary: Laura E. Pasinetti p. 3
Abstracts of Papers: Blankenberge Standardization meeting p. 4

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From the editor

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The surprising length of the current newsletter (23 pages, a record) reflects the vitality of our field and underlines the need for an effective means of communication amoung the many workers in standardization. This was the topic of our discussion at the Working Group on Standard Stars (WGSS) meeting in Prague, during the recent IAU General Assembly. The chair of the working group, Chris Corbally, reports on that meeting below, but I want to emphasize two important innovations that are being introduced. First, the Standard Star Newsletter is going over to a web format. What this means is that contributions to the newsletter will be published almost immediately on the Standard Star Newsletter website (http://stellar.phys.appstate.edu/ssn), and then the newsletter will be ``crystallized'' into PDF format on the regular 6 month schedule. This, we hope, will make the newsletter a much more valuable and immediate tool for communication. This issue is the first to be available, not only in the traditional PS and PDF formats, but also in web format. After this, draft versions of the newsletter will be available on the web, and will be updated everytime a new submission is made.

Second, to make communication even more rapid, a ``Quick Topic'' web discussion site has been opened up; all readers of the SSN are invited to become subscribers and to contribute to the discussion. To do so, visit the Standard Star Newsletter website. I have started a topic that I hope will be sufficiently controversial to attract alot of discussion. That topic is: ``Is it time to abandon Vega and find another primary spectrophotometric standard?''. I say yes, we should have done it a long time ago. What is your opinion? Extracts from the discussions, if they materialize, will be printed in the next newsletter.

The abstracts in this newsletter all come from the standardization meeting organized by Chris Sterken and held last May in Blankenberge, Belgium. Chris very kindly submitted all these abstracts using our new web submission form; there are 46 abstracts in all. After the submission of the last one, Chris collapsed on his keyboard and has not been heard from since. Many thanks, Chris (and the LOC), for organizing such a valuable and well-run meeting, and thank you for preparing these abstracts for the Standard Star Newsletter! The proceedings of this meeting will appear as an Astronomical Society of the Pacific Conference series sometime in 2007.

On a more solemn note, it is my sad duty to report the passing of the first editor of the SSN, Laura Pasinetti, on 12 September, 2006. A tribute to her life and career in astronomy is printed below.

Richard Gray, editor
grayro@appstate.edu

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A Note From the Chair

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International Astronomical Union

Working Group on Standard Stars (WGSS)

This is literally just a note since this newsletter includes my report of the WGSS's meeting in Prague. I mainly should like to thank those couple of dozen people who managed to attend this meeting.

The result of their ``ideas and wisdom", as I put it in the last newsletter when I invited your attendance, was (1) to affirm the importance of the WGSS as a facilitator of communication among those engaged in work on standards and (2) to make practical suggestions as how this role could be made more effective. Some of these suggestions have already been implemented, as you will have heard from Richard Gray in his message to us all on September 14.

Of course, none of these web-based aids to communicate among ourselves, and to broaden the network, will work without your regular participation and without your encouraging your co-workers in standards to participate. For a start, do please sign up right now as a "subscriber" at http://www.quicktopic.com/37/H/eUSTyWsHcXJ (or via the link at the SSN webpage).

I wish you pleasant and productive web-conversations,

Chris Corbally corbally@as.arizona.edu

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Meeting Report

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Report on the Meeting of the Working Group on Standard Stars
IAU XXVIth General Assembly, 16 August 2006, Prague

The meeting of the working group on standard stars (WGSS) opened under the chairmanship of Corbally. Gray, as editor, gave a report on the Standard Stars Newsletter (SSN). Through a round of thanks given to Gray, it was recognized by the two dozen present that this publication is the major vehicle between WGSS meetings for achieving its tasks, namely promoting and communicating work on standard stars.

The meeting broadly considered the status of work on standard stars and then was largely devoted to a discussion on evolving the WGSS and the SSN with the times. It was felt that, with the development of new photometric and spectrophotometric systems by the large surveys, there was even more need for communication among the various survey teams and from these teams to the individual researchers, who would use the survey databases. Practical proposals included:

1. promoting a web-based bibliography of papers on standard stars, such as the current Standard Objects For Astronomy site (http://sofa.astro.utoledo.edu/SOFA/);
2. expanding the WGSS membership, especially to those working on the new, large survey systems;
3. making and promoting a web-based discussion forum for standard star issues;
4. changing to the ApN's strategy of posting contributions immediately to a website, and ``crystallizing" these into a newsletter issue after 6 months;
5. communicate the changed structure and possibilities of interaction to the whole IAU membership, possibly via the IAU newsletter.

In the light of these discussions those present considered that an updated WGSS would continue to fulfil a unique service to astronomy. To help achieve this, Corbally was prevailed upon to continue as chairperson and Gray as newsletter editor.

Christopher J. Corbally

Chairperson of the Working Group

Chris Corbally
corbally@as.arizona.edu

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Obituary: Laura E. Pasinetti

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Laura E. Pasinetti (1935-2006)

On September 12 Laura E. Pasinetti suddenly passed away.

She was one of the organizers in 1982 of the ``Working Group on Standard Stars", sponsored by Commissions 25,29,30,45 of the IAU, and she chaired it for the first three years, moreover she was the founding editor (from 1982 to 1990) of the ``Standard Stars Newsletter".

Laura taught Astronomy at the Milano University for 35 years (1971-2006), and many of her students are now active researchers in Astronomy.

She was very interested in many fields of the Stellar Astronomy: determinations of stellar chemical composition, nuclear evolution and age by means of high resolution spectral analysis and multiband photometry; study of stellar plasma heating by means of X-band observations; study of variable stars and determination of fundamental stellar parameters.

In 1981 along with her husband, the late prof. Massimo Fracassini, she prepared, on behalf of Commission #5 of the IAU, the ``Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS)". This Catalogue is available at the CDS of Strasbourg and at the ADC of NASA. The Catalogue was then constantly updated (3rd edition, 2001). She was just working at a new edition of it in these days. Its completion will be a legacy to her closest collaborators.

All people who were lucky to know her remember her kindness and humanity. We have lost more than a colleague, we have lost a friend.

Luciano Mantegazza
INAF - Oss. Astron. di Brera
Via E. Bianchi, 46
I-23807 Merate
Italy

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Abstracts

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Note: The following abstracts are all from the meeting THE FUTURE OF PHOTOMETRIC, SPECTROPHOTOMETRIC AND POLARIMETRIC STANDARDIZATION, a workshop organised in Blankenberge, Belgium 8-11 May 2006, editor Christiaan Sterken, ASP Conf. Series 2007, in press.

Standardization in the Classical $UBVRI$ Photometric System

A. U. Landolt
Dept. of Physics and Astronomy, Louisiana State University, Baton Rouge, Louisiana U.S.A. 70803-4001

There occur projected against the celestial sphere a wide range of phenomena for which observers and experimenters need intensity and color information. A fundamental datum for celestial objects is their intensity measured along the electromagnetic spectrum. Through considered choices of filters, one may establish photometric systems which isolate portions of the spectrum, and such systems in turn may be used to define color indices. Therefore, there is a long-term need for accurate photometric standard stars, those with known intensities and color indices at a variety of wavelengths.

References to history will set the stage for introduction of the $UBVRI$ photometric system. It provides a common thread with previous centuries of photometry, and can provide similar continuity to the future. Much of this contribution will be a discussion of a set of procedures which may be followed, and some of the pitfalls that may be encountered, in establishing a useful optical photometric system.

