VLT calibration issues Optical instruments Martino Romaniello (ESO – HQ) on behalf of a lot of people

Why this meeting? or The fundamental importance of calibrations The end user of the data are more and more detached from the observatories (space-based telescopes, service mode, etc.) The increasing importance of archival research (aka Virtual Observatory) requires excellent archival calibrations, even beyond the scope of the original program It is not the individual dataset that should be calibrated, but, rather, the instrument Calibration are an integral part of the operations of an instrument and should maximize the scientific return and ensure sustainable operations

Today’s menu VLT/VLTI Optical Infrared(Nancy Ageorges) VLTI(Christian Hummel) VST/VISTA (Dietrich Baade, Fernando Selman) Space telescopes HST(Jeremy Walsh) JWST(Harald Kuntschner) Alma(Robert Laing) OWL(Nancy Ageorges)

The basics: photometric standards The Landolt (1992) standard stars are used for the VLT, Keck and Gemini

The Landolt standards Photometric errors of a few millimags Data obtained at the CTIO’s 1.5m telescope 298 stars with an average of 29 measures each on 19 nights

The Landolt standards: problems Too sparse for typical FOVs (few stars, narrow color range, etc.) Too bright for 8m class telescopes (saturation, scintillation, etc.) Equatorial: don’t cover the same azimuth range as science targets

The Stetson standards: better, but… No U band BVRI not available in all fields

The FORS Faint Standards Project By Appointment to the FORS Instrument Operations Team The Goal: define a homogeneous set of standard stars in the optical, suitable for large telescopes, with high photometric accuracy Stefano Bagnulo, Wolfram Freudling, Arto Järvinen, Emmanuel Jehin, Kieran O'Brien, Roberto Mignani, Palle Møller, Ferdinando Patat, Monika Petr-Gotzens, Martino Romaniello, Gero Rupprecht, Thomas Szeifert, Mario Van Den Ancker… The FSSP Working Group:

The FORS Faint Standards Project Status: proof of concept, is it possible to achieve extreme photometric precision with FORS? Dataset: B and R rotated and shifted images of a mildly crowded stellar field, plus 3 Landolt fields, plus tons of biases and sky flats

Optical polarimetry An open issue is the intrinsic variations with time of the polarization characteristics of polarimetric standards Exploit the FORS1 archive to obtain repeated polarization measurements of commonly-used polarization standards: Stefano Bagnulo, Elena Mason (PSO), plus DGDF (??) The archival data are mostly adequate for the project Gaps or missing data can be obtained virtually for free in the context of the FORS Calibration Plan

Optical polarimetry Instrumental polarization across the field is obviously an issue: FORS1 instrumental polarization map Patat & Romaniello (2004)

And also: Spectrophotometric standards Radial velocity standards Lick standards Astrometric calibration …

Finally: Need for an official channel for calibration proposals, similar to the HST: STScI does not have the resources to calibrate fully all potential capabilities of all instruments. On the other hand, the astronomical community has expressed interest in receiving support to perform calibrations for certain uncalibrated or poorly calibrated modes, or to develop specialized software for certain HST calibration and data reduction tasks. [HST Call for Proposals, section 3.6] Zum Beispiel: Faint photometric standards for WFPC2 Creation of a coronagraphic and spectroscopic PSF library for STIS/CCD NICMOS polarimetric calibrations ACS photometric zero point verification Calibration of the ACS emission line filters Monitoring of atmospheric conditions (sky transparency and seeing in the pointing direction, extinction coefficients, etc.)