Description

This step creates a catalog of source photometry and morphologies. Both aperture and isophotal (segment-based) photometry are calculated. Source morphologies are based on 2D image moments within the source segment.

Source Detection

Sources are detected using image segmentation, which is a process of assigning a label to every pixel in an image such that pixels with the same label are part of the same source. The segmentation procedure used is from Photutils source extraction. Detected sources must have a minimum number of connected pixels that are each greater than a specified threshold value in an image. The threshold level is usually defined at some multiple of the background standard deviation above the background. The image can also be filtered before thresholding to smooth the noise and maximize the detectability of objects with a shape similar to the filter kernel.

Source Deblending

Overlapping sources are detected as single sources. Separating those sources requires a deblending procedure, such as a multi-thresholding technique used by SExtractor. Here we use the Photutils deblender, which is an algorithm that deblends sources using a combination of multi-thresholding and watershed segmentation. In order to deblend sources, they must be separated enough such that there is a saddle between them.

Source Photometry and Properties

After detecting sources using image segmentation, we can measure their photometry, centroids, and morphological properties. The aperture photometry is measured in three apertures, based on the input encircled energy values. The total aperture-corrected flux and magnitudes are also calculated, based on the largest aperture. Both AB and Vega magnitudes are calculated.

The isophotal photometry is based on photutils segmentation. The properties that are currently calculated for each source include source centroids (both in pixel and sky coordinates), isophotal fluxes (and errors), AB and Vega magnitudes (and errors), isophotal area, semimajor and semiminor axis lengths, orientation of the major axis, and sky coordinates at corners of the minimal bounding box enclosing the source.

Source Catalog Table

The output source catalog table is saved in ECSV format.

The table contains a row for each source, with the following columns:

Column

Description

id

Unique source identification number

xcentroid

X pixel value of the source centroid

ycentroid

Y pixel value of the source centroid

sky_centroid

Sky coordinate of the source centroid

aper_bkg_flux

The local background value calculated as the sigma-clipped median value in the background annulus aperture

aper_bkg_flux_err

The standard error of the sigma-clipped median background value

aper30_flux

Flux within the 30% encircled energy circular aperture

aper30_flux_err

Flux error within the 30% encircled energy circular aperture

aper50_flux

Flux within the 50% encircled energy circular aperture

aper50_flux_err

Flux error within the 50% encircled energy circular aperture

aper70_flux

Flux within the 70% encircled energy circular aperture

aper70_flux_err

Flux error within the 70% encircled energy circular aperture

aper_total_flux

Total aperture-corrected flux based on the 70% encircled energy circular aperture; calculated only for stars

aper_total_flux_err

Total aperture-corrected flux error based on the 70% encircled energy circular aperture; calculated only for stars

aper30_abmag

AB magnitude within the 30% encircled energy circular aperture

aper30_abmag_err

AB magnitude error within the 30% encircled energy circular aperture

aper50_abmag

AB magnitude within the 50% encircled energy circular aperture

aper50_abmag_err

AB magnitude error within the 50% encircled energy circular aperture

aper70_abmag

AB magnitude within the 70% encircled energy circular aperture

aper70_abmag_err

AB magnitude error within the 70% encircled energy circular aperture

aper_total_abmag

Total aperture-corrected AB magnitude based on the 70% encircled energy circular aperture; calculated only for stars

aper_total_abmag_err

Total aperture-corrected AB magnitude error based on the 70% encircled energy circular aperture; calculated only for stars

aper30_vegamag

Vega magnitude within the 30% encircled energy circular aperture

aper30_vegamag_err

Vega magnitude error within the 30% encircled energy circular aperture

aper50_vegamag

Vega magnitude within the 50% encircled energy circular aperture

aper50_vegamag_err

Vega magnitude error within the 50% encircled energy circular aperture

aper70_vegamag

Vega magnitude within the 70% encircled energy circular aperture

aper70_vegamag_err

Vega magnitude error within the 70% encircled energy circular aperture

aper_total_vegamag

Total aperture-corrected Vega magnitude based on the 70% encircled energy circular aperture; calculated only for stars

aper_total_vegamag_err

Total aperture-corrected Vega magnitude error based on the 70% encircled energy circular aperture; calculated only for stars

CI_30_50

Concentration index calculated as aper30_abmag - aper50_abmag

CI_50_70

Concentration index calculated as aper50_abmag - aper70_abmag

CI_30_70

Concentration index calculated as aper30_abmag - aper70_abmag

is_star

Flag indicating whether the source is a star

sharpness

The DAOFind source sharpness statistic

roundness

The DAOFind source roundness statistic

nn_dist

The distance in pixels to the nearest neighbor

nn_abmag

The AB magnitude of the nearest neighbor. If the object is a star it is the total aperture- corrected AB magnitude, otherwise it is the isophotal AB magnitude.

isophotal_flux

Isophotal flux

isophotal_flux_err

Isophotal flux error

isophotal_abmag

Isophotal AB magnitude

isophotal_abmag_err

Isophotal AB magnitude error

isophotal_vegamag

Isophotal Vega magnitude

isophotal_vegamag_err

Isophotal Vega magnitude error

isophotal_area

Isophotal area

semimajor_sigma

1-sigma standard deviation along the semimajor axis of the 2D Gaussian function that has the same second-order central moments as the source

semiminor_sigma

1-sigma standard deviation along the semiminor axis of the 2D Gaussian function that has the same second-order central moments as the source

ellipticity

1 minus the ratio of the 1-sigma lengths of the semimajor and semiminor axes

orientation

The angle (degrees) between the positive X axis and the major axis (increases counter-clockwise)

sky_orientation

The position angle (degrees) from North of the major axis

sky_bbox_ll

Sky coordinate of the lower-left vertex of the minimal bounding box of the source

sky_bbox_ul

Sky coordinate of the upper-left vertex of the minimal bounding box of the source

sky_bbox_lr

Sky coordinate of the lower-right vertex of the minimal bounding box of the source

sky_bbox_ur

Sky coordinate of the upper-right vertex of the minimal bounding box of the source