Improved Photometric Accuracy and the Creation of an All-sky High-Accuracy Stellar Standard System

Aleksey Mironov, Andrey Zakharov and Anna Ambartsumyan
Sternberg Astronomical institute, Moscow, Russia

Some astrophotometry problems require accuracy within 0.001-0.003 mag, both in the random and systematic sense. Many research teams are planning now to carry out new sky surveys. These surveys will be made in different photometric systems and each photometric system will need its own standards. Thus the main problem of photometric standardization becomes a task to construct the fundamental photometric catalog. Magnitudes of stars in the catalog have to be easily transformed to any specified photometric system without sacrifice of accuracy.

Definition of the SPM Set of Secondary Standards for the Strömgren $uvby\beta$ Photometric System

Omar López-Cruz$^1$, William J. Schuster$^2$, Carlos Chavarría$^2$ and J. Gabriel García$^2$
$^1$ Instituto Nacional de Astrofísica, Optica y Electrónica (INAOE), Coordinación de Astrofísica, Tonantzintla, Puebla, México
$^2$ Universidad Nacional Autónoma de México (UNAM), Instituto de Astronomía, Ensenda, Baja California, México

Intermediate-band photometry has clear advantages over broad-band photometry: its results are closer to those of low resolution spectroscopy. Specifically, the Strömgren $uvby\beta$ photometric system (SPS) was carefully defined to measure physical stellar properties through color indices sensitive to temperature, metallicity, and surface gravity. Additionally, a large number of standards have been defined to guarantee its accessibility. Nevertheless, most of those standards are too bright for even small telescopes, when equipped with CCDs.

In preparation for large aperture facilities such as the Gran Telescopio Canarias (http:// www.gtc.iac.es/home.html) and the forthcoming large telescopes to the Mexican National Observatory in San Pedro Martír, Baja California, México (OAN-SPM, http://www.astrossp.unam.mx), we are conducting a comprehensive survey to define a set of secondary standard stars for the SPS. For this aim, we are conducting simultaneous observations using the OAN-SPM 84-cm telescope + CCD + the Strömgren set of filters and the OAN-SPM 1.5-m telescope + the Danish Spectrophotometer. This strategy has been adopted to make precise transformations to the original standard system with a high degree of accuracy. This is a long-term project that is making efficient use of the smaller OAN-SPM's telescopes and their instrumentation.

In this talk, we present the status of this calibration project, after one year of observations. We also discuss details on the observing strategy, data reductions and transformations. During the presentation, we touch upon extensions and limitations of the SPS as well as applications of faint Strömgren photometry for galactic and extragalactic studies.

The Walraven $VBLUW$ Photometric System
32 Years of 5-Channel Photometry

Jan Willem Pel$^1$ and Jan Lub$^2$
$^1$Kapteyn Astronomical Institute, University of Groningen P.O.Box 800 9700AV Groningen, The Netherlands
$^2$Sterrewacht, Leiden University, P.O.Box 9513 2300RA Leiden, The Netherlands

An overview is given of the Walraven $VBLUW$ photometric system. We concentrate on the aspects of stability, internal and external precison of the standard system, flux calibration and theoretical model grids.

Photometric and Astrometric Calibration of the JWST Instrument Complement

Rosa I. Diaz-Miller Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

In preparation for the James Webb Space Telescope, a set of cross calibration programs with HST and Spitzer for suitable primary photometric standards and astrometric fields was developed. NICMOS/HST and IRAC/Spitzer photometric observations of new solar analog standards in NGC6791 and Melotte 66 were executed. These new photometric standards will provide $\sim 5$% photometric precision at $V\sim 19$ from the near-IR to the mid-IR wavelength range for efficient on-orbit calibration and measuring of photometric stability of the JWST complement. For the astrometric calibration, a field in the LMC has been selected. This field falls within the JWST continuous viewing zone, within $5^{\circ}$ from the ecliptic poles, and has the stellar density necessary to achieve accuracies better than 1 mas with HST/ACS. These independent observations will play a key role in meeting the mission requirements and will allow a fast commissioning of the observatory.

Development of the ${u'g'r'i'z'}$ Standard System

J. Allyn Smith, D. Tucker, S. Allam, B. Yanny, D. Finkbeiner, D. Eisenstein, J. Gunn, G. Knapp and Z. Ivezic

We will describe the development of the $u'g'r'i'z'$ standard star system developed to support the Sloan Digital Sky Survey (SDSS) and how it is used to support the survey. Issues related to the SDSS will be discussed including the offset between $u'g'r'i'z'$ and $ugriz$, the offset to the AB system, and the overall calibration of the SDSS. We will describe the northern hemisphere primary standard system, the southern hemisphere extension, and the current observational program designed to enhance the system.

Variable Stars and Uncalibrated Photometric Data

K. Oláh$^1$ & C. Sterken$^2$
$^1$ Konkoly Observatory, 1525 Budapest, P.O. Box 67, Hungary
$^2$ Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium

We discuss the effects of using unfiltered and uncalibrated photometric data studying different types of variable stars. Photometry without any filter has of very limited value in most cases. The standardization procedure is not an easy task, a wrong transformation may result in spurious variation of the given object, and this error can hardly be discovered. A not suitably chosen comparison star may cause extra light variation of the given variable due to mistreated extinction. Solving eclipsing binary lightcurves the badly (or not) calibrated data would give erroneous results.

The Secondary Standards Programme for OmegaCAM at the VST

Gijs Verdoes Kleijn & Ronald Vermeij, for the Astro-Wise consortium
Kapteyn Astronomical Institute, Groningen, The Netherlands

Early 2007 the OmegaCAM wide-field imager will start operations at the ESO VLT Survey Telescope at Paranal, Chile. A significant fraction of the observing time is dedicated to public surveys covering large areas of the southern sky using mostly the $u'g'r'i'z'$ filters. The photometric calibration of OmegaCAM data through these filters depends on standard star measurements that cover the complete FoV of OmegaCAM.

A catalog fullfilling this requirement has been obtained by extending eight Landolt equatorial fields: SA92, SA95, SA98, SA101, SA104, SA107, SA110, and SA113 with secondary standards. Here we present a first version of this catalog based on data obtained with the WFC at the INT at ING on La Palma. This version will be used for the early verification of OmegaCAM+VST performance. A second, final, version of the catalog will be constructed from OmegaCAM observations during the first year of operations.

Photometry with FORS at the ESO VLT

W. Freudling, M. Romaniello, F. Patat, P. Møller, E. Jehin & K. O'Brien
European Southern Observatory
Karl-Schwarzschild-Strasse 2
85748 Garching bei München,Germany

ESO's two FOcal Reducer and low dispersion Spectrographs (FORS) are the primary imaging cameras for the VLT. Since they are not direct-imaging cameras, the accuracy of photometry which can routinely be obtained is limited by significant sky concentration and other effects.

Photometric standard observations are routinely obtained by ESO, and nightly zero points are computed mainly for the purpose of monitoring the instrument performance. The accuracy of these zero points is about 10%.

Recently, we have started a program to investigate, if and how percent-level absolute photometric accuracy with FORS1 can be achieved. The main results of this project are presented in this paper. We first discuss the quality of the flatfields and how it can be improved. We then use data with improved flat-fielding to investigate the usefulness of Stetson standard fields for FORS calibration and the accuracy which can be achieved.

