1. Institute of Cosmos Sciences - University of Barcelona
Gaia Photometry
Institute for Space Studies of Catalonia
and
Institute of Cosmos Sciences - University of Barcelona
Claus Fabricius
On behalf of DPAC CU5

2. Photometric data in Gaia DR1
Phot_g_mean_flux
[e– /s]
Phot_g_mean_flux_error
s.d./ 𝑛
Phot_g_mean_mag
[mag]
Vega system
Phot_variable_flag
string
mostly N/A
Phot_g_n_obs –
# CCD transits
Flux_error is a coarse uncertainty measure
Zero-point (DR1)
Vega:
AB:
Small set of variables

3. Airbus DS

4. Gaia passbands
Jordi et al. 2010

5. Gaia focal plane

6. Observation strategy Sky Mapper detection: magnitude
Exposure time can be restricted by gates:
16, 126, 252, 503, 1006, 2013, ms G ≲ 12 mag
4417 ms G ≳ 12 mag
Only small windows are read out
G < 13 mag
G: 13 – 16 mag
G: 16 – 21 mag

7. Windows read out around stars
0.7 arcsec
G = 14.9
2.1 arcsec
Scan direction
Windows measure 12 × 12 pixels (typically)
Pixels are binned on chip before reading
Lower readout noise
Less telemetry

8. One sample masked (saturation)
Recent obs.
Bright star
2D window
1006 ms exp.
One sample masked (saturation)
Not DR1 !
G = 10.8

9. Recent obs. Bright detect 2D window 252 ms exp. Binary Poor centroid
~ 0.25 ̋ sep.
Poor centroid
G = 9.4

10. Simulated BP / RP spectra
Jordi et al. 2010

11. BP spectra
G = 17.5
600
400

12. RP spectra
G = 16.7

13. Windows in case of conflict
0.7 arcsec
Fainter detection
Truncated window – not used for DR1
2.1 arcsec
Brighter detection
Complete window – with contamination
Window for brighter detection is the winner
Window for the fainter detection will lose
Truncated windows are not used in Gaia DR1

14. Conflicts between spectra
Brighter detection
Complete window
3.5 arcsec
2.1 arcsec
4.1 arcsec
Fainter detection
Truncated window
Window for brighter detection is the winner
We need at least one “good” spectrum for a source
Fainter source of a pair closer than 2 ̋ is lost in DR1

15. A dense field from DR1 Drop at 4 arcsec separation
Very few below 2 arcsec separation

16. A sparse field from DR1 No drop at 4 arcsec separation
Small peak of binaries

17. AF CCD flat field – pre launch
Response non-uniformities wavelength & gate dependent
400 nm
550 nm
900 nm
Carrasco et al. 2016

18. Simulated AC de-centring flux loss
Carrasco et al. 2016

19. Calibration model Response variation Large scale Small scale Gate
FoV & CCD level
Small scale
CCD column level, ~ 500 per CCD
Gate
8 levels of exposure time (ranges of pixel rows)
Colour
6 bands
Time
Strong variations: mirror contamination
Large scale: calibration valid for 1 day
Small scale: single calibration for DR1

20. Calibration “units” Concerns
Scale
CCDs
Strategy
Telescopes
N_AC
N_time
N_CU AF LS 62 10 2 1
420 SS
492
BP/RP 7 6
35 280
20 664
Concerns
Enough observations for the shorter gates ?
Enough sources observed with different gates ?
Enough sources observed with different window classes ?

21. Spectral shape coefficients
𝑪 𝟑
𝑪 𝟐
𝑪 𝟏
𝑪 𝟒
𝑪 𝟓
𝑪 𝟔
Carrasco et al. 2016

22. Flux extraction PSF fitting (G < 13 mag)
Insensitive to AC de-centring
LSF fitting (G > 13 mag)
Wide windows (G mag)
Narrow windows (G mag)
For BP & RP
Simple diaphragm photometry
NB! Not included in DR1

23. Calibration model 𝑓𝑜𝑏𝑠=𝑓𝑖𝑛𝑠𝑡 𝐿 𝑆 𝐿 = 1 6 𝐴 𝑚 𝐶 𝑚 + 0 2 𝐵 𝑗 𝜇 𝑗
𝑓𝑜𝑏𝑠=𝑓𝑖𝑛𝑠𝑡 𝐿 𝑆
𝐿 : large scale (CCD level)
𝑆 : small scale (CCD column level)
𝐿 = 𝐴 𝑚 𝐶 𝑚 𝐵 𝑗 𝜇 𝑗
𝐶 𝑚 : spectral shape coefficients
𝜇 : CCD column
𝑆 : one coefficient every 4 columns
Coefficients 𝐴 𝑚 , 𝐵 𝑗 , 𝑆 : valid over a calibration unit
Carrasco et al. 2016

24. Response monitoring using Tycho-2
decontaminations
M. Hauser 2016

25. Monitoring vs LargeScale calibration
Evans et al. 2016

26. Small Scale calibr., three time ranges
Evans et al. 2016

27. Scatter of individual CCD observations
Gaia science performance
Predicted accuracy
Evans et al. 2016

28. Magnitude zeropoint Spectro-photometric standard stars, SPSS
Nominal passband
Calculate synthetic G magnitudes for SPSS
Compare with instrument G magnitudes
Details:
Carrasco et al. 2016, A&A, in press
Gaia DR1 on-line documentation

29. Zeropoint, Vega-system
Why colour dependence ?
Passband ?
PSF ?
Aperture ?
Carrasco et al. 2016

30. Comparison to APASS
Carrasco et al. 2016

31. G-RP-f(colour) versus G
Arenou et al. 2016

32. Completeness of TGAS Missing bright stars Several stars too faint for
Tycho-2

33. A very faint TGAS stars
Tycho epoch
Gaia epoch

34. A couple of RR Lyr: LMC / MW
Clementini et al. 2016

35. Some limitations of DR1 
Processing
Simple cross match: source duplication
Saturated samples not excluded
No aperture correction (astrometry not yet known)
Dense fields: few faint stars
Extreme colours excluded (fuzzy limit)
Many poorly scanned areas

36. Coming to you in DR2  Processing Dense fields: more faint stars
Much improved cross match
Saturation library enabled
Aperture correction using preliminary DR2 astrometry
Dense fields: more faint stars
Extreme colours included
Few poorly scanned areas