Photometric calibration with Aladin

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Photometric calibrationwith Aladin
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Method in 4 steps

• From an astrometrical calibrated image
• The first step will be to use S-extractor on your
  image to extract sources and estimate photometry
• After that you will compare this first result
  with a photometric catalog such GSC2.2
• And you will adjust S-extractor parameters for
  fixing unknown ones

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Requirements..

• You have Aladin install on your machine
  (http//aladin.u-strasbg.fr/AladinJava?framedownl
  oading)
• If you have a firewall, you will have to open
  temporary the port 4128 that Aladin will be using
  for S-extractor remote usage).

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Load your image

• Launch Aladin and load your image.Here a SDSS
  image centered on NGC1087

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Use S-extractor

• Use the S-extractortool provided with Aladin for
  getting a first approximation

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Load a photometric catalog

• Load a photometric catalog such as the GSC2.2

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X-match the result

• Xmatch your result with the GSC to have common
  objects to compare

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Compare both mags

• Compare the 2 mag values in a 2D graph via VOplot.

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Adjust the zero point

• Extrapolate thezero point-14.5 ?
  12.5ZeroPoint 27
• And refine photometry with this new value

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You have it !
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Just for the fun

• Your result compared to the SDSS original catalog

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Remarks

• S-extractor is developed by E. Bertin from
  Terapix (Paris) and this version is running on a
  PC cluster at CDS (Strasbourg).
• If your calibration requires better tuning that
  the default Aladin one, download and use
  S-extractor directly on your machine.
• NB Aladin can read directly S-extractor catalog
  outputs.