The main finding of the FORS Absolute Photometry Project program are as follows. There are significant differences between the sky flats and the true photometric response of the instrument which partially depend on the rotator angle. A second order correction to the sky flat significantly improves the relative photometry within the field. Percent level photometric accuracy can be achieved with FORS1. To achieve this accuracy, observers need to invest some of the assigned science time for imaging of photometric standard fields in addition to the routine nightly photometric calibration.

Overview of Photometric Calibration of LSST Instruments and Data

David L. Burke, for the LSST Collaboration
Kavli Institute for Particle Astrophysics and Cosmology
Stanford Linear Accelerator Center,
Stanford University, Stanford, CA 94309, USA

Science studies made by the Large Synoptic Survey Telescope will reach systematic limits in nearly all cases. Requirements for accurate photometric measurements are particularly challenging. Advantage will be taken of the rapid cadence and pace of the LSST survey to use celestial sources to monitor stability and uniformity of photometric data. A new technique using a tunable laser is being developed to calibrate the wavelength dependence of the total telescope and camera system throughput. Spectroscopic measurements of atmospheric extinction and emission will be made continuously to allow the broad-band optical flux observed in the instrument to be corrected to flux at the top of the atmosphere. Calibrations with celestial sources will be compared to instrumental and atmospheric calibrations.

All-Sky Photometric Surveys

Arne A. Henden$^1$ & Michael P. Sallman$^2$
$^1$ AAVSO, 25 Birch St., Cambridge MA 02138 USA
$^2$ 981 County Rd. C2 W., Roseville MN 55113 USA

There currently exists only one optical all-sky photometric multiwavelength survey: Tycho2. There are pieces of other surveys that can be usable (such as the Sloan Digital Sky Survey) but there exists a need for deeper photometry. This paper describes the problems and a possible solution for an intermediate-depth survey.

Calibration of the Pan-STARRS $3\pi$ Survey

Eugene Magnier and the Pan-STARRS Team
Univerisity of Hawaii, Institute for Astronomy, 2680 Woodlawn Dr., Honolulu, HI 96822, USA

The Pan-STARRS project will begin a large-scale survey in early-2007 using the prototype telescope PS1 currently being commissioned on the summit of Haleakala. A major goal of this survey is to provide a calibration reference grid, the Pan-STARRS Astrometric & Photometric (AP) Reference Catalog, for eventual observations from the full Pan-STARRS 4 telescope system, which will perform a survey beginning in the 2009 timeframe. The precision requirements for internal relative photometry for this reference catalog are an ambitious 10 millimagnitudes for the $riz$ filters, only slightly relaxed for the $g$ and $y$ filters. We have been working with the Canada-France-Hawaii Telescope (CFHT) Legacy Survey / MegaPrime photometric standards team to guide the design of the photometric calibration procedures and to assess the likely sources of systematic errors.

The current photometry from the CFHT MegaPrime standard star analysis demonstrates relative photometric residuals of better than 10 millimagnitudes, with systematic errors close to the Pan-STARRS goals. The Pan-STARRS $3\pi$ Survey will implement the lessons learned from CFHT and make use of additional external atmospheric transparency measurements, as well as an innovative flat-field screen, in an attempt to push the photometric accuracy below the 10 millimagnitude level. In this article, we discuss the plans for the Pan-STARRS PS1 Survey photometry calibration. The resulting photometric reference catalog will provide a highly accurate and dense reference system for future science observations in the entire $3\pi$ steradians north of Declination $-30$.

A Comparison of SDSS Standard Star Catalog for Stripe 82 with Stetson's Photometric Standards

Z. Ivezic$^{1}$, J. A. Smith$^{2}$, G. Miknaitis$^{3}$, H. Lin$^{3}$, D. Tucker$^{3}$, R. Lupton$^{4}$, G. Knapp$^{4}$, J. Gunn$^{4}$, M. Strauss$^{4}$, J. Holtzman$^{5}$, S. Kent$^{3}$, B. Yanny$^{3}$, D. Schlegel$^{6}$, D. Finkbeiner$^{7}$, N. Padmanabhan$^{6}$, C. Rockosi$^{8}$, M. Juric$^{4}$, N. Bond$^{4}$, B. Lee$^{6}$, S. Jester$^{9}$, H. Harris$^{10}$, P. Harding$^{11}$, J. Brinkmann$^{12}$, D. York$^{13}$, for the SDSS Collaboration

$^1$ Department of Astronomy, University of Washington, Seattle, WA 98115
$^2$ Department of Physics & Astronomy, Austin Peay State University, Clarksville, TN 37044
$^3$ Fermi National Accelerator Laboratory, P.O. Box 500, Batavia, IL 60510
$^4$ Princeton University Observatory, Princeton, NJ 08544
$^5$ New Mexico State University, Box 30001, 1320 Frenger St., Las Cruces, NM 88003
$^6$ Lawrence Berkeley National Laboratory, MS 50R5032, Berkeley, CA, 94720
$^7$ Department of Astronomy, Harvard University, 60 Garden St., Cambridge, MA 02138
$^8$ University of California-Santa Cruz, 1156 High St., Santa Cruz, CA 95060
$^{9}$ School of Physics and Astronomy, University of Southampton, Highfield, Southampton, SO17 1BJ, UK
$^{10}$ U.S. Naval Observatory, Flagstaff Station, P.O. Box 1149, Flagstaff, AZ 86002
$^{11}$ Department of Astronomy, Case Western Reserve University, Cleveland, Ohio 44106
$^{12}$ Apache Point Observatory, 2001 Apache Point Road, P.O. Box 59, Sunspot, NM 88349-0059
$^{13}$ University of Chicago, Astronomy & Astrophysics Center, 5640 S. Ellis Ave., Chicago, IL 60637

We compare Stetson's photometric standards with measurements listed in a standard star catalog constructed using repeated SDSS imaging observations. The SDSS catalog includes over 700,000 candidate standard stars from the equatorial stripe 82 ($\vert$Dec$\vert<$ 1.266$^{\rm o}$) in the RA range 20h 34$^\prime$ to 4h 00$^\prime$, and with the $r$ band magnitudes in the range 14-21. The distributions of measurements for individual sources demonstrate that the SDSS photometric pipeline correctly estimates random photometric errors, which are below 0.01 mag for stars brighter than (19.5, 20.5, 20.5, 20, 18.5) in ugriz, respectively (about twice as good as for individual SDSS runs). We derive mean photometric transformations between the SDSS gri and the BVRI system using 1165 Stetson stars found in the equatorial stripe 82, and then study the spatial variation of the difference in zeropoints between the two catalogs. Using third order polynomials to describe the color terms, we find that photometric measurements for main-sequence stars can be transformed between the two systems with systematic errors smaller than a few millimagnitudes. The spatial variation of photometric zeropoints in the two catalogs typically does not exceed 0.01 magnitude. Consequently, the SDSS Standard Star Catalog for Stripe 82 can be used to calibrate new data in both the SDSS ugriz and the BVRI systems with a similar accuracy.

