6 September 2008NVO Summer School 2008 – Santa Fe1 DAL Clients: Scripting Data Access with Python Ray Plante T HE US N ATIONAL V IRTUAL O BSERVATORY

6 September 2008NVO Summer School 2008 – Santa Fe2 The Data Access Layer: from data discovery to data retrieval Standard interfaces for finding and retrieving particular types of data –Simple Cone Search (object catalog data) –Simple Image Access Protocol (SIAP) –Simple Spectral Access Protocol (SSAP) –Table Access Protocol (TAP)* –Simple Line Access Protocol (SLAP)* *not covered here Common mechanisms accessing data –Search query returns matching items Simple: Query expressed as a URL (a base URL + name=value arguments) Result returned as a VOTable –SIAP/SSAP: record includes URL for downloading data item –Bridge from discovery to retrieval –Differences reflect maturing interface designs 2 nd generation will have more unified behavior

6 September 2008NVO Summer School 2008 – Santa Fe3 Simple Cone Search (SCS) Retrieve a table of objects located within a given distance from a sky position Ultra-simple interface: RA= &DEC= &SR= –RA – right ascension of central search position in degrees –DEC – declination of central search position in degrees –SR – search radius in degrees –Optional argument: VERB= 1=minimum # of columns, 3=maximum # Examples: Just want the table metadata? –Use SR=0 –Service will return VOTable header with no rows

6 September 2008NVO Summer School 2008 – Santa Fe4 Simple Cone Search (SCS) VOTable results –Only one RESOURCE element with one TABLE –Each record represents an object found within the cone –Must contain columns with the following UCDs*: ID_MAIN – a name for the object RA_POS_EQ_MAIN – the objects J2000 RA DEC_POS_EQ_MAIN – the objects J2000 Dec *Note: these are Version 1.0 UCDs

6 September 2008NVO Summer School 2008 – Santa Fe5 Simple Image Access (SIA) Retrieve a table of available images that intersect a rectangular region of sky Pretty simple interface: POS=, &SIZE= [, ][…] –POS – R.A. and dec. in degrees, separated by a comma –SIZE – width & height in degrees One value: square region –INTERSECT – restrict how image intersects the region COVERS, ENCLOSED, CENTER, or OVERLAPS –FORMAT – restrict the types of images returned Mime types: e.g. image/fits or image/jpeg ALL – all available types GRAPHIC – any computer graphics format (JPEG, GIF, PNG,…) –Image generation parameters –Anything else: service-specific parameters Just want metadata? –Use FORMAT=METADATA –Service will return VOTable header with no rows

6 September 2008NVO Summer School 2008 – Santa Fe6 Simple Image Access (SIA) VOTable results –RESOURCE element with attribute type="results" Contains exactly one TABLE element –Each record represents an available image –Must contain columns with the following UCDs*: VOX:Image_Title – a concise description of the image RA_POS_EQ_MAIN – the RA of the image center DEC_POS_EQ_MAIN – the Dec of the image center VOX:Image_Naxes – the # of axes in image VOX:Image_Naxis – the # pixels along each axis (comma-separated) VOX:Image_Scale – the pixel size along each axis, in degrees VOX:Image_Format – the MIME type of the image –E.g. image/fits, image/gif VOX:Image_AccessReference – the URL for retrieving the image –Other columns are recommended World Coordinate System (WCS) metadata Spectral information

6 September 2008NVO Summer School 2008 – Santa Fe7 Simple Spectral Access Retrieve a table of available spectra that intersect a rectangular region of sky Fairly simple interface: REQUEST=queryData&POS=, &SIZE= […] –REQUEST – what to do with inputs Other operations coming: e.g. getData, stageData, getAvailability, … –POS – R.A. and dec. in degrees, separated by a comma –SIZE – width & height in degrees –FORMAT – restrict to spectra encoded particular format –BAND – restrict to wavelength range Range format: 2.7E-7/0.13 (optical to radio) –TIME – restrict to observation time range Range format: /1999 (May 21, 1998 to 1999) –Other metadata parameters –Anything else: service-specific parameters Just want metadata? –Use FORMAT=METADATA –Future: will be replaced by REQUEST=getCapabilities

6 September 2008NVO Summer School 2008 – Santa Fe8 Simple Spectral Access (SSA) VOTable results –RESOURCE element with attribute type="results" Contains exactly one TABLE element –Each record represents an available image –Semantic meaning of a column identified by its UTYPE More specific in meaning than UCD –Must contain columns with the following UTYPEs: Dataset.Length – # of values in spectrum DataID.Title – dataset name Curation.Publisher – name of the spectrums publisher Char.SpatialAxis.Coverage.Location.Value – observed position for the spectrum (RA & Dec.) Char.SpatialAxis.Coverage.Bounds.Extent – angular diameter of spectrums apeture Char.TimeAxis.Coverage.Location.Value – midpoint of observation time Char.SpectralAxis.Coverage.Location.Value – wavelength spectrum midpoint Char.SpectralAxis.Coverage.Bounds.Extent – wavelength bandwidth of spectrum –UTYPEs are available for many columns of spectrum metadata

6 September 2008NVO Summer School 2008 – Santa Fe9 Caution: pre-v1.0 SSA SSA is relatively new –Fairly few, fully compliant services Majority of currently registered SSAs pre- date the final v1.0 version of standard –Registered as version 0.5 compliant See interfaceVersion column in Registry Results browser

6 September 2008NVO Summer School 2008 – Santa Fe10 What is simple anyway? With a simple DAL service, –You can express the query as a single URL Can be easily submitted via a web browser Easy to call via a scripting language –Results are returned as a VOTable Can use any XML or VOTable viewer/parser to open it up