PVPhotFluxPACS Photometer photometric calibration

MPIA

PACS Commissioning and PV Phase Plan Review21st – 22nd January 2009, MPE Garching

Markus Nielbock (MPIA)Marc Sauvage (CEA/SAp)

M. N

ielb

ock, M

. Sauvage – P

VPhotFlu

xInstrumental 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

• additional optical elements

– filter wheel

– mirrors (external and internal)

– chopper

M. N

ielb

ock, M

. Sauvage – P

VPhotFlu

xPHOT photometric calibration topics

• PACS Calibration Document (PCD) requirements 3.2

• mainly covered by observations of PVPhotFlux proposal

• partly fulfilled by interdependent PCD requirements

• partly covered by related observations of other PV PHOT proposals

M. N

ielb

ock, M

. Sauvage – P

VPhotFlu

xPCD 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

• fully covered by PVPhotBol (PHOT detector characterisation)

M. N

ielb

ock, M

. Sauvage – P

VPhotFlu

xPCD 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

– 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. N

ielb

ock, M

. Sauvage – P

VPhotFlu

xPCD 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

◦ 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. N

ielb

ock, M

. Sauvage – P

VPhotFlu

xPCD 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)

– 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

M. N

ielb

ock, M

. Sauvage – P

VPhotFlu

xPCD req. 3.2.4Establish full system linearity

• calibrate the linear approximation

• verify valid flux range of linear approximation of detector response

• 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. N

ielb

ock, M

. Sauvage – P

VPhotFlu

xPCD 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

– not only depends on detector properties

– easy to implement (standard point-source AOT)

– easy to analyse (SPG, pipeline)

– suitable weak calibration targets from ISO GBPP / ISOPHOT Cohen

– observe set of targets to minimise impact of flux uncertainties

– estimated time required: approx. 10 h

M. N

ielb

ock, M

. Sauvage – P

VPhotFlu

xPCD 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

• Implementation:

– internal CS

◦ calibration block during slew to target prior to AOR execution

◦ chopping between two CSs having different brightness (temperatures)

◦ minimised or no down time for satellite

◦ individual CS illumination pattern available from FOV scans

M. N

ielb

ock, M

. Sauvage – P

VPhotFlu

xPCD req. 3.2.8Full system flat field

• Status: fully defined and implemented

– 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

• Implementation:

• Analysis: SPG (pipeline), additional work based on SOVT-2 results

M. N

ielb

ock, M

. Sauvage – P

VPhotFlu

xPCD 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

• fully covered by PVPhotSpatial

• fulfilled by PCD req. 3.1.7 (FOV characterisation)

M. N

ielb

ock, M

. Sauvage – P

VPhotFlu

xSummary

• 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.

• required observation time in total: 9.6 h

• Interdependent requirements are partly covered by different calibration programmes (PVPhotBol, PVPhotSpatial).

• Optional additional observations (lower flux limit check) are easy to implement and may add another 10 hours.