Catalog Generation

The Hubble Advanced Products (HAP) project generates two source list catalogs, colloquially referred to as the Point and Segment catalogs. Both catalogs are generated using utilities from Photutils with the Point catalog created based upon functionality similar to DAOPhot-style photometry, and the Segment catalog created with Source Extractor segmentation capabilities and output in mind.

1: Support Infrastructure for Catalog Generation

1.1: Important Clarifications

As previously discussed in Single-visit Mosaic Processing, AstroDrizzle creates a single multi-filter, detector-level drizzle-combined image for source identification and one or more detector/filter-level drizzle-combined images (depending on which filters were used in the dataset) for photometry. The same set of sources identified in the multi-filter detection image is used to measure photometry for each filter. We use this method to maximize the signal across all available wavelengths at the source detection stage, thus providing photometry with the best quality source list across all available input filters.

It should also be stressed here that the point and segment photometry source list generation algorithms identify source catalogs independently of each other and DO NOT use a shared common source catalog for photometry.

1.2: Generation of Pixel Masks

Every multi-filter, detector-level drizzle-combined image is associated with a boolean footprint mask which defines the illuminated (True) and non-illuminated (False) portions of the image based upon its constituent exposures and the corresponding WCS solution. The boundary of the illuminated portion is iteratively eroded or contracted to minimize the impact of regions where signal quality is known to be degraded, and thereby, could affect source identification and subsequent photometric measurements. The erosion depth is approximately ten pixels and is done by using the ndimage.binary_erosion scipy tool. For computational convenience, an inverse footprint mask is also available for functions which utilize masks to indicate pixels which should be ignored during processing of the input data.

1.3: Detection Image Background Determination

For consistency, the same background and background RMS images are used by both the point and segment algorithms. To ensure optimal source detection, the multi-filter detection image must be background-subtracted. In order to accommodate the different types of detectors, disparate signal levels, and highly varying astronomical image content, three background computations are used as applicable. The first category of background definition is a special case situation, and if it is found to be applicable to the detection image, the background and background RMS images are defined and no further background evaluation is done.

It has been observed that some background regions of ACS SBC drizzle-combined detection images, though the evaluation is done for all instrument detection images, are measured to have values identically equal to zero. If the number of identically zero pixels in the footprint portion of the detection image exceeds a configurable percentage threshold value (default is 25%), then a two-dimensional background image is constructed and set to the value of zero, hence the Zero Background Algorithm. Its companion constructed RMS image set to the RMS value computed for the non-zero pixels which reside within the footprint portion of the image.

If the Zero Background Algorithm is not applicable, then sigma-clipped statistics are computed, known as the Constant Background Algorithm, for the detection image using the astropy.stats.sigma_clipped_stats Astropy tool. This algorithm uses the detection image and its inverse footprint mask, as well as a specification for the number of standard deviations and the maximum number of iterations to compute the mean, median, and rms of the sigma-clipped data. The specification for the number of standard deviations and the maximum number of iterations are configurable values which are set to 3.0 and 3 by default, respectively.

At this point the Pearson’s second coefficient of skewness is computed.

\[skewness = 3.0 * (mean - median) / rms\]

The skewness compares our sample distribution with a normal distribution where the larger the absolute value of the skewness, the more the sample distribution differs from a normal distribution. The skewness is computed in this context to aid in determining whether it is worth computing the background by other means (i.e., our third option of a two-dimensional background). For example, a high positive skew value can be indicative of there being a significant number of sources in the image which need to be taken into account with a more complex background.

Since the median and rms values will be used to generate the two-dimensional background and RMS images, respectively, the values need to be deemed reasonable. A negative mean or median value is reset to zero, and a minimum rms value is computed for comparison to the sigma-clipped statistic based upon FITS keyword values in the detection image header. For the CCD detectors only, a-to-d gain (ATODGN), read noise (READNSE), number of drizzled images (NDRIZIM), and total exposure time (TEXPTIME) are employed to compute a minimum rms, with the larger of the two rms values (sigma-clipped rms or minimum rms) adopted for further use. Once viable background and background RMS values are determined, two-dimensional images matching the dimensions of the detection image are constructed. Through a configuration setting, a user can specify the sigma-clipped statistics algorithm be the chosen method used to compute the background and RMS images, though the special case of identically zero background data will always be evaluated and will supersede the user request when applicable.

