General Instructions

Instructions and Notebooks Specific to Data Set

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CosmoDC2 is a large synthetic galaxy catalog designed to support precision dark energy science with LSST, covering 440 sq. deg. of sky area to a redshift of z = 3, with a magnitude depth of 28 in the r band. A wide range of galaxy properties are available in cosmoDC2. To learn more about cosmoDC2, please see

Follow the steps below to access cosmoDC2. If you use cosmoDC2 in your work, we ask that you cite the publication above.

Step 1: Download cosmoDC2 catalog files

Follow these instructions to download “CosmoDC2 v1.1.4” data files. You can download the full data set or, if space is at a premium, a recommended subset of files. The data files you downloaded are in HDF5 format, partitioned into sky areas (by healpixels) and redshift ranges. The data schema can be found here.

While you can access these files by standard tools, we recommend that you use GCRCatalogs to access the catalog because some quantities described in the cosmoDC2 schema are only available when you access with GCRCatalogs, but not in the raw files.

Step 2: Prepare GCRCatalogs

See these instructions to install and configure GCRCatalogs.

If you use GCRCatalogs as described here to access cosmoDC2, please also cite Mao et al. (LSST DESC), ApJS, 234, 36 (2018).

Step 3: Load cosmoDC2 with GCRCatalogs

import GCRCatalogs
cat = GCRCatalogs.load_catalog("desc_cosmodc2")

Step 4: Get familiar with GCRCatalogs and cosmoDC2

There are many tutorial notebooks showing you how to use GCRCatalogs and how to play with the cosmoDC2 data. Note that if you see the tutorial notebooks load cosmoDC2_v1.1.4_small or cosmoDC2_v1.1.4_image, you will need to change the catalog name to desc_cosmodc2.

Note that in some tutorials you may see instructions for loading other catalogs with GCRCatalogs. Those instructions will not work on your local machine. You will only be able to access public DESC catalogs that you have downloaded to your machine.