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Simple Spectral Access Protocol
Version 0.91

IVOA Working Draft Sep 20, 2005

This version:

Latest version:

Previous version(s):

Version 0.90 May 2005


D.Tody, M. Dolensky


M.Dolensky, D.Tody, T.Budavari, I.Busko, J.McDowell, P.Osuna, F.Valdes


The purpose of the Simple Spectral Access (SSA) protocol is to define a uniform interface to remotely access regularly or irregularly sampled, tabular spectrophotometric data, including 1D spectra, time series, and spectral energy distributions (SEDs).

The term simple in Simple Spectral Access refers to the design goal of defining a simple, uniform interface for retrieving spectrophotometric data from distributed data collections. It does not necessarily mean that implementing an SSA service is trivial, but the effort to make a data collection SSA compliant has been minimized. The Data Access Layer (DAL) working group supports the implementation of reference code for services which can serve as a starting point for data providers when implementing a new service.
The SSA interface is similar to that of Simple Image Access (SIA) and Simple Cone Search (SCS). In particular the query/response protocol and data access method follow the same approach:

  1. A Query method returns a table (VOTable format) describing candidate datasets which can be retrieved, including metadata describing each dataset, and an access reference which can be used to retrieve the data.

  2. A getData method is used to access an individual dataset. The accessed data may be generated on-the-fly by the service at access time, e.g., to reformat or subset the data.

  3. A metadata query method is defined by which an external client (such as the Registry) can determine the capabilities of a given SSA service instance.

SSA includes both a service protocol, and a spectral data model describing all the data types dealt with by SSA. External data is actively mediated at access time to return data to a client application which is conformant to the SSA data model.

Status of This Document

This is a Working Draft. The first release of this document was in April 2005.

This is an IVOA Working Draft for review by IVOA members and other interested parties. It is a draft document and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use IVOA Working Drafts as reference materials or to cite them as other than "work in progress”.

Comments on this document can be posted to the mailing list, uploaded to the collaborative web page IvoaDAL, or sent to the authors directly.

It is expected that the Simple Image Access, Simple Cone Search and Simple Spectrum Access specifications will be homogenized prior to promotion to the proposed recommendation level. Additional changes are planned when support for SOAP and the Astronomy Data Query Language (ADQL) is integrated. Other changes such as a generalized metadata extension mechanism (Proposal for an evolution of the SIA protocol, Bonnarel et al. 2004) are also planned.

A list of current IVOA Recommendations and other technical documents can be found at


This document has been developed with support from the 5th Framework Programme of the European Community for research, technological development and demonstration activities, contract HPRI-CT-2001-50030 and via a grant from the National Science Foundation's Information Technology Research program to develop the U.S. National Virtual Observatory.

Many of the ideas in this document originated from others involved in developing Virtual Observatory concepts and standards. In particular, the idea of using a logical name to group similar datasets was originally proposed by Roy Williams. Arnold Rots originated the idea of ranking query results via a score heuristic, and helped put the coordinate systems used in SSA on firm theoretical foundation via the development of STC. The technique for using dimensional analysis to consistently scale the physical axes of a spectrum came from Pedro Osuna, Jesus Salgado, and others at ESAC. Francois Bonnarel, Mireille Louys, Alberto Micol, and others made notable contributions to the representation of astronomical metadata. Laszlo Dobos contributed early implementations of the access protocol using the spectral archive at JHU.
Many thanks to all who contributed to the DAL survey among spectral data providers and consumers (Dolensky/Tody 2004): Ivo Busko, Mike Fitzpatrick, Satoshi Honda, Stephen Kent, Tom McGlynn, Pedro Osuna Alcalaya, Benoît Pirenne, Raymond Plante, Phillipe Prugniel, Enrique Solano, Alex Szalay, Francisco Valdes and Andreas Wicenec.


1 Overview 3

2 Requirements for Compliance 7

3 Query Interface 9

4 Data Access 25

5 Service Metadata 26

Appendix A: Sample Query Response for Mixed Service 28

Appendix B: Query Response for Spectrum/TimeSeries Service 30

References 31

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