This article describes how to make effective searches for data by astrophysical source name, and how avoid problems with the underlying translations from name to sky coordinates.
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Source Identifiers in MAST
Most MAST search interfaces, apart from those specifically devoted to source catalogs, query the MAST archive catalog of Observations. Observations are collections of products and metadata related to exposures by a given mission at a given sky position. The archive catalog:
- is a database of Observations from missions hosted by MAST
- is not a database of individual astrophysical sources
Thus, searching MAST for sources (or positions) amounts to querying the archive catalog for Mission data that overlap the astrophysical source position or extent.
Note that these queries apply to fixed astrophysical sources—i.e., not solar system bodies. (See Finding Solar System Bodies in MAST for non-fixed sources, which uses the term target to describe planned pointings of the telescope.) This article describes how MAST translates searches of positions and source names to queries of matching Observations. The following terms take on a specific meaning for searches of MAST holdings:
| Term | Usage |
|---|---|
| Identifier | Designation is the official IAU term for the name of a unique astrophysical radiation source; SIMBAD uses the term identifier. An astrophysical source may have multiple identifiers (most often this means it is listed in multiple published catalogs), but the key point is that all identifier synonyms refer unambiguously to a single, unique source. Any of these names can be resolved (i.e., translated) to sky coordinates of the fixed source, provided the name string matches the syntax rules of the resolver service. SIMBAD syntax rules permit partial identifiers (using wildcard characters), but this feature is not supported in MAST. |
| Object | Refers to a generic source or field name, which is translated to sky coordinates via MAST or another name resolution service. MAST syntax rules are somewhat more restrictive than SIMBAD. MAST also uses the term Object to refer to an Observation identifier (a name for a collection of related files, rather than the name of an astrophysical source), which one of the MAST-hosted services can resolve to a spatial position. This conflation of terms can lead to confusion in some contexts. |
| Source | A generic term used in the literature or in MAST documentation to refer to an astrophysical entity. Can refer to (often spatially unresolved) aggregate objects. |
| Target | The name of a target or field associated with an observation by a MAST-hosted mission. The target name is provided by the observer, and may or may not match an official designation. Targets are string-matched to a field in the MAST observation database. See warning below. |
Searching by target name in MAST can problematic because:
- it must match exactly to values in the database field, unless wildcard characters are used
- it may or may not match an official designation
- it may or may not match the name given for a different Observation by another observer of the same source or field
Because of these issues, searches for data by target name may not match every entry in the MAST catalog that one might expect.
Search by source name
Searches by source name in MAST are translated to searches by sky coordinates using a back-end web service called SANTA (STScI Archive Name Translation Applications). SANTA itself depends on multiple name resolution services: the major astronomical name services SIMBAD (Set of Identifications, Measurements and Bibliography for Astronomical Data; curated by CDS) and NED (the NASA/IPAC Extragalactic Database), in addition to internal services for identifiers specific to MAST.
NED, as its name implies, contains extensive holdings on extra-Galactic sources. Though their database does contain entries for point-sources, it is not intended to be complete for stars. SIMBAD includes both Galactic and extra-Galactic sources, with excellent completeness on the former. Both services harvest source catalogs in the published literature.
Name translation
The MAST SANTA service uses coordinates directly, or queries one or more back-end name translation services, or name resolvers, to determine source coordinates. The services are searched in the order of the table below until a match is found.
The order is fixed, and MAST applications do not currently offer users the option to specify an alternative name resolver, except when using the SANTA service directly.
Problem cases
There are cases where the returned coordinates differ between SIMBAD and NED, though the coordinates usually agree to within 0.25 arcsec among services. But there are many cases where the returned coordinates between SIMBAD and NED are quite discrepant. These cases fall into a few categories.
Imprecise source specification
There are cases where source identifiers (expressed in ASCII text) for different objects are very similar, but refer to very different astrophysical sources. Examples include:
Ambiguous names
Names of sources can be ambiguous for a resolver service, or the name can be conflated by the service. Examples include:
Angularly large sources
The exact center of certain extended sources can be uncertain. Examples include:
MAST Search Interfaces
Searches by source name may be executed with one of the MAST web user interfaces, with user-written software by calling an API, or by using the SANTA interface directly. Note that MAST does not support the full syntax allowed by SIMBAD, and in particular partial identifiers with wildcards are not allowed in source names. Spaces in source names, including between the catalog abbreviation and the member number, are allowed but often not necessary in MAST.