The SkyMapper Southern Sky Survey

S. Keller, M. Bessell, B. Schmidt & P. Francis
RSAA, The Australian National University, Mount Stromlo, Cotter Rd, Weston, ACT 2611, Australia

SkyMapper is an automated 1.3-m telescope with an 5.7-square degree field of view being built at Siding Spring Observatory. It will have 16K$\times16K$ 0.5-arcsec pixels and conduct a multi-color ( $u,v{_s},g,r,i,z$), multi-epoch (4hr, 1 day, 1 week, 1 month, 1 yr sampling) survey of the southerly 2$\pi$ steradian to below 22 magnitude. It will provide star and galaxy photometry to better than 3% global accuracy and astrometry to better than 50 mas. Data will be supplied without proprietary period as part of Virtual Observatory work. The survey will take 5 years to complete.

Photometric Calibration of the MOSAICII and WFI Mosaic Cameras

M. Monelli$^1$, G. Bono$^2$, A. R. Walker$^3$ A. Munteanu$^4$, R. Buonanno$^5$, F. Caputo$^2$, C. E. Corsi$^2$, P. Francois $^6$, M. Nonino$^7$, L. Pulone$^2$, H. A. Smith$^8$, P. B. Stetson$^9$ & F. Thévenin$^{10}$
$^1$ IAC-Instituto de Astrofísica de Canarias
$^2$ INAF-Osservatorio Astronomico di Roma
$^3$ Cerro Tololo Inter-American Observatory
$^4$ ICREA, University of Barcelona
$^5$ Dipartimento di Fisica, Universitá di Roma Tor Vergata
$^6$ Observatoire de Paris
$^7$ INAF-Osservatorio Astronomico di Trieste
$^8$ Dept. of Physics, Michigan State University
$^9$ Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics
$^{10}$ Observatoire de la Côte Azur

In the framework of a large program aimed at studying the static and variable stars in the Carina dSph galaxy, we present the photometric calibration of two mosaic cameras adopted in this study, namely the MOSAICII camera available on the 4m CTIO telescope, and the WFI of the 2.2-m ESO telescope. In particular, we investigated the occurrence of a positional effect, which affects the magnitude estimates according to the position of the star in the chip. Such effect appears to be relevant for the WFI only, and we present polynomial functions to correct it, for different passbands.

Calibration of Wide Field Imagers

G. Bono
INAF - Rome Astronomical Observatory

We discuss both absolute and relative calibration of mosaic CCD cameras. In particular, we show that accurate stellar photometry based on broad band data requires a thorough investigation of deceptive systematic errors (positional effects, color drifts). We also show recent findings concerning the absolute calibration of predicted intermediate-band Strömgren magnitudes and the comparison with cluster photometry. The impact of alpha-enhanced atmosphere models is also mentioned.

The Photometric Calibration of the Dark Energy Survey

Douglas Tucker and James Annis for the DES Collaboration
Fermilab, MS 127, P.O. Box 500, Batavia, IL 60510-0500, USA

The Dark Energy Survey (DES) is a 5000 sq. deg. $griz$ imaging survey to be conducted using a proposed 3 sq. deg. wide-field mosaic camera on the CTIO Blanco 4m telescope. The primary scientific goal of the DES is to constrain dark energy cosmological parameters via 4 complementary methods: galaxy clusters, weak lensing, galaxy angular correlations, and Type Ia supernovae, supported by precision photometric redshifts.

Here we present the photometric calibration plans for the DES, including a discussion of standard stars and field-to-field calibrations.

The HST Photometric Systems

M. Sirianni
Space Telescope Science Institute, Baltimore MD 21218, USA

For more than fifteen years the Hubble Space Telescope has been producing high quality photometric data from the Near UV to the near IR with several instruments. By its own definition the HST photometric systems are based on the absolute fluxes of the HST primary spectrophotometric standards. However HST photometric measurements are reported both in the language of absolute fluxes and as magnitude, in particular when the results need to be compared with established ground based photometric systems.

As a consequence two different types of calibration are usually derived for the Scientific Instruments of HST: the classic flux based calibration based on spectrophotometric standards, and the transformations between HST magnitudes and magnitudes in more traditional photometric systems. These transformations are based on synthetic photometry and observations of secondary standard fields. We will examine the different approaches to the photometric calibration of main cameras of HST and their limitation when converting the HST-non standard filters to the ground system. We finally discuss the central role of synthetic photometry in the calibration of all instrument photometric modes.

Photometry with Gaia

C. Jordi$^{1,2}$ and J. M. Carrasco$^{1}$
$^{1}$ Dept. Astronomia i Meteorologia, Universitat de Barcelona, Martí i Franqués 1, E-08028 Barcelona, Spain
$^{2}$ Institut d'Estudis Espacials de Catalunya (IEEC), Edif.Nexus, Gran Capità, 2-4, E-08034 Barcelona, Spain

The ESA Gaia spacecraft has been approved for launch by a Soyuz-Fregat launcher in December 2011. After five years continuously scanning the sky, astrometric, photometric and spectroscopic data for a billion objects down to $V\sim20$ will be acquired. That huge survey of the Galaxy content and beyond will provide insights to the origin and evolution of the Galaxy, to stellar astrophysics, to solar system objects, to QSOs, and so on. Gaia is the successor of the successful Hipparcos mission and it means a big step forward, both in terms of number of objects and in terms of precision.

The final industrial approach to photometry includes two slitless spectrographs, covering the blue and the red parts of the radiation spectrum from 330 to 1050nm. Photometry will be acquired by a white photometric band, and by two low-dispersion spectra. Now, there is a unique focal plane integrating astrometry, photometry and spectroscopy.

In this paper, we review the goals of Gaia photometry, and present as well the new low-dispersion spectrophotometry approach. With the redesign of the mission, the reduction process changes considerably and a great effort must be devoted to this issue in the next years.

A Uniform Set of Optical/NIR Photometric Zero Points to be Used with CHORIZOS
J. Maíz Apellániz
Space Telescope Science Institute, Baltimore, MD 21218, USA
Instituto de Astrofísica de Andalucía, Granada 18008, Spain (current)

I have recently combined HST/STIS spectrophotometry with existing photometric data to analyze the calibration of three standard optical photometry systems: Tycho-2 $B_{\rm T}V_{\rm T}$, Strömgren $uvby$, and Johnson $UBV$. In this contribution I summarize those results, present new ones for 2MASS $JHK_s$, and combine them with recent literature results to generate a uniform set of zero points for six photometric systems, the above mentioned plus Cousins $RI$ and SDSS $ugriz$. With the exception of the latter system, the zero points use the new Vega spectrum presented at this meeting by Ralph Bohlin. I also discuss the implementation of these results in CHORIZOS, a Bayesian photometric code that compares multi-filter observational data with spectral energy distributions to solve the inverse problem of finding the models which are compatible with the observations.

Unified Absolute Spectrophotometry for Star Clusters

R. J. Dodd
School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington, New Zealand

Uniform, dereddened, absolute, flux density versus frequency, low-resolution spectra were constructed for stars in star clusters. Photometric and spectrophotometric observations were extracted from printed papers and catalogues, CDROM and on-line databases, for ten stars selected, on the basis of their positions, proper motion components and photometry, as members of the young open cluster IC2391. The units of measurement used in the original publication were converted, where necessary, to apparent flux densities in Janskys and frequencies in Hertz. Given measured values for interstellar extinction and distances to the stars, absolute flux densities at the standard 10pc distance were readily computed from the apparent values.

Plots were prepared for each of the member stars showing the mean frequency, the bandwidth, the absolute monochromatic flux density and a total error estimation, where possible, for each observed passband. Absolute spectrophotometry for Vega from Hubble Space Telescope observations was also shown on each plot to serve as a reference.

