Groups¶

This document describes the meta data of a specdb database and methods to query it.

Description¶

Each group in a specdb database contains a meta data table and the corresponding set of spectra. The two are aligned, row by row.

Within the HDF5 file, each group is stored in a data group which contains two datasets:

meta data in hdf[group+’/meta’]
spectra in hdf[group+’/spec’]

InterfaceGroup¶

The InterfaceGroup Class is used to interface to a group. It stores the meta data, provides a method to query the data, and provides methods to extract the spectra.

The class is instantiated with the point to an HDF5 object, the name of the datagroup, and the IDKEY:

igroup = InterfaceGroup(hdf, 'GGG', 'IGM_ID')

Upon instantiation, the meta data table is stored in the .meta attribute as an astropy.table.Table.

Querying a Single Meta Table¶

Using the InterfaceGroup class, one queries a single meta data table using a Query dict object. Here is an example:

qdict = {'TELESCOPE': 'Gemini-North', 'INSTR': 'GMOS-N', 'NPIX': (1580,1583), 'DISPERSER': ['B600', 'R400']}
matches, sub_meta, IDs = igroup.query_meta(qdict)

This query requests all spectra taken at the Gemini-North telescope with the GMOS-N instrument having between 1580-1583 pixels (inclusive) and with either the B600 or R400 grating.

The method returns a bool array (matches) indicating which rows of the catalog are matched, the sub-table of those rows, and the IDKEY values of those rows.

Retrieving Spectra from a Group¶

See Spectra for a full description of the spectra contained in a specdb database. Here we describe the methods in the InterfaceGroup class that can be used to retrieve spectra. In all cases, the spectra are returned in memory as an XSpectrum1D object.

grab_specmeta¶

The lowest level method for retrieving spectra from a group is ~specdb.interface_group.grab_specmeta. This method takes as input the row(s) desired and returns an XSpectrum1D object containing all of the spectra and a Table of the meta data (the latter may be DEPRECATED). The items in each object are aligned with the input row(s). Here is an example:

# One spectrum
row = 3
spec, meta = igroup.grab_specmeta(row)
# Multiple
rows = np.array([2,5,5,11])
spec, meta = igroup.grab_specmeta(rows)

In the latter call, the 5th row is retrieved twice.

spec_from_meta¶

A more convenient method within InterfaceGroup is to input a Table of meta data to retrieve the desired spectra. This Table must include a column of GROUP_ID values. Using the sub_meta Table from the above Querying a Single Meta Table:

spec = igroup.spec_from_meta(sub_meta)

Again, an XSpectrum1D object is returned with spectra aligned to the input meta data Table.

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