PVPhotFlux PACS Photometer photometric calibration

1. MPIA PVPhotFluxPACS Photometer photometric calibration PACS Commissioning and PV Phase Plan Review 21st – 22nd January 2009, MPE Garching Markus Nielbock (MPIA) Marc Sauvage (CEA/SAp)

2. Instrumental background • PACS photometer (PHOT) two bolometer arrays • blue detectors: 4 x 2 matrices of 16 x 16 pixels • red detectors: 2 matrices of 16 x 16 pixels M. Nielbock, M. Sauvage – PVPhotFlux • additional optical elements • filter wheel • mirrors (external and internal) • chopper

3. PHOT photometric calibration topics • PACS Calibration Document (PCD) requirements 3.2 • mainly covered by observations of PVPhotFlux proposal M. Nielbock, M. Sauvage – PVPhotFlux • partly fulfilled by interdependent PCD requirements • partly covered by related observations of other PV PHOT proposals

4. PCD req. 3.2.1Determination of detector responsivity • establish relation between voltage output and absolute sky brightness • internal calibration sources (CS), celestial standards • measure irradiation power vs. detector signal M. Nielbock, M. Sauvage – PVPhotFlux • fully covered by PVPhotBol (PHOT detector characterisation)

5. PCD req. 3.2.2Monitor stability 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 M. Nielbock, M. Sauvage – PVPhotFlux • 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

6. PCD req. 3.2.2Monitor 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 M. Nielbock, M. Sauvage – PVPhotFlux • 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

7. PCD req. 3.2.3Calibrate non-linearity • 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) M. Nielbock, M. Sauvage – PVPhotFlux • 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 • 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

8. PCD req. 3.2.4Establish full system linearity • 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 M. Nielbock, M. Sauvage – PVPhotFlux • 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

9. PCD req. 3.2.6Noise and minimum detectable flux • establish NEP depending on detector biasing • internal calibration sources • fully covered by PVPhotBol (PHOT characterisation) • independent confirmation of minimum flux may be desirable M. Nielbock, M. Sauvage – PVPhotFlux • 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

10. PCD req. 3.2.8Full system flat field • 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 M. Nielbock, M. Sauvage – PVPhotFlux • 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

11. PCD req. 3.2.8Full 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 M. Nielbock, M. Sauvage – PVPhotFlux • Status: fully defined and implemented • Analysis: SPG (pipeline), additional work based on SOVT-2 results

12. PCD req. 3.2.9Telescope background and stability • telescope will be major flux source • determine spatial and temporal stability of telescope contribution • assessed by frequent field-of-view scans with chopper M. Nielbock, M. Sauvage – PVPhotFlux • fulfilled by PCD req. 3.1.7 (FOV characterisation) • fully covered by PVPhotSpatial

13. Summary • PV plan regarding the photometric calibration of the PACS photometer is fully prepared as it is currently defined. • All relevant calibration requirements (PCD) are met. M. Nielbock, M. Sauvage – PVPhotFlux • 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.