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Radiometric performance

Radiometric performance. ESTEC – SEPTEMBER 10, 2002 Instrument performance session. Gilbert BARROT. GOMOS radiometric performance. Non-linearity Non-uniformity Radiometric calibration. GOMOS radiometric performance. Non-linearity determination.

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Radiometric performance

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  1. Radiometric performance ESTEC – SEPTEMBER 10, 2002 Instrument performance session Gilbert BARROT

  2. GOMOS radiometric performance • Non-linearity • Non-uniformity • Radiometric calibration

  3. GOMOS radiometric performance Non-linearity determination Observation of the same star outside the atmosphere with different integration times (use of the GOMOS linearity monitoring mode) Use of a dedicated tool (CALEX in mode NL) to compute the non-linearity factors for each gain setting of the GOMOS instrument

  4. GOMOS radiometric performance Non-linearity Instrument set in Linearity monitoring mode Star 18 mv=1.17 T=9700K Gain setting SPA 4/4 SPB 0/0 Nominal gain setting for the star observation (except Sirius)

  5. GOMOS radiometric performance Non-linearity Star 18 mv=1.17 T=9700K Example: for column 250 (SPA1) Deviation wrt Linear regression Electronic chain Offset Gain setting SPA 4/4

  6. GOMOS radiometric performance Electronic chain offset Star 18 mv=1.17 T=9700K Example: for SPA1 gain 4 Mean offset 122.4 ADU St. dev. 1.2 ADU Very stable < 0.1 ADU

  7. GOMOS radiometric performance Non-linearity Example: Non-linearity for SPA1 gain 4 fnlin<0.1% on full range

  8. GOMOS radiometric performance Non-linearity calibration Conclusions  Specification: 0.1% per decade Measured: <0.1% on full dynamic range  Determination of the electronic chain offset during the linearity calibration

  9. GOMOS radiometric performance Non-uniformity An accurate PRNU map is needed when mixing information from different spatial bands (e.g. background correction) Analysis of GOMOS observations in bright limb conditions GOMOS is set in Uniformity Monitoring Mode (no star) Use of a dedicated tool (CALEX in modes QP/QB) to compute the Pixel Response Non-Uniformity Correction of PRNU LUT stored in the GOMOS calibration database

  10. GOMOS radiometric performance Non-uniformity results • Non-uniformity has been measured at pixel and at band level. Measurements are made in bright limb. • ‘Bad pixel’ in DMSA2 (-26%) already seen on ground has not moved • Interferences in IR present the same shape and amplitude than on ground (anyway the CCD temperature affects the location of the inteferences) • Slit profile measurement is consistent with ground • Performance is consistent with expected

  11. GOMOS radiometric performance Non-uniformity map of IR2 Interferences

  12. GOMOS radiometric performance Impact of a ‘bad’ PRNU map in bright limb Using on-ground PRNU map SPB2 Using updated PRNU map Star 5 / Orbit 2130 Bright limb TGP alt = 42 km

  13. GOMOS radiometric performance Non uniformity Conclusions • Non-uniformity shall be monitored every month, especially in IR, where interferences position is influenced by temperature. • No variation is expected if temperature is stable. • No trend has been noticed yet : no pixel response degradation even for ‘’hot pixels’’.

  14. GOMOS radiometric performance Radiometric calibration Comparison of GOMOS measurements (star spectra) with existing calibrated star spectra Use of a dedicated tool (CALEX in mode RS) to compute the deviation between the two spectra Correction of the radiometric sensitivity LUT stored in the GOMOS calibration database

  15. GOMOS radiometric performance Radiometric calibration Mesured star flux has been converted from electrons to physical units using the Radiometric Sensitivity LUT SPA2 Observed relative variations are used to correct the LUT

  16. GOMOS radiometric performance Radiometric calibration SPA1

  17. GOMOS radiometric performance Radiometric calibration Radiometric sensitivity in the invalid spectral range 375-405 nm

  18. GOMOS radiometric performance Radiometric stability Sirius spectrum measured between orbit 2154 and 2186 (3 days)

  19. GOMOS radiometric performance Radiometric stability Sirius spectrum measured between orbit 2154 and 2186 (3 days) Variation level between orbits is compatible with the theoretical noise

  20. GOMOS radiometric performance Radiometric calibration Conclusions  Very stable radiometry. No trend observed yet  Absolute radiometric calibration has no impact on the level 2 products (species density profiles) Transmission and associated error are computed from relative variations of the star spectra

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