NHSC SPIRE Point Source Spectroscopy Webinar
21 March 2012
Extracting Photometry
from SPIRE Maps
David Shupe, Bernhard Schulz, Kevin Xu
on behalf of the SPIRE ICC
PACS
NHSC SPIRE Point Source Photometry Webinar
21 March 2012
SPIRE Point Source Photometry
• SPIRE Calibration and Map Units
• Fitting Functions to Point Sources
• Considerations for Aperture Photometry
• Source Extraction
• Summary: Checklists for Astronomers
• Demos of the HIPE Photometry Tools
page ‹#›
PACS
NHSC SPIRE Point Source Photometry Webinar
21 March 2012
SPIRE Point Source Photometry
• SPIRE Calibration and Map Units
– How Calibration is Defined
– Units and Beam Areas
• Fitting Functions to Point Sources
• Considerations for Aperture Photometry
• Source Extraction
• Summary: Checklists for Astronomers
• Demos of the HIPE Photometry Tools
page ‹#›
PACS
SPIRE calibration is defined by the
peak deflection when scanning
• 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
The timeline-based definition
leads to some accounting corrections
• Units are in Janskys per beam
• Most software wants Jy/pixel or MJy/sr
• Map-making lowers the peaks…
• …relative to the timelines
• Calibration is for point source peaks
• Some detectors have bigger areas
• Calibration is for nu*Fnu = constant
• Color correction for other spectral shapes
5
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
– 250um: 423 sq arcsec
– 350um: 751 sq arcsec
– 500um: 1587 sq arcsec
• These values may change slightly in the
near future (by a few percent)
• The “convertImageUnit” task will
change maps to Jy/pixel or MJy/sr
6
Divide peaks measured in maps
by the pixelization correction
– 0.951 (250)
– 0.931 (350)
– 0.902 (500)
Curves obtained by integrating skybins/pixels
over Gaussian approximations to the average beam
Factor by which peak is lowered
• See Sec 5.2.11
of the SPIRE
Observer’s
Manual
• For nominal
pixel sizes,
divide by
factors of:
1.0
500
250
0.9
350
0.8
6”
10”
14”
Pixel size in arcseconds
7
Applying the Relative Gains equalizes
the detector areas (instead of the peaks)
• Gains are provided in the SPIRE cal tree
• These gains are applied before
destriping and map-making
• More details when we talk about
Aperture Photometry
8
Single-detector Neptune maps show the gains
reduce the spread in aperture photometry
Neptune, Operational Day 168, PSW (250 microns)
Number of Detectors in Flux Bin
Timeline-fitting
Aperture Photometry
Measured Flux (Jy)
9
Aperture radius = 150”
Aperture correction = 1.027
Color corrections are described in the SPIRE
Observers’ Manual & Data Reduction Guide
10
Multiplicative Correction
• See Sec 5.2.8
• Multiply by the
value
appropriate for
your source
• For specific
cases, compute
your own
correction from
the filter RSRFs
Color Correction Factor vs. Assumed
Source Spectral Index (Point Source)
250
1.0
0.98
500
350
0.94
0.90
-3
-2
-1
0
1
2
3
Source Spectral Index (in frequency)
NHSC SPIRE Point Source Photometry Webinar
21 March 2012
SPIRE Point Source Photometry
• SPIRE Calibration and Map Units
• Fitting Functions to Point Sources
– Peak-fitting techniques
– Estimating uncertainties
• Considerations for Aperture Photometry
• Source Extraction
• Summary: Checklists for Astronomers
• Demos of the HIPE Photometry Tools
page ‹#›
PACS
Peak-fitting is recommended
for point sources
• Fitting Gaussians to timelines is the best
method for sources visible on the maps
– Jython script: bendoSourceFit_v0.9.py
– Java task: timelineSourceFitterTask
• Gaussian-fitting on maps is serviceable as
a second choice
– Java task: sourceFitting
– Gaussian is good approximation for radii up to
(22, 30, 42) arcsec
To estimate the total uncertainty in flux,
combine these terms in quadrature
• Uncertainty in the fitted amplitude
– Includes the instrument and confusion
noise (minimum of about 5 mJy)
• 7% of flux density for
calibration uncertainty
– 2% statistical reproducibility
– 5% absolute level of Neptune model
(this term does not apply to SPIRE colors)
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NHSC SPIRE Point Source Photometry Webinar
21 March 2012
SPIRE Point Source Photometry
• SPIRE Calibration and Map Units
• Fitting Functions to Point Sources
• Considerations for Aperture Photometry
– Applying the relative gains
– Aperture corrections
• Source Extraction
• Demos of the HIPE Photometry Tools
page ‹#›
PACS
The relative gains may be applied using a
Useful Script, or the SPIA
• SPIRE official pipeline scripts
– applyExtendedEmissionGains option
• SPIRE Useful Script: Photometer Baseline
Removal and Destriper
– Example is at the end (5th one)
• spiaLevel2 task
– Relative gains are applied by default
• In the last 2 cases, the original Level 1 timelines
are left untouched
– The point-source calibration still based on peaks
– The best of both worlds: timeline-fitting on
Level 1, and aperture photometry on maps
15
Official aperture corrections are now in
the SPIRE Data Reduction Guide!