For the final background determination algorithm, Conformal Background Algorithm, the photutils.background.Background2d Astropy tool is only invoked if the Zero Background Algorithm has not been applied, the user has not requested that only the Constant Background Algorithm computed, and the skewness value derived using the sigma-clipped statistics is less than a pre-defined and configurable threshold (default value 0.5).

The Conformal Background Algorithm uses sigma-clipped statistics to determine background and RMS values across the image, but in a localized fashion in contrast to Constant Background Algorithm. An initial low-resolution estimate of the background is performed by computing sigma-clipped median values in 27x27 pixel boxes across the image. This low-resolution background image is then median-filtered using a 3x3 pixel sample window to correct for local small-scale overestimates and/or underestimates. Both the 27 and 3 pixel settings are configurable variables for the user.

Once a background and RMS image are determined using this final technique, a preliminary background-subtracted image is computed so it can be evaluated for the percentage of negative values in the illuminated portion of the image. If the percentage of negative values exceeds a configurable and defined threshold (default value 15%), the computation of the background and RMS image from this algorithm are discarded. Instead the background and RMS images computed using Constant Background Algorithm, with the associated updates, are ultimately chosen as the images to use.

Attention

It cannot be emphasized enough that a well-determined background measurement, leading to a good threshold definition, is very crucial for proper and successful source identification.

1.3.1: Configurable Variables

Through-out this section variables have been mentioned which can be configured by the user. The values used for these variables for generating the default catalogs are deemed to be the best for the general situation, but users can tune these values to optimize for their own data. To this end, users can adjust parameter values in the <instrument>_<detector>_catalog_generation_all.json files in the following path: /drizzlepac/pars/hap_pars/default_parameters/<instrument>/<detector>/.

1.4: Image Kernel

In an attempt to optimize the source detection for the specific image being processed, the software attempts to derive a custom image kernel based upon the data. The multi-filter detection image is analyzed to find an isolated, non-saturated point source away from the edge of the image to use as a template for a source detection kernel. If no suitable source is found, the algorithm falls back to the use of a two-dimensional Gaussian kernel based upon the supplied FWHM and the astropy.convolution.Gaussian2DKernel Astropy tool.

2: Point (Aperture) Photometric Catalog Generation

2.1: Source Identification with DAOStarFinder

We use the photutils.detection.DAOStarFinder Astropy tool to identify sources in the background-subtracted multi-filter detection image. This would be where the background computed using one of the algorithms discussed in Section 1.3 is applied to the science data to initialize point-source detection processing. This algorithm works by identifying local brightness maxima with roughly gaussian distributions whose peak values are above a predefined minimum threshold. Full details of the process are described in Stetson 1987; PASP 99, 191. The exact set of input parameters fed into DAOStarFinder is detector-dependent. The parameters can be found in the <instrument>_<detector>_catalog_generation_all.json files mentioned in the previous section.

2.2: Aperture Photometry Measurement - Flux Determination

Aperture photometry is then preformed on the previously identified sources using a pair of concentric photometric apertures. The sizes of these apertures depend on the specific detector being used, and are listed below in table 1:

Table 1: Aperture photometry aperture sizes
Instrument/Detector Inner aperture size (arcsec) Outer aperture size (arcsec)
ACS/HRC 0.03 0.125
ACS/SBC 0.07 0.125
ACS/WFC 0.05 0.15
WFC3/IR 0.15 0.45
WFC3/UVIS 0.05 0.15

Raw (non-background-subtracted) flux values are computed by summing up the enclosed flux within the two specified apertures using the photutils.aperture.aperture_photometry tool. Input values are detector-dependent, and can be found in the *_catalog_generation_all.json files described above in section 1.3.