The difficulties experienced in producing the plots are discussed and ways in which these may be ameliorated are suggested.

The ASTRA Spectrophotometer: Design and Overview

Saul J. Adelman$^{1}$, Austin F. Gulliver$^{2}$, Barry Smalley$^{3}$, John S. Pazder$^{4}$, P. Frank Younger$^{5}$, Louis J. Boyd$^{6}$, Donald Epand$^{6}$, and Thomas Younger$^{5}$
$^1$Department of Physics, The Citadel, 171 Moultrie Street, Charleston, SC 29409, USA
$^2$Department of Physics & Astronomy, Brandon University, Brandon, MB R7A 6A9, Canada
$^3$Astrophysics Group, School of Chemistry and Physics, Keele University, Straffordshire ST5 5BG, United Kingdom
$^4$Dominion Astrophysical Observatory, Herzberg Institute for Astrophysics, 5071 W. Saanich Road, Victoria, BC V9E 2E7, Canada
$^5$Aurora Astronomical Services, 585 Aurora Way, Victoria, BC V8Z 3J8, Canada
$^6$Fairborn Observatory, HCR5 2, Box 256, Patagonia, AZ, 85624,USA

The ASTRA Cassegrain Spectrophotometer and its automated 0.5-m $f/16$ telescope will soon be working together at the Fairborn Observatory near Nogales, Arizona. Scientific observations are expected to begin in 2007. We provide an overview of this project and review the design of the system. A separate paper in these Proceedings presents details of the data reduction and flux calibrations. The Nogales site averages 150 photometric nights per year. ASTRA should observe stars whose declinations are in the range +80$^{\circ}$ to $-35^{\circ}$.

In an hour the system should obtain $S/N = 200$ observations of stars as faint as 9.5 mag after correction for instrumental errors. Vega will require about 25 seconds for observation and CCD readout. Usually the telescope will find its next target in less than a minute. A small CCD camera finds and centers the target and a second then guides on the zeroth order spectrum. The spectrophotometer uses both a grating and a cross-dispersing prism to produce spectra from both the first and the second orders simultaneously. 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 $\lambda\lambda$3300-9000. We are initially using about 10 minutes/hour to observe Vega and secondary standard candidates.

Our scientific CCD is electronically cooled to $-50^{\circ}$ C with a water recirculation system heat sink. The same $4^{\circ}$ C recycling water system provides thermal stabilization of the instrument. Our flat fielding system uses a second 0.5-m telescope to produce a collimated beam from a 100 $\mu$m pinhole illuminated by a quartz halogen lamp. When the two telescopes point at one another this ``artificial star" is focused by the ASTRA telescope which is then rocked to expose the image from the top to the bottom of the entrance aperture.

A LINUX HP server at The Citadel will have databases of ASTRA observations. Each observing request has its own priority and observing window, ASTRA can observe standard stars at a regular rate throughout the night, any accessible target at a given time, and variable stars. ASTRA will produce considerable high quality data.

ASTRA Spectrophotometer: Reduction and Flux Calibrations

Barry Smalley$^{1}$, Austin F. Gulliver$^{2}$, Saul J. Adelman$^{3}$
$^1$Astrophysics Group, Keele University, Staffordshire ST5 5BG, UK
$^2$Department of Physics & Astronomy, Brandon University, Brandon, MB R7A 6A9, Canada
$^3$Department of Physics, The Citadel, 171 Moultrie Street, Charleston, SC 29409, USA

The ASTRA Cassegrain Spectrophotometer and its automated 0.5-m telescope at Fairborn Observatory in Arizona will produce a large quantity of high-precision stellar flux distributions. A separate paper presented a review of the design criteria for the system and an overview of its operation. This paper discusses the techniques used in the data reduction to final flux calibrations.

Extraction of 1-d spectra from the 2-d images will be performed by a highly automated version of CCDSPEC. The characteristics of the CCD are automatically applied to the images, including the location of dead rows and hot pixels. In order to achieve the goal of better than 1% precision, large numbers of bias and flat field frames will be used in the reduction process. There will be a continual programme to monitor the image quality. Finally, optimally extracted spectra will be obtained, including the removal of scattered light and cosmic rays.

The Earth's atmosphere has a considerable effect on the stellar flux as measured from the surface. The principal sources of extinction, Rayleigh and aerosol scattering, ozone and telluric line absorption, are discussed, along with methods used to determine their effects on the observed spectra. Correction for telluric lines is the most problematical, due to their non-linear variation with airmass. By using a large network of constant stars to monitor atmospheric extinction it is possible to determine the extinction coefficients to generally better than 1% and to assess their temporal variability.

The spectrophotometric observations are placed on an absolute flux scale by reference to stars with known values of true flux at top of Earth's atmosphere. These standard stars have been calibrated against terrestrial sources of known properties. Unfortunately, very few stars have been calibrated directly. The ASTRA fluxes will be calibrated against the best available Vega flux distribution. The constant stars used in the extinction determinations will provide the internal calibration network of secondary flux standards.

The available absolute calibrations are accurate to typically 1-2%. Ultimately this uncertainty will limit the accuracy of the final fluxes of other stars. However, the internal precision will be significantly higher, and should more-accurate absolute calibrations become available the fluxes can be re-calibrated to higher accuracy.

Removal of Instrumental Signatures in Echelle Spectra

Herman Hensberge
Royal Observatory of Belgium, Ringlaan 3, B-1180 Brussels, Belgium

The use of echelle spectra in astronomy is occasionally hampered by imprecise merging of spectral orders. Analysis of time-dependent instrumental signatures in extracted spectral orders, before merging them, allows to identify various types of bias and to suggest improvements in the data reduction process. Experience with several echelle spectrographs during 20 years is used to define the needs for a performant data reduction pipeline, differential in nature and evaluating the validity of the data reduction procedure continuously.

SNAP Candidate Standard Stars
S. Allam$^1$, N. Mostek$^2$, M.W. Richmond$^3$ and the SNAP Calibration Team
$^1$University of Wyoming
$^2$University of Indiana
$^3$Rochester Institute of Technology

SNAP (Supernova Acceleration Probe) proposes to investigate the dark energy in the Universe by observing over 2000 Type Ia supernovae to a redshift of $z=1.7$, with high precision and accurate absolute colors over the wavelength range between 0.35 to 1.70 microns.

Although many of the same calibration principles and techniques as used on the ground-based telescope and the HST will be reused for the flux calibration of the SNAP instruments, SNAP calibration is much more challenging due to the large number of SNAP filters (9) and the wide focal plane, and its need for fainter ($V~19$ mag) sets of spectrophotometric standard stars over the SNAP wavelength range.

In this talk, we summarize our ongoing projects for the calibration of the SNAP candidate spectrophotometric standard stars, selected from the Sloan Digital Sky Survey, using the Double Imaging Spectrographs (optical) and the CorMass spectrograph (NIR) on the ARC 3.5m telescope, and using the WIYN 0.9m telescope.

