1. CU5-DU13: Ground-based support observations for Gaia absolute photometric calibration BOLOGNA: C. Cacciari, G. Altavilla, M. Bellazzini, A. Bragaglia, E. Diolaiti, L. Federici, F. Fusi Pecci, P. Montegriffo, E. Pancino, E. Rossetti BARCELONA: C. Jordi, G. Anglada, J.M. Carrasco, F. Figueras GRONINGEN: S. Trager [A. Helmi, E. Tolstoy]

2. Preliminary assessment - 1  Definition of requirements ( see GAIA-C5-TN-OABO-MBZ-001-1 ): general description of criteria for ● selection of SpectroPhotometric Standard Stars (SPSS) – number, spectral types, magnitude range, spatial distribution ● selection of observing modes – spectrophotometry, spectroscopy & photometry, spectral resolution, etc.  Definition of observing protocol (draft, in preparation by OABO)  Definition of tests for pilot program (draft, in preparation by OABO)  Definition & construction of database for SPSS obs. (draft, in preparation by OABO)

3. Preliminary assessment - 2  Simulations by the Barcelona team ( see GAIA-C5-TN-UB-JMC-039-1, draft ): S/N estimates for BP/RP spectra and crowding due to FoV overlapping vs. ● magnitude ● interstellar extinction ● n. of observations ● calibration errors  quantitative estimate of limiting magnitude, position on the sky and spectral type of candidate SPSS  aiming at S/N > 100 over entire spectral range for both BP and RP

4. Preliminary assessment - 3  Total n. of uncrowded transits i.e. observations at the end of the mission as a function of position in the sky (equatorial coordinates) Average n. of observations on the full sky is 80

5. Preliminary assessment - 4  end of mission SNR for B5, A0, F0, G2, K5 stars with V=10, 13, 16. Assumed zero reddening, 80 observations and calibration error =10 mmag

6. Work in progress: SPSS selection - 1  Evaluation and selection criteria for SPSS (see GAIA-C5-TN- OABO-MBZ-001-1): ● as featurless as possible (WDs, hot sd, very metal-poor) ● need for spectral type coverage  metal-poor red stars ● initial estimate of magnitude range V=10-16 ● position on the sky: for a mean value of 80 transits  stars at V=16 do not reach S/N=100 in the bluest part of BP and RP (exact fraction depending on spectral type)  stars at V=13 require more than the average 80 transits to reach S/N=100 in the bluest range  for stars brighter than V=13 no requirements on the number of transits are needed  Need to restrict magnitude range to V=10-14

7. Work in progress: SPSS selection - 2 Construction of database for Gaia SPSS:  Scan of literature  HST CalSpec primary SPSS ● based on 3 pillar stars + best existing Vega cal. (~ 2% vis & 4% ir) [ Bohlin & Gilliland 2004 ] ● entire wavelength coverage ● 1% internal photometric accuracy ● incomplete spectral type coverage (few red stars) ● few stars with high enough S/N in BP & RP (mag. + sky location)  Altogether, including other suitable sources in addition to HST CalSpec, < 40 stars are candidate primary SPSS  Need to expand with additional secondary SPSS to reach ~ 100 stars (exact number TBD based on simulations & pilot program)

8. Work in progress: ground-based observations -1 The observing campaign: Both primary and secondary SPSS need to be (re)observed from the ground to ensure homogeneous treatment (spectral coverage, resolution, telluric lines removal, data reduction, etc.) and same zero-point of the flux scale The observing criteria (to match BP/RP characteristics):  Spectrophotometry: ideal, but difficult (ph. conditions are rare)  Spectroscopy & Photometry ● Spectral coverage 330-1050 nm, possibly in no more than 2 steps with partial overlap ● Spectral resolving power R ~ 500-1000 ● Aiming at S/N=100 per res. element over the entire range ● Use of standard (Johnson UBV + Cousins RI) filters ● Aiming at 1% accuracy in flux

9. Work in progress: ground-based observations -2 The observing campaign needs to be preceded by a pilot program ● to perform all necessary tests to ensure feasibility and homogeneity of procedures and quality of the final database ● to start building the database of primary SPSS  Test Methods: ● calibration (e.g. SP vs S&P) ● observing (strategies, e.g. are all UBVRI needed?) ● data reduction (e.g. essential calibration frames)  Test Quality: ● night, run, telescope, instrument (e.g. systematics) ● site (e.g. photometric conditions, extinction curve)

10. Work in progress: ground-based observations -3 The pilot program  Evaluation of sites/telescopes/instruments (see GAIA-C5-TN- OABO-LF-001-1, draft) ● Calar Alto: CAFOS@CAHA ● Canary Islands: ISIS@WHT; IDS@INT; ALFOSC@NOT; LRS@TNG ● ESO: EMMI@NTT; FORS2@UT1 ► LRS@TNG, CAFOS@CAHA and FORS2@UT1 are ranked highest for pilot program, ranking may change after pilot program results  At least three facilities need to be used for the observing campaign (for risk management policy)

11. Work in progress: ground-based observations - 4  Observing proposal for pilot program ready for submission at Calar Alto (deadline Sept. 15)  Observing proposal for pilot program in preparation for submission at TNG (deadline around Oct. 15)  Possible proposal for ESO considered for spring 2007 deadline (depending on coordination with other CUs, TBD)