Local background values are computed based on the 3-sigma-clipped mode of pixel values present in a circular annulus with an inner radius of 0.25 arcseconds and an outer radius of 0.50 arcseconds surrounding each identified source. This local background value is then subtracted from the raw inner and outer aperture flux values to compute the background-subtracted inner and outer aperture flux values found in the output .ecsv catalog file by the formula

\[f_{bgs} = f_{raw} - f_{bg} \cdot a\]
where
  • \(f_{bgs}\) is the background-subtracted flux, in electrons per second
  • \(f_{raw}\) is the raw, non-background-subtracted flux, in electrons per second
  • \(f_{bg}\) is the per-pixel background flux, in electrons per second per pixel
  • \(a\) is the area of the photometric aperture, in pixels

The overall standard deviation and mode values of pixels in the background annulus are also reported for each identified source in the output .ecsv catalog file in the “STDEV” and “MSKY” columns respectively (see Section 3 for more details).

2.3: Calculation of Photometric Errors

2.3.1: Calculation of Flux Uncertainties

For every identified source, the photutils.aperture_photometry() tool calculates standard deviation values for each aperture based on a 2-dimensional RMS array computed using the photutils.background.Background2d() tool that we previously utilized to compute the 2-dimensional background array in order to background-subtract the detection image for source identification. We then compute the final flux errors as seen in the output .ecsv catalog file using the following formula:

\[\Delta f = \sqrt{\frac{\sigma^2 }{g}+(a\cdot\sigma_{bg}^{2})\cdot (1+\frac{a}{n_{sky}})}\]
where
  • \({\Delta} f\) is the flux uncertainty, in electrons per second
  • \({\sigma}\) is the standard deviation of photometric aperture signal, in counts per second
  • \({g}\) is effective gain in electrons per count
  • \({a}\) is the photometric aperture area, in pixels
  • \({\sigma_{bg}}\) is standard deviation of the background
  • \({n_{sky}}\) is the sky annulus area, in pixels

2.3.2: Calculation of ABmag Uncertainties

Magnitude error calculation comes from computing \({\frac{d(ABMAG)}{d(flux)}}\). We use the following formula:

\[\Delta mag_{AB} = 1.0857 \cdot \frac{\Delta f}{f}\]
where
  • \({\Delta mag_{AB}}\) is the uncertainty in AB magnitude
  • \({\Delta f}\) is the flux uncertainty, in electrons per second
  • \({f}\) is the flux, in electrons per second

2.4: Calculation of Concentration Index (CI) Values and Flag Values

2.4.1: Calculation of Concentration Index (CI) Values

The Concentration index is a measure of the “sharpness” of a given source’s PSF, and computed with the following formula:

\[CI = m_{inner} - m_{outer}\]
where
  • \({CI}\) is the concentration index, in AB magnitude
  • \({m_{inner}}\) is the inner aperture AB magnitude
  • \({m_{outer}}\) is the outer aperture AB magnitude

We use the concentration index to classify automatically each identified photometric source as either a point source (i.e. stars), an extended source (i.e. galaxies, nebulosity, etc.), or as an “anomalous” source (i.e. saturation, hot pixels, cosmic ray hits, etc.). This designation is described by the value in the “flags” column

2.4.2: Determination of Flag Values

The flag value associated with each source provides users with a means to distinguish between legitimate point sources, legitimate extended sources, and scientifically dubious sources (those likely impacted by low signal-to-noise ratio, detector artifacts, saturation, cosmic rays, etc.). The values in the “flags” column of the catalog are a sum of a one or more of these values. Specific flag values are defined below in table 2:

Table 2: Flag definitions
Flag value Meaning
0 Point source \({(CI_{lower} < CI < CI_{upper})}\)
1 Extended source \({(CI > CI_{upper})}\)
2 Bit value 2 not used in ACS or WFC3 sourcelists
4 Saturated Source
8 Faint Detection Limit
16 Hot pixels \({(CI < CI_{lower})}\)
32 False Detection: Swarm Around Saturated Source
64 False detection due proximity of source to image edge or other region with a low number of input images
2.4.2.1: Assignment of Flag Values 0 (Point Source), 1 (Extended Source), and 16 (Hot Pixels)

Assignment of flag values 0 (point source), 1 (extended source), and 16 (hot pixels) are determined purely based on the concentration index (CI) value. The majority of commonly used filters for all ACS and WFC3 detectors have filter-specific CI threshold values that are automatically set at run-time. However, if filter-specific CI threshold values cannot be found, default instrument/detector-specific CI limits are used instead. Instrument/detector/filter combinations that do not have filter-specific CI threshold values are listed below in table 3 and the default CI values are listed below in table 4.

