Extracting Photometry From SPIRE Maps - ESAherschel.esac.esa.int/twiki/pub/Public/DataProcessingWorkshop2012/... · Extracting Photometry From SPIRE Maps Wednesday, ... • The ﬁne-scale

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Luca ConversiHerschel Science Centre

OT2 DP WorkshopESAC, Madrid, 20-24/02/2012

Luca Conversion behalf of the SPIRE ICC

Extracting PhotometryFrom SPIRE Maps

Wednesday, 22 February 12


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• Map Calibration and Units– How Calibration is Applied– Units and Beam Areas

• Bright Point Source–Which technique to use?– Estimating uncertainties– Peak fitting demo

• Source Extraction– Inside HIPE: DAOPhot & Sussextractor– Correction factors

• Extended Emission– Spectral response for extended sources– Caveats

Presentation Outline



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Calibration And Units - 1

• SPIRE Calibration is defined in a straightforward way

• When a detector is scanned directly over a point source, the peak deflection of the signal timeline equals the brightness of the source.

Scan of detector PSWE8 over

Neptune,obsid 1342187440

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Calibration And Units - 2

The timeline-based definition leads to some accounting corrections

• Units are in Jy/beam• Peak value is nominally the brightness

• Calibration applies to timelines• Map-making lowers the peaks

• Calibration is for ν*Fν = constant• Color correction for other spectral shapes

• Calibration is for point sources• Correction for extended sources

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• Pixelization corrections are described in the SPIRE Observer’s Manual: see Sec 5.2.11

• For nominal pixel sizes,divide by factors of:

– 250µm: 0.941

– 350µm: 0.917

– 500µm: 0.903

250 350

500

Pixelization Correction

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• Colour corrections are described in the SPIRE Observer’s Manual: see Sec 5.2.8

• Multiply by the value appropriate for your source

• Different corrections for point and extended sources

• For specific cases (e.g., grey body), compute your own colour correction.Filters RSRF are available in the calibration tree:– rsrf = obs.calibration.phot.rsrf

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Colour Correction



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Unit Conversion

• Use the fine-scale beam areas to convert Jy/beam to other units

• The fine-scale beam areas were measured from Neptune maps with 1-arcsec pixels

– 250µm: 423 arcsec2

– 350µm: 751 arcsec2

– 500µm: 1587 arcsec2

• However, these values may be updated in the upcoming weeks. Have a look at the SPIRE Twiki http://herschel.esac.esa.int/twiki/bin/view/Public/SpireCalibrationWeb?template=viewprint

• The convertImageUnit task will change maps to Jy/pixel or MJy/sr

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http://herschel.esac.esa.int/twiki/bin/view/Public/SpireCalibrationWeb?template=viewprint

http://herschel.esac.esa.int/twiki/bin/view/Public/SpireCalibrationWeb?template=viewprint


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Gaussian Fitting

Peak-fitting is recommended for point sources

• Gaussian-fitting on maps gives good results for bright sources– Gaussian is good approximation for radii up to (22, 30, 42) arcsec for (PSW, PMW, PLW)

respectively

• Fitting Gaussians to timelines is under development by ICC– Better results for 100mJy+ sources– Prototype available as script on the HSC public TWiki:

http://herschel.esac.esa.int/twiki/pub/Public/SpireCalibrationWeb/bendoSourceFit_v0_9.py.txt

• Aperture photometry is possible– Official corrections not yet available– Derive your own

• Beam profiles are available in the calibration tree:– beamPSW = obs.calibration.phot.getBeamProf('PSW')– beamPMW = obs.calibration.phot.getBeamProf('PMW')– beamPLW = obs.calibration.phot.getBeamProf('PLW')

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Flux Uncertainties

Combine these in quadrature to estimate the uncertainty

• Uncertainty in the fitted amplitude– Includes the instrument and confusion noise

(minimum of ~5 mJy)

• 2%-3% of flux density for uncertainty in pixelization correction

• 7% of flux density for calibration uncertainty– 2% relative (band-to-band) uncertainty– 5% is Neptune model (does not apply to SPIRE colours)

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Source Extraction in HIPETwo source extractors are available within HIPE:

1. sourceExtractorSussextractor implements the SUSSEXtractor algorithm, as described by Savage & Oliver (2007), ApJ, 661, 1339. The image is smoothed with a convolution kernel, derived from the point response function, and the resulting smoothed image is searched for peaks, which are taken to be the positions of the point sources. The intensity in the smoothed image at the position of a point source is taken as the estimate of that source's flux density. Note that "NaN" pixels near point sources in the image or error extensions of the input image may prevent sourceExtractorSussextractor from finding that source

