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MPIA PVPhotFlux PACS Photometer photometric calibration PACS Commissioning and PV Phase Plan Review 21st – 22nd January 2009, MPE Garching Markus Nielbock (MPIA) Marc Sauvage (CEA/SAp) Instrumental background – blue detectors: 4 x 2 matrices of 16 x 16 pixels – red detectors: 2 matrices of 16 x 16 pixels • additional optical elements – filter wheel – mirrors (external and internal) – chopper M. Nielbock, M. Sauvage – PVPhotFlux • PACS photometer (PHOT) two bolometer arrays PHOT photometric calibration topics • mainly covered by observations of PVPhotFlux proposal • partly fulfilled by interdependent PCD requirements • partly covered by related observations of other PV PHOT proposals M. Nielbock, M. Sauvage – PVPhotFlux • PACS Calibration Document (PCD) requirements 3.2 • establish relation between voltage output and absolute sky brightness • internal calibration sources (CS), celestial standards • measure irradiation power vs. detector signal • fully covered by PVPhotBol (PHOT detector characterisation) M. Nielbock, M. Sauvage – PVPhotFlux PCD req. 3.2.1 Determination of detector responsivity • identify amplitude and timescales of responsivity drifts • possible significantly contributing sources: – internal stray light (incl. self-emission) – temperature changes between individual cooling cycles – variation in efficiency of cryo pumping – bias voltage supply – thermal conductance – particle irradiation – interference by other satellite components • calibration targets used: – internal CS – stable celestial flux standard (S/N ≥ 20), repeatedly during PV phase M. Nielbock, M. Sauvage – PVPhotFlux PCD req. 3.2.2 Monitor stability of detector responsivity • Implementation: – internal CS ◦ calibration block during slew to target prior to AOR execution ◦ chopping between two CSs having different temperatures ◦ minimised or no down time for satellite – celestial flux standard ◦ point-source AOR on ε Car (5 repetitions, always visible, ~10 Jy) ◦ estimated time required: 0.5 h • Status: fully defined and implemented • Analysis: SPG (pipeline), additional work based on SOVT-2 results M. Nielbock, M. Sauvage – PVPhotFlux PCD req. 3.2.2 Monitor stability of detector responsivity • characterise the non-linear range of PHOT detectors • non-linearity for very bright sources • calibration targets: very bright flux standards (e.g. bright stars, asteroids) • Implementation: – point-source photometry with reduced gain (avoid electronic saturation) – flux grid of celestial flux standards (2, 10, 50, 200, 500, 1000 Jy) – measure all three filters (simultaneous coverage where possible) – accuracy goal: S/N ≥ 30 – caveat: difficult to find bright and non-variable sources M. Nielbock, M. Sauvage – PVPhotFlux PCD req. 3.2.3 Calibrate non-linearity – estimated time required: 1.3 h • Status: fully defined and implemented (some discussion on target selection) • Analysis: SPG (pipeline), additional work based on SOVT-2 results • verify valid flux range of linear approximation of detector response • calibrate the linear approximation • calibration targets: celestial flux standards • Implementation: (similar to PCD req. 3.2.3) – point-source photometry with default gain setting – flux grid of celestial flux standards (20 mJy to 200 Jy) – measure all three filters (simultaneous coverage where possible) – accuracy goal: S/N ≥ 30 – estimated time required: 5.0 h • Status: fully defined and implemented • Analysis: SPG (pipeline), additional work based on SOVT-2 results M. Nielbock, M. Sauvage – PVPhotFlux PCD req. 3.2.4 Establish full system linearity • establish NEP depending on detector biasing • internal calibration sources • fully covered by PVPhotBol (PHOT characterisation) • independent confirmation of minimum flux may be desirable – not only depends on detector properties – suitable weak calibration targets from ISO GBPP / ISOPHOT Cohen – observe set of targets to minimise impact of flux uncertainties – easy to implement (standard point-source AOT) – easy to analyse (SPG, pipeline) – estimated time required: approx. 10 h M. Nielbock, M. Sauvage – PVPhotFlux PCD req. 3.2.6 Noise and minimum detectable flux • determine (in)homogeneity of PHOT FOV and temporal variation • detector and optical flat field indistinguishable • calibration targets: internal CS, point source or small extended source • Implementation: – internal CS ◦ calibration block during slew to target prior to AOR execution ◦ chopping between two CSs having different brightness (temperatures) ◦ individual CS illumination pattern available from FOV scans ◦ minimised or no down time for satellite M. Nielbock, M. Sauvage – PVPhotFlux PCD req. 3.2.8 Full system flat field • Implementation: – celestial flux standard ◦ scan map AOR all three filters on NGC 6543 and Arp 220 ◦ covering all detector pixels redundantly ◦ estimated time required: 2.9 h • Status: fully defined and implemented • Analysis: SPG (pipeline), additional work based on SOVT-2 results M. Nielbock, M. Sauvage – PVPhotFlux PCD req. 3.2.8 Full system flat field • telescope will be major flux source • determine spatial and temporal stability of telescope contribution • assessed by frequent field-of-view scans with chopper • fulfilled by PCD req. 3.1.7 (FOV characterisation) • fully covered by PVPhotSpatial M. Nielbock, M. Sauvage – PVPhotFlux PCD req. 3.2.9 Telescope background and stability Summary • All relevant calibration requirements (PCD) are met. • Interdependent requirements are partly covered by different calibration programmes (PVPhotBol, PVPhotSpatial). • required observation time in total: 9.6 h • Optional additional observations (lower flux limit check) are easy to implement and may add another 10 hours. M. Nielbock, M. Sauvage – PVPhotFlux • PV plan regarding the photometric calibration of the PACS photometer is fully prepared as it is currently defined.