Table 3: Instrument/detector/filter combinations that do not have filter-specific CI threshold values
Instrument/Detector Filters without specifically defined CI limits
ACS/HRC F344N
ACS/SBC All ACS/SBC filters
ACS/WFC F892N
WFC3/IR None
WFC3/UVIS None

Note

As photometry is not performed on observations that utilized grisms, prisms, polarizers, ramp filters, or quad filters, these elements were omitted from the above list.

Table 4: Default concentration index threshold values
Instrument/Detector \({CI_{lower}}\) \({CI_{upper}}\)
ACS/HRC 0.9 1.6
ACS/SBC 0.15 0.45
ACS/WFC 0.9 1.23
WFC3/IR 0.25 0.55
WFC3/UVIS 0.75 1.0
2.4.2.2: Assignment of Flag Value 4 (Saturated Source)

A flag value of 4 is assigned to sources that are saturated. The process of identifying saturated sources starts by first transforming the input image XY coordinates of all pixels flagged as saturated in the data quality arrays of each input flc/flt.fits images (the images drizzled together to produce the drizzle-combined filter image being used to measure photometry) from non-rectified, non-distortion-corrected coordinates to the rectified, distortion-corrected frame of reference of the filter-combined image. We then identify impacted sources by cross-matching this list of saturated pixel coordinates against the positions of sources in the newly created source catalog and assign flag values where necessary.

2.4.2.3: Assignment of Flag Value 8 (Faint Detection Limit)

A flag value of 8 is assigned to sources whose signal-to-noise ratio is below a predefined value. We define sources as being above the faint object limit if the following is true:

\[\Delta ABmag_{outer} \leq \frac{2.5}{snr \cdot log(10))}\]
Where
  • \({\Delta ABmag_{outer}}\) is the outer aperture AB magnitude uncertainty
  • \({snr}\) is the signal-to-noise ratio, which is 1.5 for ACS/WFC and 5.0 for all other detectors.
2.4.2.4: Assignment of Flag Value 32 (False Detection: Swarm Around Saturated Source)

The source identification routine has been shown to identify false sources in regions near bright or saturated sources, and in image artifacts associated with bright or saturated sources, such as diffraction spikes, and in the pixels surrounding saturated PSF where the brightness level “plateaus” at saturation. We identify impacted sources by locating all sources within a predefined radius of a given source and checking if the brightness of each of these surrounding sources is less than a radially-dependent minimum brightness value defined by a pre-defined stepped encircled energy curve. The parameters used to determine assignment of this flag are instrument-dependent, can be found in the “swarm filter” section of the *_quality_control_all.json files in the path described above in section 1.3.

2.4.2.5: Assignment of Flag Value 64 (False Detection Due Proximity of Source to Image Edge or Other Region with a Low Number of Input Images)

Sources flagged with a value of 64 are flagged as “bad” because they are inside of or in close proximity to regions characterized by low or null input image contribution. These are areas where for some reason or another, very few or no input images contributed to the pixel value(s) in the drizzle-combined image. We identify sources impacted with this effect by creating a two-dimensional weight image that maps the number of contributing exposures for every pixel. We then check each source against this map to ensure that all sources and flag appropriately.

3: The Output Point Catalog File

3.1: Filename Format

Source positions and photometric information are written to a .ecsv (Enhanced Character Separated Values) file. The naming of this file is fully automatic and follows the following format: <TELESCOPE>_<PROPOSAL ID>_<OBSERVATION SET ID>_<INSTRUMENT>_<DETECTOR>_ <FILTER>_<DATASET NAME>_<CATALOG TYPE>.ecsv

So, for example if we have the following information:
  • Telescope = HST
  • Proposal ID = 98765
  • Observation set ID = 43
  • Instrument = acs
  • Detector = wfc
  • Filter name = f606w
  • Dataset name = j65c43
  • Catalog type = point-cat
The resulting auto-generated catalog filename will be:
  • hst_98765_43_acs_wfc_f606w_j65c43_point-cat.ecsv

