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Calibration Faint Stars and the RVS

Calibration Faint Stars and the RVS. Carlos Allende Prieto CU6 Workshop #8 Nice 23-25 November 2009. Calibration Faint Stars. 2D windows available only for Grvs<7 To facilitate calibration, 2D windows can also be available for fainter sources: CFS

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Calibration Faint Stars and the RVS

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  1. Calibration Faint Starsand the RVS Carlos Allende Prieto CU6 Workshop #8 Nice 23-25 November 2009

  2. Calibration Faint Stars • 2D windows available only for Grvs<7 • To facilitate calibration, 2D windows can also be available for fainter sources: CFS • The rules: cannot choose particular objects, tables are limited in size and changing patterns requires stopping a VPU, CFS shared by all instruments (not RVS when G>17) • Useful for many purposes: OGA1 (attitude calib. in IDT), RP/BP calibration of ungated observations, background verification, AC LSF calibration • MB-025 gives a specific proposal (in fact, three) addressing these issues, but there are (I have) concerns about RVS needs not being met

  3. RVS case: CFS and Serial Register CTI • CFS may be critical to deal with LSF distortions due to charge transfer inefficiency in the serial Register • Note that we probably can/should keep LSF definition independent of magnitude: SR CTI handled as AL CTI, by means of CDM which acts on AC LSF to produce the profiles used in deblending Figure credit: EADS/Astrium

  4. CFS and Serial Register CTI • Significant flux loss • Mitigation involves shifting and enlarging windows, with implications for blending (currently at ~10% of all RVS observations) • AC LSF critical to correct for charge loss and deblending Figure credit: EADS/Astrium

  5. How many CFS do we need to calibrate the AC LSF? • Grvs = -2.5 log S + 22.5866 S=total signal in the spectrum (e-) • Tests with a Gaussian LSF indicate 1% precision at G~9 (10 wavelength bins)

  6. How many CFS? • We calculate how many CFS are needed by equating the S/N at Grvs=9 to the combined S/N for n observations at a fainter G

  7. How many CFS? • Universe model: log N = 4.56 + 0.18 Grvs • 40 visits on average, 4 CCD rows, 10 AC strips, time variations (10 independent samples: calibration unit = 6 months)

  8. CFS proposal 17<G<20

  9. Summary • MB-025 proposal does not match RVS needs • Some (2/3) of the schemes proposed in MB-025 take all stars within 14.-14.5 mag for calibration • Hardest and probably most important item for RVS is to get enough signal on the faint bin. That may require ~10% of data (this estimate depends on a number of assumptions that must be checked/relaxed) • Watch for blurring when co-adding all the spectra in a bin • AL binning for faint CFS could help to reduce overhead • See draft GAIA-C6-SP-MSSL-CAP-006 for details

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