Flux calibration of the Photometer

1
Flux calibrationof the Photometer

• Koryo Okumura, Marc Sauvage,
• Nicolas Billot, Bertrand Morin
• DSM/DAPNIA/Sap

2
Stable signal for a better photometry

• How stable is the signal?
• Response?
• Offset?
• Response variation is quite small
• less than 0.1
• Offset is the main component of the drift seen in
  the noise spectral density
• about some hundreds of micro volts in 3 hours

3
Response and offset monitoring
4
Drift seen in low frequency measurements
5
Linearity and non-linearity of the response
1 of responsivity loss for 0.1 pW/pixel 3
Jy/pixel
6
Flat field derivation using data from ILT3 (1)
OGSE BB
CS2
CS1
Chopper scan
Median value for each pixel on this interval
Responsivity image
Responsivity ltresponsivity imagegt, Flat_field
Responsivity
7
Flat field derivation using data from ILT3 (2)

• Chopper scan on OGSE of the field of view
  measurements are used
• Median 2D image is considered to have a flat flux
  level at the central chopper interval
• Difference of 2 median images of different fluxes
  (pixel to pixel response to a uniform brightness
  distribution through the central field-of-view
  distortion)
• This divided by the flux difference computed from
  OGSE temperatures provides a pixel to pixel
  responsivity 2D map
• The average over the valid pixels gives the mean
  responsivity (scalar) and the flat field

8
Field of view measurements during ILT3Blue with
2 different filters
T_ogse kelvin 70 um pW/pixel 100 um pW/pixel dFlux pW/pixel
10 0.0000 0.0004 0.0003
15 0.002 0.029 0.026
20 0.055 0.281 0.227
22 0.132 0.530 0.399
25 0.382 1.142 0.761
30 1.424 2.953 1.529
35 3.694 5.884 2.191
40 7.612 9.951 2.340
9
Flux difference on the blue detectorwith 2
different filters and different OGSE temperatures
Flux 1 pW/pixel Flux 2 pW/pixel dFlux pW/pixel
0.0000 0.0004 0.0003
0.0004 0.0023 0.0020
0.0023 0.0287 0.0264
0.0287 0.0548 0.0260
0.0548 0.1317 0.0769
0.1317 0.2815 0.1498
0.2815 3815 0.1000
0.3815 0.5303 0.1488
0.5303 1.1421 0.6118
1.1421 1.4238 0.2817
1.4238 2.9532 1.5293
2.9532 3.6937 0.7405
3.6937 5.4843 2.1906
5.8843 7.6117 1.7275
7.6117 9.9514 2.3396
10
Flux difference on the red detectorwith 2
different OGSE temperatures
Flux 1 pW/pixel Flux 2 pW/pixel dFlux pW/pixel
0.043 0.712 0.669
0.712 3.107 2.395
3.107 4.693 1.586
4.693 7.760 3.067
7.760 14.574 6.814
14.574 23.207 8.633
23.207 33.294 10.088
11
Noise propagation in the photometry

• Signal flat_field x Flux offset
• dSignal flat_field x dFlux
• Each time we do a multiplication or division, a
  relative variance is added
• dFlux should be large to reduce the noise, but
  should be small enough to stay within the valid
  linear range

12
Which responsivity and flat field to use?

• Standard flat field and CS flux
• OGSE flat field dSignal / dFlux
• CSs flux dSignal / flatField
• Calibration block flat field
• CSs flat field dSignal / dFlux

13
How do we use the CS calibration blocks?

• Standard flat field
• Flux Signal / OGSE_flat_field
• Sum of relative variances of
• OGSE flux
• OGSE signal
• Data signal

• CSs flat field
• Flux Signal / CSs_flat_field
• Sum of relative variances of
• OGSE flux
• OGSE signal
• Cal CSs signal
• Data CSs signal
• Data signal

14
Flat field and responsivity as calibration file

• The standard flat field should be used for the
  flux calibration of the data
• The flat field depends on the flux level
• We need flat fields with low noise
• The flat field has to be interpolated from
  available flux level to a real background flux
  level
• Flat field of the red detector contains the
  electrical cross-talk
• Flat field of the red detector is a poor quality
  because of the offset drift in the measurements
• How can we measure in orbit a good flat field, if
  necessary?