The MAST Discovery Portal is the primary web interface for discovering, visualizing, assessing, and retrieving calibrated data products from the JWST calibration pipeline, as well as calibration and engineering data products and services.
Accessing JWST data
There are a variety of ways for researchers to access JWST data of interest, including the MAST Discovery Portal (hereafter, Portal) which is the primary web interface for discovering, visualizing, assessing, and retrieving archived data. The portal also provides access to ancillary and engineering data related to the observations, and data quality summaries that enable users to assess any anomalies during the observations, the achieved exposure depth, and the calibration processing. Users may evaluate contemporaneous calibration reference data that were used to remove the instrumental signature from the science data products. They may also discover data through a programmatic (software-based) interface such as the MAST Applications Programming Interface (API), and through various community tools. Users may elect to be notified as data from new JWST observations become available through MAST.
Data product types
The JWST Data Management System (DMS) produces many products for each JWST observation, including the science files produced by the data reduction pipeline. The exact type and number of products depends upon the instrument, its configuration, and operating mode. Consult the Understanding Data Files article for a detailed description of each science product and the concomitant data. Most of the science data files are images or tables in FITS format (Pence, et al. 2010), while others are in structured or unstructured ASCII format. Table 1 contains a short summary of the data product types that may be included with each dataset. Table 1 below summarizes the semantic content of the various data products, including some that are produced outside the science data reduction pipeline.
Table 1. Summary of data product types that may be included with each dataset
|Science data||FITS MEF images||Science data products for imaging, spectroscopy, coronagraphy, and (early stages of) time-series observations. The image extensions contain multi-dimensional science pixel arrays, concomitant data quality flags, and variance arrays.|
|FITS binary table||Extracted spectra are stored as tables, and include fields for spectral coordinates, flux, and concomitant quality information. AMI fringe parameters are also stored as binary tables of coefficients.|
|ECSV||Source catalogs, as well as TSO light curves are stored in an ASCII enhanced character-separated variable format.|
|ASDF||Metadata for images or spectra are stored in ASDF format, but are packaged as BYTE arrays in a FITS extension of type BINTABLE.|
|Associations||ECSV||A collection of metdata for all exposures that match a combination of instrument, program ID, and observation from which associations are generated based on program information.|
|JSON||Enumeration of relationships between all levels of science data products, and related calibration reference products, that were used during the course of calibration pipeline processing.|
|WSS||FITS||Wavefront-sensing Optical Path Difference (OPD) images that describe the state of the mirror segment alignments for a particular epoch. The OPD files can be used with a tool such as WebbPsf to determine the estimated PSFs for any given instrument aperture.|
|Engineering data||CSV||Engineering data are provided in the form of a time series of values, in CSV format, with one mnemonic per file.|
|CAL reference files||ASDF, ECSV, FITS, JSON||Reference files used in calibration processing come in various formats. See the article on Calibration Reference Files.|
|Previews||JPEG or PNG||Preview and thumbnail images for presenting data products in the Portal.|
Minimum recommended data products
Of the many different data products produced by the calibration pipeline, a subset has been identified as essential for extracting the intended science from the data. These are termed the "minimum recommended products" (MRP). The selection of data products that are included in this set depends upon the instrument used to obtain the data, and its configuration and operating mode. Generally, products in the MRP include the lowest-level science product for which the instrumental signature has been removed along with the associated data products, but exclude calibration, preview, and certain ancillary products.
The MRP checkbox in the Download Manager must be de-selected in order to view or retrieve raw or intermediate-level data products.
Data from JWST are likely to be reprocessed with the calibration pipeline from time to time, particularly early in the mission lifetime. Searches and retrievals will always access data from the most recent processing. Users may also wish to reprocess JWST science data themselves with the JWST CAL pipeline software for many of the same reasons, including:
- Additional observations of the target/field become available
- Better or more appropriate contemporaneous calibration reference files are created
- New software or calibrations are implemented to more completely remove instrumental signatures
- Custom processing is required to achieve the optimal scientific result
The provenance metadata in the file headers will indicate the date, software version, and (unique) calibration file identifiers.