3.2: File Format and Comparison to the HLA Catalog

The .ecsv file format is quite flexible and allows for the storage of not only character-separated datasets, but also metadata. The first section (lines 4-17) contains a mapping that defines the datatype, units, and formatting information for each data table column. The second section (lines 19-27) contains information explaining STScI’s use policy for HAP data in refereed publications. The third section (lines 28-48) contains relevant image metadata. This includes the following items:

  • WCS (world coordinate system) name
  • WCS (world coordinate system) type
  • Proposal ID
  • Image filename
  • Target name
  • Observation date
  • Observation time
  • Instrument
  • Detector
  • Target right ascension
  • Target declination
  • Orientation
  • Aperture right ascension
  • Aperture declination
  • Aperture position angle
  • Exposure start (MJD)
  • Total exposure duration in seconds
  • CCD Gain
  • Filter name
  • Total Number of sources in catalog

The next section (lines 50-66) contains important notes regarding the coordinate systems used, magnitude system used, apertures used, concentration index definition and flag value definitions:

  • X, Y coordinates listed below use are zero-indexed (origin = 0,0)
  • RA and Dec values in this table are in sky coordinates (i.e. coordinates at the epoch of observation and fit to GAIADR1 (2015.0) or GAIADR2 (2015.5)).
  • Magnitude values in this table are in the ABMAG system.
  • Inner aperture radius in pixels and arcseconds (based on detector platescale)
  • Outer aperture radius in pixels and arcseconds (based on detector platescale)
  • Concentration index (CI) formulaic definition
  • Flag value definitions

Finally, the last section contains the catalog of source locations and photometry values. It should be noted that the specific columns and their ordering were deliberately chosen to facilitate a 1:1 exact mapping to the_daophot.txt catalogs produced by Hubble Legacy Archive. As this code was designed to be the HLA’s replacement, we sought to minimize any issues caused by the transition. The column names are as follows (Note that this is the same left-to-right ordering in the .ecsv file as well):

  • X-Center: 0-indexed X-coordinate position
  • Y-Center: 0-indexed Y-coordinate position
  • RA: Right ascension (sky coordinates), in degrees
  • DEC: Declination (sky coordinates), in degrees
  • ID: Object catalog index number
  • MagAp1: Inner aperture brightness, in AB magnitude
  • MagErrAp1: Inner aperture brightness uncertainty, in AB magnitude
  • MagAp2: Outer aperture brightness, in AB magnitude
  • MagErrAp2: Outer aperture brightness uncertainty, in AB magnitude
  • MSkyAp2: Outer aperture background brightness, in AB magnitude
  • StdevAp2: Standard deviation of the outer aperture background brightness, in AB magnitude
  • FluxAp2: Outer aperture flux, in electrons/sec
  • CI: Concentration index (MagAp1 – MagAp2), in AB magnitude
  • Flags: See Section 2.4.2 for flag value definitions

3.3 Rejection of Cosmic-Ray Dominated Catalogs

Not all sets of observations contain multiple overlapping exposures in the same filter. This makes it impossible to ignore all cosmic-rays that have impacted those single exposures. The contributions of cosmic-rays often overwhelm any catalog generated from those single exposures making recognizing astronomical sources almost impossible amongst the noise of all the cosmic-rays. As a result, those catalogs can not be trusted. In an effort to only publish catalogs which provide the highest science value, criteria developed by the Hubble Legacy Archive (HLA) has been implemented to recognize those catalogs dominated by cosmic-rays and not provided as an output product.

Note

This rejection criteria is NOT applied to WFC3/IR or ACS/SBC data since they are not affected by cosmic-rays in the same way as the other detectors.

3.3.1 Single-image CR Rejection Algorithm

An algorithm has been implemented to identify and ignore cosmic-rays in single exposures. This algorithm has been used for ignoring cosmic-rays during the image alignment code used to determine the a posteriori alignment to GAIA.

This algorithm starts by evaluating the central moments of all sources from the segment catalog. Any source where the maximum central moment (as determined by photutils.segmentation.SourceProperties is 0 for both X and Y moments gets identified as cosmic-rays. This indicates that the source has a concentration of flux greater than a point-source and most probably represents a ‘head-on cosmic-ray’.

