DESI Legacy Survey

Query DESI Legacy Surveys data

This tool can be used to query the DESI Legacy Surveys data, i.e. catalogs of sources and sky Images. Additionally, using the third sub-tool, one can compute the total flux per filter (band) for a catalog of sources queried around a given region.

Input

All sub-tools share the following required input parameters:

  1. R.A. - Right Ascension in degrees (float)
  2. Dec – Declination in degrees (float)
  3. Radius – Image size or cone search radius in arcminutes (float)
  4. Data Release (DR) – Legacy Surveys data release, e.g., 9 or 10

In order to query images, the following parameters are required as well:

  1. Pixel size – Image pixel size in arcseconds (float)
  2. Band – Photometric filter for the image:
  3. i (for DR10)
  4. g, r, z

Output

Catalog of sources (first sub-tool)

The resulting catalog includes the following columns (see full descriptions on the Legacy Surveys tractor catalog page):

  • Coordinates & Object Info:
  • RA, DEC – Right ascension and declination in degrees

  • Type – Object type (e.g., PSF, EXP, DEV, COMP)

  • Fluxes (in µJy):

  • flux_g, flux_g_err – g-band flux and associated error
  • flux_r, flux_r_err – r-band flux and associated error
  • flux_z, flux_z_err – z-band flux and associated error
  • flux_i, flux_i_err – i-band flux and associated error (only in DR10)
  • flux_w1, flux_w1_err – WISE W1 band flux and error
  • flux_w2, flux_w2_err – WISE W2 band flux and error
  • flux_w3, flux_w3_err – WISE W3 band flux and error

  • flux_w4, flux_w4_err – WISE W4 band flux and error

  • Additional Information:

  • ebv – Galactic extinction E(B–V)
  • ref_cat – Reference catalogs the stars in the catalog
  • shape_r – Half-light radius of galaxy model for galaxy type (in arcseconds)

All fluxes are provided in microJanskys (µJy).

In addition, this sub-tool outputs a dictionary that maps the flux column names to the corresponding filter names expected by the mmoda photoz-euclid tool. This facilitates seamless integration between the catalog output and the photometric redshift estimation pipeline.

Image querying (second sub-tool)

The output of the sky image query from the Legacy Survey includes:

  • FITS file – The raw image data as returned by the Legacy Survey servers, stored in standard FITS format.
  • PNG image – A visual rendering of the FITS image generated using matplotlib.pyplot.imshow() for quick inspection and visualization.

The PNG image provides a convenient preview, while the FITS file can be used for scientific analysis and further processing.

Spectral flux computation (third sub-tool)

The output includes:

  • Table of integrated fluxes – A table listing the total fluxmeasured in each photometric band over the queried region.
  • Figure – A plot showing the flux as a function of the average energy (or wavelength) of each filter band, providing a basic spectral energy distribution (SED) overview.

Acknowledgement

This project uses a simplified version of the astroquery DESI module, implemented in desi.py.