Difference Image Analysis at OAC

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Difference Image Analysis at OAC
Groningen, 1st Dec 2004
AW-OAC team
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• The DIA is a software written by P.R.Wozniak
  based on the Alard Lupton optimal PSF matching
  algorithm.
• DIA was originally created to search variable
  objects on the OGLE-II bugle microlensing data
• The original version is not well documented
  (parameters optimization is not easy) and it is
  optimized for OGLE data
• We present a modified version that includes
• New tool to prepare the images using the
  astrometry
• New functionalities in the core (masks, external
  domains)
• Python interface to send processes on a BEOWULF
  cluster
• New tool to study the candidates (ascii
  catalogues, light curves, frequency analysis,
  phase diagrams, stamps)

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Overview Original DIA
Cross_regrid Registration and correction of input images
mstack Creation of the reference image with best seeing frames
getpsf Global PSF on REF (used in getvar and phot steps)
Aga Creates difference images
Getvar Finds variable candidates
Phot Light Curve with PSF and aperture photometry on the difference images
Not used in the new version New
functionalities added
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AGA step Image Subtraction

• Once the reference frame ref.fits is created it
  is convoluted with a kernel (spatially variable
  in general) in order to match as close as
  possible each image. This convoluted image is
  then subtracted to the current frame, producing a
  serie of subtracted images.

• The Algorithm
• A list of objects is found on the REF (domains)
• A list of objects is found on the single image
  (domains)
• It matchs the domains and calculates the kernels
  (3 Gaussians of costant widths
  multipied by polynomials)
• The best solution is taken to produce the
  difference image

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Getvar step variable objects detection

• Variable objects are detected using some
  preliminary variability measurements based on the
  entire serie of difference images for a given
  field. Final measurements are made
  only for these candidates.
• The program starts by rejecting some fraction of
  the frames with the worst seeing (in our case
  10)
• The pixel is declared as variable if one of these
  two conditions are met
• 1. There are at least 3 consecutive points
  departing at least 3s from the base line in the
  same direction (up or down), or
• 2. There are at least 10 points in total
  departing at least 4s from the base line in the
  same direction, not necessarily consecutive.
• NB The extension to other types of
  variables is straightforward!

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Phot Step Photometry

• For each variable object the program performs
  both profile and aperture photometry on
  difference images keeping the centroid fixed
• The format of the catalog is
• Flux profile photometry
• Flux error profile photometry
• Flux aperture photometry
• Flux error aperture photometry
• Background
• FWHW of the PSF profile

• Where
• Pi is PSF for pixel i
• fi flux for pixel i on differece images
• fi,0 flux for pixel on original images
• G is Gain and

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New DIA Version step 1 wcs2pix

• A C program that reads astrometric informations
  from the headers (CRVAL,CRPIX,CD) and through the
  WCS library finds the common part of the images.
• The WCS library is also used to extract the
  object coordinates in alfa e delta from the
  output files.

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New DIA Version new functionalities

• Added in Aga
• Mask for each input image
• The kernel can be calculated using an external
  list of objects
• Added in Getvar
• Ascii catalogs of variable objects
• Psf image is created and saved (QC check)
• VAR and ABS images are saved (see ISIS)
• Added in Phot
• Ascii light curves with phases

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The New DIA Version how it works on the BEOWULF
Hardware
Software
MASTER
storage
wcs2pix
prepare
Network SWITCH
mstack
Node 1
BeoRunner
getpsf
Node 2
aga
getvar
phot
Node n
lc
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Step 1 cut all images using the astrometric
solution
MASTER
storage
wcs2pix
prepare
Network SWITCH
mstack
Node 1
BeoRunner
getpsf
Node 2
aga
getvar
phot
Node n
lc
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Step 2 split the images in sub frames and copy
them on nodes
MASTER
storage
wcs2pix
prepare
Network SWITCH
mstack
Node 1
BeoRunner
getpsf
Node 2
aga
getvar
phot
Node n
lc
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Step 3 mstack the subframes indipendently on the
nodes
MASTER
storage
wcs2pix
prepare
Network SWITCH
mstack
Node 1
BeoRunner
getpsf
Node 2
aga
getvar
phot
Node n
lc
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Step 4 getpsf
MASTER
storage
wcs2pix
prepare
Network SWITCH
mstack
Node 1
BeoRunner
getpsf
Node 2
aga
getvar
phot
Node n
lc
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Step 5 aga
MASTER
storage
wcs2pix
prepare
Network SWITCH
mstack
Node 1
BeoRunner
getpsf
Node 2
aga
getvar
phot
Node n
lc
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Step 6 getvar
MASTER
storage
wcs2pix
prepare
Network SWITCH
mstack
Node 1
BeoRunner
getpsf
Node 2
aga
getvar
phot
Node n
lc
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Step 7 phot
MASTER
storage
wcs2pix
prepare
Network SWITCH
mstack
Node 1
BeoRunner
getpsf
Node 2
aga
getvar
phot
Node n
lc
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Step 8 lc
MASTER
storage
wcs2pix
prepare
Network SWITCH
mstack
Node 1
BeoRunner
getpsf
Node 2
aga
getvar
phot
Node n
lc
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LC Step Analysis of the results
For each variable candidate the software produces
a light curve in a file called LC_NAMEFIELD_Xpixel
_Ypix_alfa_delta-subframe.data An automatic
Fourier transform is done and an ascii file is
created with frequency, power and s/n ratio
(power/standard deviation). This file is called
as the LC file plus Max_fre_sn in order to
have the main informations directly in the
filename. The frequency of the max peak is used
to add a phase column.
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The format of the LIGHT CURVE

• Flux profile photometry
• Flux error profile photometry
• Flux apeture photometry
• Flux error aperture photometry
• Background
• FWHM of the PSF Profile
• MJD-OBS
• PHASE

• Where
• Pi is PSF for pixel i
• fi flux for pixel i on differece images
• fi,0 flux for pixel on original images
• G is Gain and

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• TESTs
• 69 Images VLT-FORS GC 2kx2k B Band
• 46 Images WFI Carina 8kx9k B Band
• 15 Images WFI OACDF 7kx7k V Band

Beo2 1 Master 16 nodes
Beo0/1 2 Master 8 nodes
69 2kx2k 15 7kx7k 46 8kx9k
wcs2pix 1m 14m
prepare 1m 3m 21m
mstack 3s 10s 1m10s
getpsf 2s 5s 10s
aga 3m 3m50s 19m31s
Getvar 10s 14s 1m37s
Phot 3s 10s 1m7s
Lc 8s 12s 1m
TOTAL 4m26s 8m42s 59m35s
69 2kx2k 15 7kx7k
wcs2pix 1m15s
prepare 2m 8m
mstack 10s 37s
getpsf 7s 12s
aga 4m20s 5m23s
Getvar 1m11s 1m59
Phot 59s 1m32
Lc 2m 1m
TOTAL 6m47s 19m58
it depends from the threshold in Getvar
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VLT-FORS Images
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VLT-FORS Image
VLT-FORS difference Image
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Light curves VLT-FORS
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Light curves VLT-FORS
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Light curves WFI-Carina
Light curves WFI-Carina
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Light curves WFI-Carina
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Light curves WFI-Carina
Light curves WFI-Carina
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An object from the OACDF
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Other Light curves from FORS data
Other Light curves WFI-CARINA data
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Open Points

• Improve the throughput in the preparation steps
• Photometry (PSF, aperture on original images)
  and relative amplitudes.
• User Manual
• AW integration