HST Stellar Standards with 1% Accuracy in Absolute Flux

Ralph C. Bohlin
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218

Free of any atmospheric contamination, the Hubble Space Telescope provides the best available spectrophotometry from the far-UV to the near-IR for stars as faint as $V\sim16$. The HST CALSPEC standard star network is based on three standard candles: the hot, pure hydrogen white dwarf (WD) stars G191B2B, GD153, and GD71, which have Hubeny NLTE model flux calculations that require the atomic physics for only one atom. These model flux distributions are normalized to the absolute flux for Vega of $3.46\times10^{-9}$ erg cm$^{-2}$ s$^{-1}$ Å$^{-1}$ at 5556 Å using precise Landolt $V$ band photometry and the $V$ bandpass function corrected for atmospheric transmission by M. Cohen. The three primary WD standards provide absolute flux calibrations for FOS, STIS, and NICMOS spectrophotometry from these instruments on the HST. About 32 stellar spectral energy distributions (SEDs) have been constructed with a primary pedigree from the STIS data, which extends from 1150 Å for the hot stars to a long wavelength limit of 1 $\mu$m. NICMOS grism spectrophotometry provides an extension to 1.9 $\mu$m in the IR for 17 of the HST standards and longward to 2.5 $\mu$m for a few of the brighter stars. Included among these HST standards are Vega, the Sloan standard BD+17$^{\circ}$4708, three bright solar analog candidates, three cool stars of type M or later, and five hot WDs. In addition, four K giants and four main sequence A-stars have NICMOS spectrophotometry from 0.8-2.5 $\mu$m. The WD fluxes are compared to their modeled SEDs and demonstrate an internal precision of 1-2%, while the A-stars agree with the Cohen IR fluxes to $\sim$2%. Three solar analog candidate stars differ from the solar spectrum by up to 10% in the region of heavy line blanketing from 3000-4000 Å and show differences in shape of $\sim$5% in the IR around 1.8 $\mu$m.

Networks of Absolute Calibration Stars for SST, AKARI, and WISE

Martin Cohen Radio Astronomy Laboratory, 601 Campbell Hall, University of California, Berkeley,CA 94720, USA & Monterey Institute for Research in Astronomy, 200 8th Street, Marina, CA 93933, USA

I describe the Cohen-Walker-Witteborn network of absolute calibration stars built to support ground-based, airborne, and space-based sensors, and how they are used to calibrate instruments on the Spitzer Space Telescope (SST) and Japan's AKARI (formerly ASTRO-F), and to support NASA's planned MidEx WISE (the Wide-field Infrared Survey Explorer). All missions using this common calibration share a self-consistent framework embracing photometry and low-resolution spectroscopy. CWW also underpins COBE/DIRBE, several instruments used on the Kuiper Airborne Observatory (KAO), the joint Japan-USA ``IR Telescope in Space" (IRTS) Near-IR and Mid-IR spectrometers, the European Space Agency's IR Space Observatory (ISO), and the US Department of Defense's Midcourse Space eXperiment (MSX). This calibration now spans the far-UV to mid-infrared range with Sirius (one specific Kurucz synthetic spectrum) as basis, and zero magnitude defined from another Kurucz spectrum intended to represent an ideal Vega (not the actual star with its pole-on orientation and mid-infrared dust excess emission). Precision 4-29 $\mu$m radiometric measurements on MSX validate CWW's absolute Kurucz spectrum of Sirius, the primary, and a set of bright K/MIII secondary standards. Sirius is measured to be 1.0% higher than predicted. CWW's definitions of IR zero magnitudes lie within 1.1% absolute of MSX measurements. The US Air Force Research Laboratory's independent analysis of on-orbit MSX stellar observations compared with emissive reference spheres show CWW primary and empirical secondary spectra lie well within the $\pm$1.45% absolute uncertainty associated with this 15-year effort. Our associated absolute calibration for the InfraRed Array Camera (IRAC) on the SST lies within $\sim$2% of the recent extension of the calibration of the Hubble Space Telescope's STIS instrument to NICMOS (Bohlin, this volume), showing the closeness of these two independent approaches to calibration.

Dark Energy Science Constraints on Calibration: Design of the SNAP Calibration System
Susana Deustua

The determination of dark energy properties is the goal of a current set of proposed space missions. SNAP (Supernova Acceleration Probe) proposes to investigate the dark energy by going "beyond Lambda", beyond constant equation of state to the dynamical variation of dark energy, and testing the geometry (flatness) and gravity beyond Einstein. This is accomplished through comprehensive supernova distance and weak gravitational lensing surveys and a multi-color, wide-field, imaging program. This science demands high precision and accuracy, placing stringent requirements particularly on the the absolute color calibration of supernovae. The overall calibration for the SNAP focal plane will be conducted through several routes.

I shall present the basic, inherent issues and possible strategies to meet these requirements over the SNAP wavelength range between 350 nm to 1700 nm.

A Rocket-borne Transfer of the NIST Absolute Calibration Standards to the Stars
Mary Elizabeth Kaiser and the ACCESS Team

We present an astrophysics experiment to effect a transfer of the National Institute of Standards and Technology (NIST) absolute calibration standards to the bright stars Vega and Sirius. This experiment, ``Absolute Color Calibration Experiment for Standared Stars'' (ACCESS), is a proposed series of rocket-borne sub-orbital missions whose purpose is to establish a network of standard stars with absolute fluxes that are directly traceable to ground based laboratory standards. The goal is to obtain an absolute spectophotometric calibration accuracy of $<1$% in the 0.35-1.7 micron bandpass at a spectral resolution greater than 500 for each of these two stars, thus establishing the the first links in a chain of stellar calibrators.

It is our intent to include NIST traceable observations of standard stars ($\sim 10$th magnitude), which are observable by major telescopes, thus enabling the ultimate calibration to extend to faint magnitudes.

Preliminary Results from Detector-Based Throughput Calibration of the CTIO Mosaic Imager and Blanco Telescope Using a Tunable Laser

Christopher W. Stubbs$^1$, Sara K. Slater$^1$, Yorke J. Brown$^1$, Daniel Sherman$^1$,R. Chris Smith$^2$, Nicholas Suntzeff$^2$, Abi Saha$^2$,John L. Tonry$^3$, Joseph Masiero$^3$ and Stephen Rodney$^3$
$^1$ Department of Physics, Harvard-Smithsonian Center for Astrophysics, Harvard University,17 Oxford Street, Cambridge MA 02138, USA
$^2$ Cerro Tololo Interamerican Observatory, NOAO Casilla 603, La Serena, Chile
$^3$ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI96822, USA

We describe the scientific motivation for achieving photometric precision and accuracy below the 1% level, and we present a calibration philosophy based on using calibrated detectors rather than celestial sources as the fundamental metrology reference. A description of the apparatus and methodology is presented, as well as preliminary measurements of relative system throughput vs. wavelength for the Mosaic imager at the CTIO Blanco 4m telescope. We measure the throughput of the optics, filter, and detector by comparing the flux seen by the instrument to that seen by a precisely calibrated monitor photodiode, using a tunable laser as the illumination source. This allows us to measure the transmission properties of the system, passband by passband, with full pupil illumination of the entire optical train. These preliminary results are sufficiently promising that we intend to further pursue this technique, particularly for next-generation survey projects such as PanSTARRS and LSST.

Standardization and the Enhancement of IR Precision
E. F. Milone & A. T. Young
University of Calgary and San Diego State University

The standardization of infrared photometry is poorly practiced at present. Papers containing infrared photometry typically provide minimal details of the passband system that was used to obtain the observations. Lack of standardization in the intermediate infrared is, in fact, so rampant as to merit characterization of the situation as ``atrocious." Yet, as a political campaign suggested a couple of years ago, ``hope is on the way." The work begun by the IRWG in 1988 at the Joint Commission meeting of Commissions 25 and 9 in Baltimore that resulted in a new passband system has borne fruit and spawned at least one offshoot system, as enlightened IR astronomers confront the standardization problem. In this paper, we review the basic problem and the solution we found, and describe the appropriate steps to realize the high precision that this area is capable of bringing to infrared photometry.