• Available in the online HIPE 8 docs
http://herschel.esac.esa.int/hcss-doc8.0/load/spire_drg/html/ch05s07.html
• Not in the docs that come with HIPE 8.1
• Values in Photometry Recipe:
– 1.2750 (250μm) 22” radius, sky 60”-90”
– 1.1933 (350μm) 30” radius, sky 60”-90”
– 1.2599 (500μm) 42” radius, sky 60”-90”
– Uncertainties at 5% level
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NHSC SPIRE Point Source Photometry Webinar
21 March 2012
SPIRE Point Source Photometry
• SPIRE Calibration and Map Units
• Fitting Functions to Point Sources
• Considerations for Aperture Photometry
• Source Extraction
– Inside HIPE: DAOPhot & Sussextractor
– DAOPhot Demo
• Summary: Checklists for Astronomers
• Demos of the HIPE Photometry Tools
page ‹#›
PACS
Two source extractors are available
within HIPE
• sourceExtractorDaophot
– Algorithms from IDL AstroLib
– FIND for detection
– APER for photometry (not PSF-fitting!)
• sourceExtractorSussextractor
– Sussextractor algorithm
(Savage & Oliver 2007)
– Flux density is peak of smoothed image
– Good for detecting sources
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The simplest operation of extractors
uses the FWHM of each band
• Averages for nominal pixels:
– 250um: 18.2 arcsec
– 350um: 24.9 arcsec
– 500um: 36.3 arcsec
• You can supply your own PRF image
• The fine-scale beam areas are needed
(or the images must be converted)
– (423, 751, 1587) sq arcsec
19
Corrections are still needed
for extractor outputs
• For sourceExtractorDaophot, need an
aperture correction appropriate to
radius supplied
• For sourceExtractorSussextractor, the
correction seems to be flux-dependent
• A nice combination for sources 20 mJy
and brighter, is to use Sussextractor for
detection and timeline-fitting for the
photometry (see demo)
20
NHSC SPIRE Point Source Photometry Webinar
21 March 2012
SPIRE Point Source Photometry
• SPIRE Calibration and Map Units
• Fitting Functions to Point Sources
• Considerations for Aperture Photometry
• Source Extraction
• Summary: Checklists for Astronomers
• Demos of the HIPE Photometry Tools
page ‹#›
PACS
Checklist for timeline-fitting photometry
Use the Level 1 timelines without
relative gains applied
Perform first-order baseline correction
Supply a starting position
Run the bendoSourceFit or the
timelineSourceFittingTask
Apply color correction as needed
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Checklist for fitting on the map
Use the map made without relative
gains applied
Run the sourceFitting task
Divide by the pixelization correction
Apply color correction as needed
23
Checklist for aperture photometry
Make the map with relative gains
applied (destriping recommended also)
Convert to Jy/pixel using
“convertImageUnit” task
Run the
annularSkyAperturePhotometry task
Apply an appropriate aperture
correction
Apply color correction as needed
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NHSC SPIRE Point Source Photometry Webinar
21 March 2012
SPIRE Point Source Photometry
• SPIRE Calibration and Map Units
• Fitting Functions to Point Sources
• Considerations for Aperture Photometry
• Source Extraction
• Summary: Checklists for Astronomers
• Demos of the HIPE Photometry Tools
page ‹#›
PACS
Demos of the photometry tools in HIPE
• Fitting Gaussians on the map
(Bernhard)
• Timeline-fitting of Gaussians (Bernhard)
• Aperture Photometry (Kevin)
• Source detection with Sussextractor
combined with timeline-fitting (David)
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