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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.