This work has been supported in part by grants to EFM from NSERC of Canada and the University Research Grants Committee at the University of Calgary.

Steps Toward a Common Near-Infrared Photometric System
A. T. Tokunaga$^1$ & W. D. Vacca$^2$
$^1$ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Dr., Honolulu, HI 96822
$^2$ Stratospheric Observatory for Infrared Astronomy/Universities Space Research Association, NASA Ames Research Center, MS N211-3, Moffett Field, CA 94035-1000

The proliferation of near-infrared (1-5 $\mu$m) photometric systems over the last 30 years has made the comparison of photometric results difficult. In an effort to standardize infrared filters in use, the Mauna Kea Observatories near-infrared filter set has been promoted among instrument groups through combined filter production runs. The characteristics of this filter set are summarized, and some aspects of the filter wavelength definitions, the flux density for zero magnitude, atmospheric extinction coefficients, and color correction to above the atmosphere are discussed.

Extending the Methods Used to Calibrate ISO to the GTC

P. L. Hammersley, M. A. Di Césare & J. M. Rodríguez-Espinosa
Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain.

An accurate and traceable photometric calibration is becoming increasingly important for large telescopes if they are to reach their full potential. The scientific community of the Gran Telescopio Canarias (GTC) has recognised this and there are programmes underway to ensure that suitable calibration standards are in place. The aim is to extend the methods used for the development of the photometric standards for the Infrared Satellite Observatory (ISO), which is described here, and develop full spectral energy distributions of the the calibration objects from which the magnitudes in any photometric system can be determined using synthetic photometry.

The Spectrum of Th-Ar Hollow Cathode Lamps in the 900-4500 nm region: Establishing Wavelength Standards for the Calibration of VLT Spectrographs

Florian Kerber$^1$, Gillian Nave$^2$, Craig. J. Sansonetti$^2$, Paul Bristow$^1$, Michael R. Rosa$^3$
$^1$ European Southern Observatory, Karl-Schwarzschild-Str.2, 85748, Garching, Germany
$^2$ National Institute of Standards and Technology, Gaithersburg, MD 20899, USA
$^3$ Space Telescope European Co-ordinating Facility, Karl-Schwarzschild-Str.2, 85748, Garching, Germany

We summarise the history, design and operation of hollow cathode lamps, including technical information that is usually not provided in literature readily available to the astronomical community. We show that modern hollow cathode lamps come close to being an ``ideal'' calibration lamp as described by a user's wish list. We provide examples of the use of such lamps for wavelength calibration of astronomical spectrographs both on the ground and in space. We then describe the joint efforts by the European Southern Observatory (ESO), the Space Telescope European Co-ordinating Facility (ST-ECF), and the US National Institute of Standards and Technology (NIST) to establish the Th-Ar hollow cathode lamp as a standard for the calibration of VLT (Very Large Telescope) spectrographs in the near infrared (IR). In the near IR only a limited number of wavelength standards are available. Th-Ar hollow cathode lamps provide a rich spectrum in the UV-visible region and have been used in astronomy for a long time. We report new measurements using the 2-m UV/visible/IR Fourier transform spectrometer (FTS) at NIST that establish more than 2000 lines as wavelength standards in the range 900 nm to 4500 nm. This line list is used as input for a physical model that provides the wavelength calibration for the Cryogenic High-Resolution IR Echelle Spectrometer (CRIRES), ESO's new high-resolution (R$\sim$100,000) IR spectrograph at the VLT. Based on these data and additional measurements investigating other properties relevant for operations we conclude that Th-Ar lamps hold the promise of becoming a standard source for wavelength calibration in near IR astronomy.

Standardisation of (Optical) Polarisation Measurements: a Simple Introduction

John D. Landstreet
Department of Physics & Astronomy, University of Western Ontario

This review will provide an overview of and introduction to astronomical polarisation, and suggest how polarisation measurements might be of use in a variety of problems. We then look at how polarisation is described by the ``Stokes vector''; how it is measured by simple polarimeters and spectro-polarimeters; and the main reduction and calibration issues for polarisation data. We will discuss the information others might need in order to model a published data set, and conclude with an illuminating example.

Polarimetric Standardization

E. Landi Degl'Innocenti$^1$, S. Bagnulo$^2$, & L. Fossati$^{2,3}$
$^1$ Dipartimento di Astronomia e Scienza dello Spazio, Università degli Studi di Firenze, Largo Enrico Fermi 2, I-50125 Firenze, Italy
$^2$ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
$^3$ Institut fuer Astronomie, Universitaet Wien, Tuerkenschanzstrasse 17, A-1180 Wien, Austria

The use of polarimetric techniques is nowadays widespread among solar and stellar astronomers. However, notwithstanding the recommandations that have often been made about the publication of polarimetric results in the astronomical literature, we are still far from having a standard protocol on which to conform. In this paper we review the basic definitions and the physical significance of the Stokes parameters, and we propose a standardization of the measurement of polarized radiation.

Low Polarization Standards

J. H. Hough$^1$, P. W. Lucas$^1$, J. A. Bailey$^2$, M. Tamura$^3$
$^1$Centre for Astrophysics Research, University of Hertfordshire, Hatfield AL10 9AB, England
$^2$Australian Centre for Astrobiology, Macquarie University, NSW 2109, Australia
$^3$National Astronomical Observatory, Osawa 2-21-1, Mitaka, Tokyo 181, Japan

There is renewed interest in high sensitivity polarimetry as a means of observing the polarization signal of extra-solar planets. We describe the measurement of low polarized standards at fractional polarizations of parts per million.

Analytical Flatfield Corrections on VATT Data

R. P. Boyle and R. Janusz
Vatican Observatory, V-00120, Vatican City State

We describe our methods to correct the initial flatfields by removing detected systematic dependency of the stellar magnitudes on their location in the CCD frames. Assuming flatfields are made with exposures on a unit input light source and that no added scattered light arrives in the focal plane, then such flatfields should be fine for calibrating the exposures on star fields. But at VATT (and probably most telescopes) the two assumptions of unit input and of absence of scattered light, are not strictly valid. And so the photometry is compromised unless correct flatfields are found.

A Grid of FASTWIND NLTE Model Atmospheres of Massive Stars

K. Lefever$^1$, J. Puls$^2$ and C. Aerts$^{1,3}$
$^1$ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium
$^2$ Universitätssternwarte München, Scheinerstrasse 1, D-81679 München, Germany
$^3$ Departement Astrofysica, Radboud Universiteit Nijmegen, PO Box 9010, 6500 GL Nijmegen, the Netherlands

In the last few years our knowledge of the physics of massive stars has improved tremendously. However, further investigations are still needed, especially regarding accurate calibrations of their fundamental parameters. To this end, we have constructed a comprehensive grid of NLTE model atmospheres and corresponding synthetic spectra in the massive star domain. The grid covers the complete B type spectral range, extended to late O on the hot side and early A on the cool side, from supergiants to dwarfs and from weak stellar winds to very strong ones. It has been calculated with the latest version of the FASTWIND code. The analysis of an extensive sample of OB stars in the framework of the COROT space mission will lead to accurate calibrations of effective temperatures, gravities, mass loss rates etc. This paper contains a detailed description of the grid, which has been baptised as BSTAR06 and which will be available for further research in the near future.

