Inputs

Inputs#

Forcepho requires several ingredients to be on hand. These are:

  • Imaging data

  • Point Spread Functions (PSFs)

  • An initial peak list

  • A Gaussian mixture approximation to Sersic profiles.

Details about each aspect are given below.

Imaging data#

There are two principal ways to interact with imaging data. One is to directly supply specifically formatted FITS images, and the other – more efficient for large datasets – is to pre-process the image data into pixel and meta-data stores

Direct FITS input#

The simplest way to provide data to be fit, appropriate for smaller datasets, is to provide a list of FITS filenames (see Configuration File) and to use the forcepho.patches.FITSPatch class.

The supplied FITS images must provide background-subtracted, photometrically calibrated fluxes (i.e. in units of nJy/pixel) in the first extension and associated uncertainty (\(=\sqrt{\rm variance}\)) images in the second extension. Masked or invalid pixels can be given inifinte or negative uncertainty values.

There must be a valid WCS in the header of each image, and each image must contain a "FILTER" keyword giving the name of the band

This corresponds approximately to the data model for crf.fits or calints.fits data products of the jwst stage 2 pipeline processing.

Image Stores#

Forcepho pixel and meta stores are classes that wrap HDF and JSON files with a particular structure. This allows for more efficient data access and processing. These stores can be created during “pre-processing’ from FITS or other data with certain properties. These properties are:

  • The images must all be the same dimension (for now), preferably square. Default is 2048 by 2048.

  • The images must have dimensions that are an integer multiple of 8.

  • The headers must contain a valid WCS such as "CRPIX", "CRVAL", and "CD" or "PC" + "CDELT" matrix keywords. In effect, astropy.wcs.WCS(astropy.io.fits.getheader(imname)) must not raise an error.) The WCS should map pixel indices (x, y) to (RA,DEC) pairs in decimal degrees.

  • The headers of all images must contain a "FILTER" keyword with string value, though this can be added in pre-processing.

  • Fluxes will be reported in image units (which should be ‘per-pixel’, not surface brightness) Therefore, when fitting several different exposures in a single FILTER it is highly desirable that all images be on the same flux scale. If the "ABMAG" header keyword is present for individual exposures, then pixel stores will automatically try to convert images to units of nJy/pixel.

  • There _must_ be a pixel-matched uncertainty image associated with every science image.

Optionally each image may also have an associated background image and pixel mask image:

  • The background image should be the same units as the science image, and will be subtracted from it during preprocessing. If not supplied then it is assumed that the images are already background subtracted, though global offsets may be supplied at runtime.

  • The mask image can be an array of bitflags or a simple array of 0 (False, do not use this pixel) and 1 (True, use this pixel.) If not supplied then the only pixels masked will be those with NaN or inf values for either the pixel data or the uncertainties.

The pre-processing script can be used to enforce the size constraints (e.g. by padding or by making cutouts), add the required header information, and parse various file formats into the ImageSet structure, which are then used to create the internal data storage.

PSFs#

The PSFs used by Forcepho are approximations based on a Gaussian mixture model fit to the actual PSFs. Tools exist within forcepho to fit user-supplied PSFs (appropriate for the supplied imaging data) with Gaussian mixture models. The PSFs, and the mixtures used to approximate them, can depend on detector coordinates.

Initial peak catalog#

There must be a FITS binary table of initial (celestial or on-sky) positions. This should have the following columns

  • ra (decimal degrees)

  • dec (decimal degrees)

  • roi (arcsec) radius of influence, used to group sources. This is usually best defined via an approximate isophote at close to the 1- or 2-sigma surface brightness

  • q (b/a, dimensionless) Use 0.8 if not known

  • pa (radians, E of N) use 0.0 if not known

  • sersic Sersic index (use 2 if not known)

  • rhalf (arcsec) half-light radius estimate, use the middle of the allowable range if not known (e.g. 0.15 arsec)

  • <band> rough flux estimate in <band>, where <band> corresponds to the "FILTER" keywords

Other columns may be present. The header of the catalog should contain the keyword "FILTERS" which is a string containing a comma separated list of all the band names.

Sersic Profiles#

A standard lookup table of Gaussian parameters will be provided.

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