2. sourceExtractorDaophot implements the DAOPHOT (classic) algorithm, following the "FIND" and "APER" procedures in the IDL Astronomy User's Library. The image is smoothed with the DAOPHOT convolution kernel to find the source positions, as in the sourceExtractorSussextractor task. Then the source flux densities are estimated using aperture photometry, as in the aperturePhotometry task

• Both runnable as Tasks

• Both can accept input source lists

• You must compute corrections yourself

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http://adsabs.harvard.edu/abs/2007ApJ...661.1339S

http://adsabs.harvard.edu/abs/2007ApJ...661.1339S

http://idlastro.gsfc.nasa.gov/contents.html#C2

http://idlastro.gsfc.nasa.gov/contents.html#C2


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Correction Factors

The simplest operation of extractors uses the FWHM of each band

• Averages for nominal pixels:– 250µm: 17.5 arcsec– 350µm: 23.9 arcsec– 500µm: 35.2 arcsec

• You can supply your own PRF image

• The fine-scale beam areas are needed (or the images must be converted)– (423, 751, 1587) arcsec2

Corrections are still needed for extractor outputs

• Need an aperture correction appropriate to FWHM– About 25% - 30%

• Still need color correction

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• The extended spectral response differs from that for point sources

• Coupling for extended source that uniformly fills the beam (Observer’s Manual)

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Extended Emission RSRF



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Extended Emission Caveats

• No absolute level– Could get from Planck

• Map-‐making technique can affect extended emission– Baseline removal vs destiper: note different flux scale on the images

• CIB fluctuations at a faint level

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Median Baseline Removal Destriper



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Ext. Emission Photometry

• Units correction and extended correction must be applied

• Correct units e.g. to Jy/pixel

– convertImageUnit task

– Fine beam areas

• Multiply by these factors for extended relative spectral response function

– 250µm 0.9939/1.0113

– 350µm 0.9898/1.0060

– 500µm 0.9773/1.0065

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To summarize...

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Point Source Photometry

Do you know where your sources are?

Yes, but they can have any flux

Yes, and they are brighter than ~20mJy in the SPIRE bands

No

Annular Sky Aperture

Timeline Fitter

Sussex or DAOPhotSource Extractor

Extended Source Photometry



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Sussex Source Extractor

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Sussex Source Extractor

Input: FWHM, beam area,threshold

Output: source catalogue Pixelization correction

Timeline Fitter Annular Sky Aperture

Loop on [RA, Dec] Colour correction[optional]

DONE!



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DAOPhot Source Extractor

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DAOPhot Source Extractor

Input: FWHM, beam area,threshold, aperture

Output: source catalogue Pixelization correction

Timeline Fitter Annular Sky Aperture

Loop on [RA, Dec]

Aperture correction

DONE!

Colour correction[optional]



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Timeline Fitter

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Timeline Fitter

Output: fitted [RA, Dec],flux, error, χ2

Colour Correction[optional]

DONE!

Input: source [RA, Dec]

Single point sourceor catalogue

Note: if you have an external source catalogue (i.e., not derived on SPIRE), it is better the first run a source extractor.Otherwise, it is likely that the timeline fitter will give wrong results for the faintest (below ~20mJy) sources



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Colour correction[optional for point sources;

mandatory for extended emission]


convertImageUnit taskInput: beam area

Convert map in Jy/pixel

Output: flux, backgroundestimation, error, etc.

Point or extended sourceor catalogue

Input: source [RA, Dec], annuli radii

Pixelization correction

Aperture correction DONE!



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Demo Time!

• Run the script OT2_Workshop_SPHOT_Photometry.py

• Target: Gamma Dra

– Model fluxes: (254, 130, 64) mJy

• Methods used to extract photometry

– Timeline fitter (bendoSourceFit) on a specific source (Gamma Dra)

– SussexExtractor on the PSW map + timeline fitter on the extracted sources

– Aperture photometry of Gamma Dra on PSW map

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Extracting Photometry From SPIRE Maps - ESAherschel.esac.esa.int/twiki/pub/Public/DataProcessingWorkshop2012/... · Extracting Photometry From SPIRE Maps Wednesday, ... • The ﬁne-scale