A Technique for a Self-Luminous Flatfield Calibration Screen

Yorke J. Brown, Christopher W. Stubbs, Edward T. Henry, & L. Bailes Brown
Department of Physics, Harvard-Smithsonian Center for Astrophysics, Harvard University, 17 Oxford Street, Cambridge, MA 02138, USA

The use of flatfield illuminators shows great promise for calibrating telescopes to be used in the next generation of Ia supernova measurements. In order to meet the physical constraints of the PanSTARRS and LSST domes, we are developing a self-luminous flatfield calibration screen which can cover the entrance aperture of these telescopes while presenting a total thickness of only 13 cm. We expect to achieve illumination uniformity and passband flatness adequate for optical throughput calibration at the 1% level. The emitting element is a ``side-emitting'' optical fiber excited by a pulsed laser tunable over the bandpass of 400 to 1100 nm. The fiber is embedded in a sheet of acrylic the size of the telescope aperture. This radiating element is backed by a mirror. In front of the radiator is an acrylic lambertian diffusing screen, and in front of the diffuser is a baffle screen that occults much of the light that would have entered the telescope by way of scattering from telescope and dome structures. The baffle screen is a sheet of black acrylic drilled with a densely packed array of threaded holes.

A Comparative Test of CCD Reduction Procedures
T. Tuvikene, M. Y. Bouzid, A. Ederoclite & C. Sterken
Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium

Three data reduction methods, applicable to time-resolved CCD photometry of open clusters, are described. A sample set of images is reduced independently by three people, using various approaches, and the results are compared. The test reveals systematic differences in the FWHM and magnitudes produced by the different software. The reduction package MOMF and pure aperture photometry are found to be equivalent in precision, but the results obtained with these methods should not be merged when high precision is required.

Time Series Photometry Data: Standard Access, Standard Formats

A. Holl
Konkoly Observatory, H-1525 P.O.Box 67, Budapest, Hungary

This paper is intended to start a debate on data access and file format aspects in photometry. We do not deal with the process of measurement, but discuss only the storage and publication of data. We review the existing standards, discuss Virtual Observatory techniques, present practice, and call for standardization where it is still lacking.

Photometric Calibration of the Supernova Legacy Survey Fields
Nicolas Regnault, for the SNLS Collaboration
LPNHE - Laboratoire de Physique Nucléaire et de Hautes-Energies, IN2P3 - CNRS - Universités Paris VI et Paris VII, 4 place Jussieu, Tour 33 - Rez de chaussée, 75252 Paris Cedex 05

The 5-year project Supernova Legacy Survey (SNLS) delivers $\sim 100$ Type-Ia supernovae (SNe Ia) per year, in the redshift range $0.3 < z < 1.0$, with well-sampled $g'r'i'z'$ lightcurves. The SNLS Collaboration uses the 1 deg$^2$ Megacam imager (36 $2048 \times 4612$ thinned CCDs) mounted on the 3.6-m Canada-France-Hawaii Telescope (CFHT) to observe four fields around the sky, in four filters. The primary goal of the project is to measure the dark energy equation of state with a final statistical precision of $\pm 0.05$. We have shown, using the first year dataset that the calibration uncertainties are currently the dominant contribution to the systematic error budget.

The calibration of the SNLS dataset is challenging in several aspects. Megacam is a wide-field imager, and only a handful of its 36 CCDs can be directly calibrated using standard star observations. Then, measuring the rest-frame $B$-band luminosity of SNe Ia over the $0.3 < z < 1.0$ redshift range requires an excellent flux intercalibration of the Megacam bands. Finally, the SN Ia SED differs significantly from that of stars and transfering the stellar calibration to the SNLS data requires a precise knowledge of the SN Ia spectra and the instrument transmissions.

We present and discuss the SNLS calibration strategy. We present the calibration aspects which impact most the cosmological measurements. We also discuss the intercalibration of the SNLS with other surveys, such as the CFHTLS-Wide and the SDSS.

The Problems with Vega
Richard O. Gray
Department of Physics and Astronomy, Appalachian State University, Boone, NC USA

The problems associated with maintaining Vega as the primary spectrophotometric standard are reviewed. These difficulties include the possible photometric variability of Vega, its status as a rapid rotator seen pole-on, which complicates the use of models to extend the optical calibration into the infrared and ultraviolet, and the presence of significant infrared excesses from the $K$-band longward due to Vega's debris disk. These problems are significant enough that the astronomical community should consider finding another fundamental calibrator.

Spectrophotometric Calibration of the 10-m GTC Telescope at La Palma Observatory
M. A. Di Césare, P. L. Hammersley and Rodriguez J. M. Espinosa
Instituto de Astrofísica de Canarias. La Laguna. Tenerife. Spain

We propose a calibration plan for the GTC based on techniques similar to the ones used for space calibration. After studying the telescope and instrument requirements, and observing during intensive campaigns we have begun to prepare the GTC catalogue of standard stars. In this article we show the calibration strategy for the telescope and the scientific instruments in the range between 0.3 and 2.7 microns. We present a sample of the results obtained and some annalysis of the data.

Mid-Infrared Calibration of a Dense Spatial Network of Stars for Accurate Absolute Spectrophotometry: a New Approach for a New Generation of Telescopes

F. Martín-Luis$^{1}$ and M. Kidger$^{2,3}$
$^{1}$Instituto de Astrofísica de Canarias, C/Vía Láctea, s/n. 38205 La Laguna, Tenerife, Spain
$^{2}$Herschel Science Centre, European Space Astronomy Centre, European Space Agency Villafranca del Castillo Satellite Tracking Station. Apartado de Correos 50727, 28080 Madrid, Spain
$^{3}$INSA, Paseo Pintor Rosales 34, 28008 Madrid, Spain

The next frontier for calibration is the new generation of 8 and 10-m telescopes. While visible and near-infrared calibration for these telescopes have relatively simple solutions, the mid-infrared is the most complicated and arguably the most important range to calibrate given the relative paucity of calibrators and the difficulties that the mid-infrared sky provides, even for a low water vapour site such as the Roque de los Muchachos Observatory in La Palma where the column of water vapour is below 2-mm for a substantial fraction of the year.

We report on our calibration programme, which is in the final stages of preparing a dense network of accurate northern-hemisphere standards for the 3.5-25 micron range for CanariCam, the mid-infrared instrument of the Spanish Gran Telescopio CANARIAS (GTC). Our goal is to provide approximately one star per ten degrees square north of declination -40 degrees, with a goal of 2% error in the absolute photometry at the short wavelength end and 10% at 25 microns. An initial network of some 100 stars is to be prepared by the end of 2006, well in advance of the entry of the GTC in service, with the final network available during telescope commissioning.

Nearly 1000 candidate stars were selected initially from a variety of sources, out of an initial sample of some 25 000 input stars. Of these we have observations of more than 650 stars from which our final sample of calibrators will be taken. We describe the selection, observation and modelling of these stars and the characteristics of our final sample of calibration sources.

Contributions to the next Newsletter, due to be ``crystallized'' in April 2007, will be welcomed at any time by grayro@appstate.edu.

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