MAST Discovery Portal
MAST implements various protocols of the Virtual Observatory (VO) including those for image, table, and spectral data access, and the Common Archive Observation Model (CAOM) that describes the core services offered by MAST. These are the mechanisms by which MAST data can be searched and retrieved by VO-aware applications, and conversely, the mechanisms for the Discovery Portal application to discover, retrieve, and operate on data not hosted in MAST.
The MAST Portal offers great flexibility in customizing queries to identify datasets from several NASA missions to explore and retrieve. See the Data Exploration article for a detailed example of a search for JWST science data using the Portal. The following tutorials may be helpful for new Portal users:
The JWST engineering database service provides an archival repository of engineering telemetry, including temperatures and other time-dependent status information, about the science instruments and the spacecraft itself. Telemetry associated with specific measurements vs. time are tagged by mnemonics which, in practice, are rather opaque to non-specialist users. Of the roughly 23,000 archived mnemonics, nearly 15,550 of them can be queried, visualized, and downloaded by general users through a Portal interface. Users may access the engineering database via a direct link to the query interface or, after querying the Portal for science data, through a link on each row of a search results table. See the JWST Engineering Database Browser for details.
Engineering data are obtained contemporaneously, but are not packaged with, the science data. Rather, EDB data are searched by mnemonic and a time range which spans the interval of the science exposure(s) of interest. The data files are provided in CSV format, which can be read by a wide variety of software applications, including MS Excel. The following tutorial may help first-time users of the EDB interface:
At any given time some observations in a program may have been executed, archived, and become available to the community; some archived observations may temporarily be restricted to those with exclusive access; while still other observations may remain to be obtained. Investigating teams and the broader community each have an interest in data availability. In order to encourage the greatest possible use of JWST data in MAST, a subscription service will notify registered and subscribed users when one of the following observation-related events occur:
- new observations are Archived,
- archived data have been reprocessed, or
- restricted-access data become available to the public.
Users may tune the notifications by mission, program ID, event type, and science product. Users may establish or cancel their subscription through the MAST Discovery Portal, change the media and frequency of notifications, and change the selection criteria for notifications. See the following tutorial:
Duplicate Observation Identification
MAST provides the capability to check one or more user targets against extant and planned observations to identify potentially duplicate observations. Duplicated observations are, in general, not allowed so it is in the proposer's best interest to perform this check prior to submitting a proposal for review. Read the article Identifying Potential Duplicate Observations to see how to query planned observations.
Virtual observatory tools
Many community software applications are capable of accessing remote data using protocols developed for the Virtual Observatory. You may have used them before without being fully aware of how such data were obtained. Table 2 provides an incomplete list of community, VO-aware applications for visualizing and exploring archived astronomical images, spectra, and catalogs:
Table 2. An incomplete list of community, VO-aware applications for visualizing and exploring archived astronomical data
|Aladin||Analysis||Interactive sky atlas, capable of overlaying archived images and source catalogs|
|DataScope||Discovery||Position-based search engine for astronomical archives|
|SAOimage/DS9||Analysis||Image display tool, capable of overlaying archived source catalogs|
|SIMBAD||Retrieval||Astronomical catalog search engine|
|SkyView||Discovery||Astronomical image search engine|
|SpecView||Analysis||1-D spectrum visualization and analysis|
|TOPCAT||Analysis||Catalog cross-match, filtering, sub-setting, and visualization tool|
|VizieR||Retrieval||Astronomical catalog search engine|
|astropy.vo.client||Discovery||Python packages for using VO services, including accessing VO registries, as well as catalog manipulation in software|
Pence, W. D., et al. 2010, A&A, 524, A42
Definition of the Flexible Image Transport System (FITS), version 3.0