In addition to these ‘head-on cosmic-rays’, ‘glancing cosmic-rays’ produce streaks across the detector. Those are identified by identifying sources with a minimum width (semiminor_axis) less than the FWHM of a point source and an elongation > 2. The width and elongation are also properties defined by photutils.segmentation.SourceProperties. The combination of these criteria allows for the identification of a vast majority of cosmic-rays. The DQ array of the single exposure then gets updated to flag those pixels identified as cosmic-rays based on these criteria. These DQ flags are then ONLY applied when creating the TotalProduct to limit the contribution of cosmic-rays from the total detection image. These flags are NOT used to generate any other product in order to avoid affecting the photometry or astrometry of any source from the total detection image any more than necessary.

3.3.2 Rejection Criteria

The rejection criteria has been defined so that if either the point source catalog or the segment catalog fails, then both catalogs are rejected and deleted.

In its simplest form the criteria for rejection is:
n_cat < thresh
where:
thresh = crfactor * (n1_residual * n1_exposure_time)**2 / texptime
and:
n_cat : Number of good point and extended sources in the catalog (flag < 2) crfactor : Number of expected cosmic-rays per second across the entire detector n1_exposure_time : amount of exposure time for all single filter exposures texptime : Total exposure time of the combined drizzle product n1_residual : Remaining fraction of cosmic-rays after applying single-image CR removal

The value of crfactor should be adjusted for sub-arrays to account for the smaller area being read out, but that logic has not yet been implemented. The values used in the processing of single-visit mosaics are:

segment-catalog crfactor : 300 point-catalog crfactor : 150

These numbers are deliberately set high to be conservative about which catalogs to keep. The CR rate varies with position in the orbit, and these are set high enough that it is rare for approved catalogs to be dominated by CRs (even though they can obviously have some CRs included.)

Finally, the n1_residual term gets set as a configuration parameter with a default value of 5% (0.05). This indicates that the single-image cosmic-ray identification process was expected to leave 5% of the cosmic-rays unflagged. This process can be affected by numerous factors, and having this as a user settable parameter allows the user to account for these effects when reprocessing the data manually. Pipeline processing, though, may still be subject to situations where this process does not do as well which can result in a catalog with a higher than expected contribution of cosmic-rays. Should this number of sources trigger the rejection criteria, these catalogs will be rejected and not written out.

Also note that we reject both the point and segment catalogs if either one fails this test. The reasoning behind that is that since the catalogs are based on the same image, it is unlikely that one catalog will be good and the other contaminated.

Should the catalogs fail this test, neither type of catalogs will be written out to disk for this visit.

4: Segmentation Catalog Generation

4.1: Source Identification with PhotUtils

For the segmentation algorithm the photutils.segmentation Astropy tool is used to identify sources in the background-subtracted multi-filter detection image. As is the case for the point-source detection algorithm, this is the juncture where the common background computed in Section 1.3, relevant for both the point and segment algorithms, is applied to the science data to begin the source detection process. To identify a signal as a source, the signal must have a minimum number of connected pixels, each of which is greater than its two-dimensional threshold image counterpart. Connectivity refers to how pixels are literally touching along their edges and corners, and the threshold image is the background RMS image (Section 1.3) multiplied by a configurable n-sigma value and modulated by a weighting scheme based upon the WHT extension of the detection image. Before applying the threshold, the detection image is filtered by the image kernel (Section 1.4) to smooth the data and enhance the ability to identify signal which is similar in shape to the kernel. This process generates a two-dimensional segmentation image or map where a segment is defined to be a number of connected pixels which are all identified by a numeric label and are considered part of the same source.

Because different sources in close proximity can be mis-identified as a single source, it is necessary to apply a deblending procedure to the segmentation map. The deblending is a combination of multi-thresholding, as is done by Source Extractor and the watershed technique.

Caution

The deblending can be problematic if the background determination has not been well-determined, resulting in segments which are a large percentage of the map footprint. In this case, the deblending can take unreasonable amounts of time (e.g., days) to conclude. Various mitigation schemes to handle this situation are being investigated (e.g., use of the evaluation strategy discussed in the following paragraph with different tolerances).

