Catalog Generation

Point (Aperture) Photometric Catalog Generation

1: Source Detection

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: Preliminaries

1.2.1: Generation of the Bad Pixel Mask

Before any source identification takes place, a bad pixel mask is created to identify regions of the detection image where signal quality is known to be degraded. These are areas near the edge of the image, areas with little to no input image contribution, and areas that contain saturated pixels. To minimize the impact of these regions on source identification and subsequent photometric measurements, the regions flagged in this bad pixel mask are iteratively “grown” for 10 steps using the ndimage.binary_dilation scipy tool. Pixels in the immediate vicinity of a given masked region may also be impacted to some degree. As we cannot be fully certain that these pixels are or are not impacted, or to the degree of the impact, they are all flagged.

1.2.2: Detection Image Background Subtraction

To ensure optimal source detection, the multi-filter detection image is background-subtracted. We computed a 2-dimensional background image using the photutils.background.Background2d Astropy tool. This algorithm uses sigma-clipped statistics to determine background and RMS values across the image. 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. It should be noted these are configurable values. Our catalogs use these values deeming them 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 “bkg_box_size” and/or “bkg_filter_size” in the <instrument>_<detector>_catalog_generation_all.json files in the following path: /drizzlepac/pars/hap_pars/default_parameters/<instrument>/<detector>/.

1.3: Source Identification with DAOStarFinder

We use the photutils.detection.DAOStarFinder Astropy tool to identify sources in the background-subtracted multi-filter detection image. Regions flagged in the previously created bad pixel mask are ignored by DAOStarFinder. 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: Aperture Photometry Measurement

2.1: 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).

2.2: Calculation of photometric errors

2.2.1: 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
2.2.2: 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

2.3: Calculation of concentration index (CI) values and flag values

2.3.1: 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 automatically classify 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

2.3.2: 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, 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
2.3.2.1: 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
2.3.2.2: 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.

2.3.2.3: 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.
2.3.2.4: 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.

2.3.2.5: 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 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

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 2.3.2 for flag value definitions

Segment Photometric Catalog Generation

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