Data Model

The DECALS imaging data are processed in “Brick”s on the sky, each Brick being a small, contiguous regions of sky (roughly 0.5x0.5 degrees). The data, including information on depths, extinction, etc. are stored in pixelized images within the FITS files. There are also catalogs of objects, produced by Tractor, which are associated with each brick.

The main way to access the DECALS imaging data is via DECALS objects defined in imaginglss.DECALS.

This is an example using the data model in an interactive Python shell.

from imaginglss import DECALS
decals = DECALS('/global/project/projectdirs/m779/imaginglss/dr2.conf.py')

dr = decals.datarelease
cat = decals.datarelease.catalogue

The DECALS object constructs the DECALS data release and dust extinction map objects from environment variables or a configuration file. Please refer to the documentation of imaginglss.DECALS for details.

The imaginglss.DECALS.datarelease attribute is represents the data release. It is an instance of imaginglss.model.datarelease.DataRelease. Important attributes are

  • footprint : the survey footprint.
  • brickindex: the Brick geometry (BrickIndex)
  • images : the images released in the DR
  • catalogue : the imaging catalogue reported by tractor

The member methods are

  • readout() : reading out pixel value of an image for coordinates on the sky.
  • create_footprint() : Creating a footprint for a given extent on the sky. The footprint will be rounded to bricks.
  • create_catalogue() : Creating a catalogue for a given footprint. Currently this is not cached and can be slow.

The full catalog information is handled by imaginglss.model.catalogue.CachedCatalogue, which stores the object catalogs associated with a data release. The catalogs are contained in many small FITS files, but this class caches the information for speed and only columns that are accessed are loaded into memory. Usually at a site where DECALS data is deployed, the cache only needs to be generated once.

The images are represented by imaginglss.model.imagerepo.ImageRepo objects. An ImageRepo object takes care of reading the image tile of a Brick. An image tile contains a padded region around the Brick, which is sometimes mentioned as a margin, a padding, or a bleeding region.

A DataRelease has several image repositories, for example,

  • DataRelease.images[‘depth’][‘u’] : the ‘u’ band inverse variance depth images
  • DataRelease.images[‘image’][‘u’] : the coadded ‘u’ band image in nanomaggies
  • DataRelease.images[‘model’][‘u’] : the model ‘u’ band image in nanomaggies;

imaginglss.model.brick.Brick holds metadata for each Brick, including the central ra, dec, and the primary area( ra1, dec1, ra2, dec2 ) covered by the brick, excluding the padding region. There are routines for converting from sky coordinates to image pixels within each brick, and routines for querying the images handled by imagerepo.

imaginglss.model.brickindex.BrickIndex holds the metadata of of the brick decomposition scheme. There are routines for converting from sky coordinates to bricks, and acts as a factory that creates Brick objects.

The imaginglss.DECALS.sfdmap attribute is represents the dust extinction map . It is an instance of imaginglss.model.sfdmap.SFDMap.

For additional information please refer to API Reference.