After deblending has successfully concluded, the resultant segmentation map is further evaluated based on an algorithm developed for the Hubble Legacy Archive to determine if big segments/blended regions persist or if a large percentage of the map is covered by segments. If either of these two scenarios is true, this is a strong indication the detection image is a crowded astronomical field. In a crowded field either the custom kernel or the Gaussian kernel (discussed in Section 1.4) can blend objects in close proximity together, making it difficult to differentiate between the independent objects. In extreme cases, a large number of astronomical objects are blended together and are mistakenly identified as a single segment covering a large percent of the image. To address this situation an alternative kernel is derived using the astropy.convolution.RickerWavelet2DKernel Astropy tool. The RickerWavelet2DKernel is approximately a Gaussian surrounded by a negative halo, and it is useful for peak or multi-scale detection. This new kernel is then used for the generation of an improved segmentation map from the multi-filter detection image.

The segmentation map derived from and when used in conjunction with the multi-filter detection image for measuring source properties is only used to determine the centroids of sources.

Note

Questionable centroids (e.g., values of nan or infinity) and their corresponding segments are removed from the catalog entirely.

4.2: Isophotal Photometry Measurements

The actual isophotal photometry measurements are made on the single-filter drizzled images using the cleaned segmentation map derived from the multi-filter detection image. As was the case for the multi-filter detection image, the single-filter drizzled image is used in the determination of appropriate background and RMS images (Section 1.3). In preparation for the photometry measurements, the background-subtracted image, as well as the RMS image, are used to compute a total error array by combining a background-only error array with the Poisson noise of sources.

The isophotal photometry and morphological measurements are then performed on the background-subtracted single-filter drizzled image using the segmentation map derived from the multi-filter detection image, the background and total error images, the image kernel, and the known WCS with the photutils.segmentation.source_properties tool. The measurements made using this tool and retained for the output segment catalog are denoted in Table 5.

Table 5: Isophotal Measurements - Subset of Segment Catalog Measurements and Descriptions
PhotUtils Variable Catalog Column Description
area Area Total unmasked area of the source segment (pixels^2)
background_at_centroid Bck Background measured at the centroid position
bbox_xmin Xmin Min X pixel in the minimal bounding box segment
bbox_ymin Ymin Min Y pixel in the minimal bounding box segment
bbox_xmax Xmax Max X pixel in the minimal bounding box segment
bbox_ymax Ymax Max Y pixel in the minimal bounding box segment
covar_sigx2 X2 Variance of position along X (pixels^2)
covar_sigxy XY Covariance of position between X and Y (pixels^2)
covar_sigy2 Y2 Variance of position along Y (pixels^2)
cxx CXX SExtractor’s CXX ellipse parameter (pixel^-2)
cxy CXY SExtractor’s CXY ellipse parameter (pixel^-2)
cyy CYY SExtractor’s CYY ellipse parameter (pixel^-2)
elongation Elongation Ratio of the semi-major to the semi-minor length
ellipticity Ellipticity 1 minus the Elongation
id ID Numeric label of the segment/Catalog ID number
orientation Theta Angle between the semi-major and NAXIS1 axes
sky_centroid_icrs RA and DEC Equatorial coordinates in degrees
source_sum FluxIso Sum of the unmasked data within the source segment
source_sum_err FluxIsoErr Uncertainty of FluxIso, propagated from input array
xcentroid X-Centroid X-coordinate of the centroid in the source segment
ycentroid Y-Centroid Y-coordinate of the centroid in the source segment

4.3: Aperture Photometry Measurements

The aperture photometry measurements included with the segmentation algorithm use the same configuration variable values and literally follow the same steps as what is done for the point algorithm as documented in Sections 2.2 - 2.4. The fundamental difference between the point and segment computations is the source position list used for the measurements.

