1. The secondary standards programme for OmegaCAM at the VST Gijs Verdoes Kleijn, Ronald Vermeij & Edwin Valentijn For the Astro-Wise consortium OmegaCEN at Kapteyn Astronomical Institute, Groningen, The Netherlands

2. The two teams Astro-Wise team OmegaCEN: Edwin Valentijn (PI) Munich, USM/MPE: Ralf Bender local PI Paris, Terapix: Yannick Mellier local PI Naples, OAC: Massimo Cappacioli local PI Munich, ESO: Peter Quinn local PI Groningen: Kor Begeman Michael Pohlen Danny BoxhoornMichiel Tempelaar Ewout HelmichGijs Verdoes Kleijn Philippe HeraudeauRonald Vermeij John MacFarlandWillem Jan Vriend Edwin Valentijn (PI) OmegaCAM team= wide-field imager USM (Munich), ESO, Padova, OmegaCEN(Groningen)/NOVA(NL): Astro-Wise team =software environment for OmegaCAM (but being used also for data from various other wide-field imagers) USM/MPE, Terapix (IAP), OAC Naples, ESO, OmegaCEN Leiden: Konrad Kuijken (OmegaCAM-PI) Erik Deul

3. Outline VST, OmegaCAM and filter specs The calibration challenges for OmegaCAM Secondary standards programme Goal of this talk: introduce the instrument and its’s analysis system to you; discuss our scheme for photometric calibration. All discussion welcomed.

4. OmegaCAM@VST, Paranal

5. VLT Survey Telescope-VST –Alt-AZ –aperture 2.610 m –corrected FOV 1.47 degree –f/5.5 –Single instrument: OmegaCAM –Alt-AZ –aperture 2.610 m –corrected FOV 1.47 degree –f/5.5 –Single instrument: OmegaCAM Artist’s impression

6. The OmegaCAM panoramic camera Successor at Paranal of WFI from LaSilla Cassegrain focus at VST 16k x 16k mosaic of 32 CCDS: 2k x4k thin CCDs pixel size 0.21 arcsec / 15μm. 330 to 1000 nm filter system contains: ugriz (core) + more Retractable Atmospheric Dispersion Compensator for u’ and large air mass Only Service Mode observations. USM Padova 1deg

7. Filters in production Sagem: u’g’r’I’z’, 4-segmented around Hα (10nm) 4-segmented narrow-band at 850nm Strömgren v Barr: B,V 4-segmented u’g’r’I’ filter Filter+ccd g’, r’, i’, z’ have been measured

8. The challenges using ΩCAM Vast amount of data Single image 550 Mbyte (~200 screens for full resolution) Sole instrument: OmegaCAM observes equivalent of Southern sky in ~3 years (30 min exp, 300 nights/year) Many filters & programs (75% public surveys, 25% GTO) Of order 10 Tbyte of raw data/year with time-varying atmosphere, instrument & telescope performance Vast amount of data Single image 550 Mbyte (~200 screens for full resolution) Sole instrument: OmegaCAM observes equivalent of Southern sky in ~3 years (30 min exp, 300 nights/year) Many filters & programs (75% public surveys, 25% GTO) Of order 10 Tbyte of raw data/year with time-varying atmosphere, instrument & telescope performance The Calibration (& Science) Wishes Good calibration/monitoring every location on each chip: (e.g., flatfield illumination correction, fringing zeropoints) Easy re-calibration as insight in instrument/telescope improves All raw and reduced data at finger tips: needle in the hay stack searches, subtle statistics Flexibility for various reductions/analyses on large data sets (quick ‘n dirty or thorough) The Calibration (& Science) Wishes Good calibration/monitoring every location on each chip: (e.g., flatfield illumination correction, fringing zeropoints) Easy re-calibration as insight in instrument/telescope improves All raw and reduced data at finger tips: needle in the hay stack searches, subtle statistics Flexibility for various reductions/analyses on large data sets (quick ‘n dirty or thorough)

9. The chosen solution: “ The 2 central ingredients of the Astro-Wise system ” 1.Make strict & ‘fully-handshaking’ procedures from scheduling through calibration/reduction Photometric calibration: every night identical procedures for a monitoring field (3x/night) and standard star field (≥1x/night) 2.Put everything in ‘smart’ database –All raw data & derived products resident in single database system –Information how a product was derived also resident: all bits are traceable –Both calibration and analysis software in database system –Database is smarter than filing system of fits tables and images with headers: internet vs library –As time goes on newly/better/differently derived products are added Photometric calibration: easy (re-)calibration & trend analysis It’s the database that is special: ‘smart’ database vs filing system == internet vs library

10. OmegaCAM photometric calibration in Astro-Wise 1 (atmosphere) monitoring field near south equatorial pole 8 equatorial standard star Landolt fields 1deg Photometric calibration / monitoring per chip: Landolt (1992) Stetson (CADC) Full FOV coverage by standards needed

11. ΩCAM secondary photometric standards Current situation: 8 Landolt fields with ● Landolt catalog converted to ugriz (Smith+02) ● Stetson catalog converted to ugriz (Smith+02) ● (SDSS non-standard stars) Secondary Standards Preparatory Programme WFC/INT @ La Palma Intermediate step ● Secondary Standards calibrated to ugriz system from OmegaCAM observations covering ~1.1x1.1 deg 2 Final situation (after ~1 year of OmegaCAM operation)

12. WFC/INT La Palma Observations WFC/INT: –D=2.5m; –FOV=0.5deg x 0.5 deg, –four CCDs, pixel size 0.333arcsec; Observations: –Jun 2002 and Feb 2003; –7/8 fields observed (not SA95) –5-dither (1.1deg x 1.1deg); –Ugriz filters; short (~10s) and long (~300s) exposure Goal: secondary standards from 14 th -19 th mag stars…..work in progress…..data reduced, tests and full error analysis in progress

13. Monitoring the quality of the night Standard extinction coefficients adopted : u' = 0.47 r' = 0.09 g' = 0.19 i' = 0.05 Stable nights

14. u' : none detected g' : 0.14 +/- 0.01 r' : none detected i' : 0.07 +/- 0.01 Color terms for WFC→ugriz system

15. The preliminary catalogue of secondary standards

16. SA92SA101 SA104SA107SA113 Illustrative test: WFC (aperture)-SDSS3 (modelmag) magnitude residuals

17. SA101

18. Conclusions OmegaCAM= wide-field 1sq degree imager operational early 2007 Astro-Wise: specifically developed analysis environment: ‘smart database’ (‘internet vs library’) concept 1.1.x1.1deg 2 fields of secondary standards in u’g’r’i’z’ system in construction centered on 8 Landolt equatorial standard fields Final outcome: deep public imaging surveys of significant (>10,000deg 2 ) part of Southern Sky and more….