An important feature of these interfaces is the ability to specify a search radius, that is, the radius of a circle centered on the resolved coordinate that will produce matches with MAST observations that overlap the circle. (These are often called cone searches, in reference to the linearly growing size of the implied physical radius with distance to the source.) If no radius is specified, a default will be used. Keep the following in mind for cone searches:
- The default for point-sources is 0.2 deg, but may be increased dynamically in the search if the source angular size (as recorded in SIMBAD or NED) is larger.
- The search results will include all data for which any part of the observing aperture overlaps the search cone.
- The observing aperture is just the extent of a (possibly composite) image, or the slit of a spectrograph. The projection of the aperture boundaries onto the sky is often called the observation footprint.
- Note: any portion of a footprint that overlaps the search cone will be matched, whether the image actually includes the source or not.
- The observing aperture is just the extent of a (possibly composite) image, or the slit of a spectrograph. The projection of the aperture boundaries onto the sky is often called the observation footprint.
- Use a search radius of r=0 to match only data that overlap that exact location in the sky.
- This is often best when searching for small or unresolved sources, provided the source does not have high proper motion.
For some very large sources where MAST-hosted missions have made many observations, the returned results may be quite large. For M31, the default search radius is over 1.6 deg, which currently matches nearly a quarter-million observations. In such cases you may find the query times out (yielding no results at all, and hanging the application), or you may find it impossible to do anything with the results, such as load rows into the download basket. See Best Practices below.
Portal
The MAST Portal enables searches for sources in two ways, summarized below. See the Portal Guide chapter Basic Search for formatting and other details.
Enter the name of a source (and optionally, the search radius) into the target search dialog box in the main Portal window (top), or into the Object Name or Position filter in the Advanced Search window (bottom).
The interface should really use the term Object rather than target, which would be in line with the vocabulary defined here, and with other MAST interfaces.
MAST Mission Search
The MAST Mission searches, like the Portal, can also search by source name. However, the results will be specific to the particular mission (e.g., HST, JWST).
Enter coordinates or the name of one or more objects (and optionally, a search radius) into the Object dialog box in the search window.
MAST API
from astroquery.mast import Observations, Mast
obs = Observations.query_object("M57", radius="1 m")
coords = Mast.resolve_object('M57')
resolver_request = {'service':'Mast.Name.Lookup',
'params':{'input':'M57',
'format':'json'},
}
headers, resolved_object_string = mast_query(resolver_request)
The Python package astroquery.mast provides a search by object name, and a way to resolve directly the source name to coordinates (top left).
A fragment of the same query is shown for the MAST API (bottom left), where the object name is resolved with the Mast.Name.Lookup service. See the MAST API Tutorial for the full query.
https://mastresolver.stsci.edu/Santa-war/query?name=EPIC 201124136&resolveAll=true
The SANTA service can be queried directly with a URL of the form shown at left. It is possible to select the back-end resolver service, or to show the results of all resolvers available to SANTA. In addition to coordinates, SANTA returns the canonical name of the source, its angular size (if extended), and the name of the back-end resolver.
Best Practices
Searches by source name in MAST will be more robust, accurate, and efficient if you embrace the following best practices:
- Query by the source designation in a major, modern catalog, rather than by common name
- If all you know is the common name, try searching SIMBAD or NED directly to find the canonical name
- Leverage the syntax rules of the back-end name resolvers (SIMBAD or NED)
- SIMBAD often follows conventions for catalog entries (where the ^ symbol indicates a blank space): catalog abbreviation(^catalog version)^member number
- Gaia^DR3^2090486618786534784
- IRAS^18517+3257
- Identifiers from major catalogs where the source name includes coordinates often follow the rule: catalog abbreviation^coordinate type ID+coordinates, where the coordinate type is J for J(2000) equatorial coordinates, G for Galactic coordinates, B for Besselian FK4 (1950) coordinates, etc.
- 2MASS^J00054775-7145571
- GB6^B1851+3257
- PN^G063.1+13.9
- SIMBAD often follows conventions for catalog entries (where the ^ symbol indicates a blank space): catalog abbreviation(^catalog version)^member number
- Always specify a search radius, with a value that is no larger than what is needed for your science purpose
- This will limit the size (and minimize the response time) of the results returned by the query
- Use
r=0to match a position exactly, assuming the source does not have significant proper motion
- Be aware of the need to re-cast coordinates to the epoch of interest for high proper-motion sources. (See the article Finding Solar System Bodies in MAST.)
For Further Reading...
- SIMBAD search by identifier
- See also the query by identifier page, which offers specific guidance on syntax
- NED search by name
- MAST SANTA name resolver
- IAU designations for astronomical radiation sources
- Portal Guide
- MAST Mission searches for HST and JWST