5: The Output Segment Catalog Files

The metadata for the catalogs, both total detection and filter, as discussed in Sections 3.1 and 3.2, is pre-dominantly the same. The differences arise with respect to the specific columns present in the catalog. The naming convention for the catalogs is also the same except the filter name is replaced by the literal total for the total detection catalog: <TELESCOPE>_<PROPOSAL ID>_<OBSERVATION SET ID>_<INSTRUMENT>_<DETECTOR>_total_<DATASET NAME>_<CATALOG TYPE>.ecsv where CATALOG TYPE is either point-cat or segment-cat. Using the same example from Section 3.1, the resulting auto-generated segment total detection catalog filename will be:

  • hst_98765_43_acs_wfc_total_j65c43_segment-cat.ecsv

and the filter catalog filename will be:

  • hst_98765_43_acs_wfc_f606w_j65c43_segment-cat.ecsv

5.1: Total Detection Segment Catalog

The multi-filter detection level (aka total) catalog contains the fundamental position measurements of the detected source: ID, X-Centroid, Y-Centroid, RA, and DEC, supplemented by some of the aperture photometry measurements from each of the filter catalogs (ABMAG of the outer aperture, Concentration Index, and Flags). Effectively, the output Total Detection Segment Catalog is a distilled version of all of the Filter Segment Catalogs.

5.2: Filter Segment Catalog and Comparison to the HLA Catalog

Section 3.2 discusses the file format for the output filter catalogs, where the latter portion of this section is specific to the point catalogs. The general commentary is still relevant for the segment catalogs, except for the specific columns. In the case of the segment filter catalogs, the specific columns and the order of the columns were designed to be similar to the Source Extractor catalogs produced by the Hubble Legacy Archive (HLA) project.

Having said this, the PhotUtils tool is not as mature as Source Extractor, and it was not clear that all of the output columns in the HLA product were relevant for most users. As a result, some measurements in the HLA Source Extractor catalog may be missing from the output segment catalog at this time. The current Segment column measurements are as follows in Table 6 with the same left-to-right ordering as found in the .ecsv:

Table 6: Segment Filter Catalog Measurements and Descriptions
Segment Column SExtactor Column Description Units
X-Centroid X_IMAGE 0-indexed Coordinate position pixel
Y-Centroid Y_IMAGE 0-indexed Coordinate position pixel
RA RA Sky coordinate at epoch of observation degrees
DEC DEC Sky coordinate at epoch of observation degrees
ID   Catalog Object Identification Number  
CI CI Concentration Index  
Flags FLAGS    
MagAp1 MAG_APER1 ABMAG of source, inner (smaller) aperture ABMAG
MagErrAp1 MAGERR_APER1 Error of MagAp1 ABMAG
FluxAp1 FLUX_APER1 Flux of source, inner (smaller) aperture electrons/s
FluxErrAp1 FLUXERR_APER1 Error of FluxAp1 electrons/s
MagAp2 MAG_APER2 ABMAG of source, outer (larger) aperture ABMAG
MagErrAp2 MAGERR_APER2 Error of MagAp2 ABMAG
FluxAp2 FLUX_APER2 Flux of source, outer (larger) aperture electrons/s
FluxErrAp2 FLUXERR_APER2 Error of FluxAp2 electrons/s
MSkyAp2   ABMAG of sky, outer (larger) aperture ABMAG
Bck BACKGROUND Background, position of source centroid electrons/s
Area   Total unmasked area of the source segment pixels^2
MagIso MAG_ISO Magnitude corresponding to FluxIso ABMAG
FluxIso FLUX_ISO Sum of unmasked data in source segment electrons/s
FluxIsoErr FLUXERR_ISO Uncertainty, propagated from input error electrons/s
Xmin XMIN_IMAGE Min X pixel in minimal bounding box segment pixels
Ymin YMIN_IMAGE Min Y pixel in minimal bounding box segment pixels
Xmax XMAX_IMAGE Max X pixel in minimal bounding box segment pixels
Ymax YMAX_IMAGE Max Y pixel in minimal bounding box segment pixels
X2 X2_IMAGE Variance along X pixel^2
Y2 Y2_IMAGE Variance along Y pixel^2
XY XY_IMAGE Covariance of position between X and Y pixel^2
CXX CXX_IMAGE SExtractor’s ellipse parameter pixel^2
CYY CYY_IMAGE SExtractor’s ellipse parameter pixel^2
CXY CXY_IMAGE SExtractor’s ellipse parameter pixel^2
Elongation ELONGATION Ratio of semi-major to semi-minor length  
Ellipticity ELLIPTICITY The value of 1 minus the elongation  
Theta THETA_IMAGE Angle between semi-major and NAXIS1 axes radians