Source Extraction Definitions

MExtract Module

The Extract Sources command produces a large number of measurements for the sources it detects. Some measurements are reported only when certain features are enabld on the Post Process page. All source measurements are defined in the table below.

Threshold Sensitivity

Note that some depend to varying degrees on the choice of threshold value. the Threshold is set on the Detect page of the Source Extraction dialog:

• Some quantities, such as Area and CI, are completely dependent upon the threshold value, since the threshold defines which pixels are included in the source. These quantities will give completely different answers for different threshold levels.

• Other quantities, such as Ellip and Angle, use the included pixels as an estimator of their true value hence they are sensitive to varying degrees on the chosen threshold.

• As a hybrid of the two cases, the Major axis and Minor axis values are dependent on the threshold, but their ratio, as presented in the ellipticity (Ellip item) is an estimate of the true value for the object.

• The optional Precision FWHM measurement is not sensitive to the threshold, when measuring unresolved sources.

• The optional Source Photometry measurements are not not sensitive to the threshold.

• Properties Listed in the Report Window

  #	A sequential number for the measurement
  Image	The name of the image that was measured
  Obj	The source number. If sources touch and are merged during the detection process, one of the numbers is discarded, making the Obj values generally not a continuous sequence.
  <X>	The mean X position, measured in world coordinates. If Precision Centroid or Precision FWHM is checked on the Post Process page, this is measured in the same way as an interactively marked centroid using the settings on the Centroid setup dialog.
  <Y>	The mean Y position, measured in world coordinates. If Precision Centroid or Precision FWHM is checked on the Post Process page, this is measured in the same way as an interactively marked centroid using the settings on the Centroid page.
  Col	The mean Column position, measured in pixel coordinates. If Precision Centroid or Precision FWHM is checked on the Post Process page, this is measured in the same way as an interactively marked centroid using the settings on the Centroid setup dialog.
  Row	The mean Row position, measured in pixel coordinates. If Precision Centroid or Precision FWHM is checked on the Post Process page, this is measured in the same way as an interactively marked centroid using the settings on the Centroid setup dialog.
  Lum	The total luminance of the source above the background. If you think of pixel value as defining a 3 dimensional surface, the Lum value is the volume of the surface above background.
  Mean Lum	The total luminance (Lum value) divided by the area.
  Std Dev	The standard deviation of the luminance.
  Ellip	The ellipticity, measured as 1 – (minor axis / major axis).
  Angle	The angle of the major axis relative to the positive X axis (0 degrees), increasing through the positive Y axis (90 degrees). The angle is adjusted to fall in the range -90 to +90 degrees.
  Area	The area, in pixels, of the source above the threshold. This is threshold dependent.
  Maximum	The maximum luminance. This is the pixel value of the highest pixel inside the source.
  Minimum	The minimum luminance. This is the pixel value for the threshold value above the background.
  FWHM (Precision FWHM)	The Full Width at Half maximum brightness. This measures the point spread function of the source profile.
  CI	Concentration Index. This is the ratio of the Effective Diameter divided by 2 times the radius of gyration. The Effective diameter is the diameter of a circle having the same area. The radius of gyration is the radius for the second moment of the intensity distribution (analogous to the moment of inertia of a rotating mass distribution). This is threshold dependent.
  Major Axis	The major axis length in units of pixels. This is threshold dependent.
  Minor Axis	The minor axis length in units of pixels. This is threshold dependent.
  Kurtosis	The source profile kurtosis. This measures the "flatness" of the luminance distribution.
  X Skew	The source profile skewness in the X direction. This measures the asymmetry of the luminance distribution.
  Y Skew	The source profile skewness in the Y direction. This measures the asymmetry of the luminance distribution.
  Background	The background value evaluated at the average position of the source (at <X> and <Y>).
  Total Lum (Source Photometry)	The total luminance of the source above its local background. This is also known as "count volume". It is integrated within an elliptical aperture surrounding the object.
  Error (Source Photometry)	The uncertainty in the Total Luminance. This is computed using the signal and an error estimate based on variation in the pixel values inside the background annulus surrounding the source.
  Error(T) (Source Photometry)	The theoretical estimate of the unertainty in the Total Luminance. This is computed using signal at each point along with the gain, readout noise, and exposure time for the image.
  S/N (Source Photometry)	The Signal To Noise ratio for the detected signal inside the aperture that measures the Total Luminance.
  Bg Local (Source Photometry)	The precision background measured from pixels inside the annulus between the inner and outer background apertures. This is more accurate for a precision luminance measurement than is obtained using the Background surface value at the position of the source. This value is effective at the center position of the object.

  Additional Properties Saved to a File

  The following quantities are listed in the text file using the [Save] button on the Procedure page. They are not listed in the Report window. Except for the two additional FWHM results, all the properties listed below are obviously dependent on the threshold value.

  FWHM Peak (Precision FWHM)	The Peak value (above background) from the FWHM calculation. This is measured only if FWHM is selected in the post processing options. This is not threshold dependent.
  FWHM Bg (Precision FWHM)	The background value from the FWHM calculation. This is measured only if FWHM is selected in the post processing options. This is not threshold dependent. Compare its value with the computed background value, which is estimated using the selected Background strategy.
  Min X	The minimum Column position of the source.
  Max X	The maximum Column position of the source.
  Min Y	The minimum Row position of the source.
  Max Y	The maximum Row position of the source.
  X at Min Y	The Column position at the minimum Row position of the source.
  X at Max Y	The Column position at the maximum Row position of the source.
  Y at Min X	The Row position at the minimum Column position of the source.
  Y at Max X	The Row position at the maximum Column position of the source.
  X at Min Lum	The Column position at the minimum Luminance of the source.
  X at Max Lum	The Column position at the minimum Luminance of the source.
  Y at Min Lum	The Row position at the minimum Luminance of the source.
  Y at Max Lum	The Row position at the minimum Luminance of the source.

   

  Note

  Source Photometry gives the best background estimates when sources are separated by empty background. This is because the local background used by the measurement samples pixels lying inside an annulus surrounding the source. The Modal Brightness Median Brightness background methods do rejection of non-background pixels but still can give a biased result when non-backgound pixels comprise a significant portion (e.g., 10% or more) of the local background sample.

  Related Topics

  Extract Sources command, Extracting Sources from Images, Centroid Parameters, FWHM Parameters, Source Photometry Parameters