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]
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CU5-DU13: Ground-based support observations for Gaia absolute photometric calibration BOLOGNA: C. Cacciari, G. Altavilla, M. Bellazzini, A. Bragaglia,
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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]
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)
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
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
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
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
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)
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
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)
Work in progress: ground-based observations -3 The pilot program
Evaluation of sites/telescopes/instruments (see GAIA-C5-TN-OABO-LF-001-1, draft)