1 Introduction

1.1 Purpose and scope

This document summarizes the ESO official data interface specification. This specification applies to all data structures produced or used by the ESO VLT and NTT telescopes. A description of the term Data Interface is given in section 2 below, together with a summary of when and how such an interface is used.

The data structures mentioned in this document reflect the concepts and objects developed for the VLT Data Flow System (DFS) as implemented in the NOV97 release of VCS and DFS software. The DFS is described in [11] , its design is addressed in [10] . The VLT Control Software (VCS) interfaces are described in [5] and [6] .

This document is issued and maintained by the ESO Data Interface Control Board (DICB). The DICB Terms of Reference are given in [4] and reprinted for convenience in appendix A.

This document is meant as a technical reference and therefore, its main audience is engineers or scientists who develop software to either produce, analyze or handle data products that conform to this specification. A corresponding set of user manuals will be available both off– and on–line to aid the final user to understand and use the data structures described hereafter.

The detailed data interface specifications are described in data dictionaries . There is one dictionary for each context i.e. instrument, telescope system, observatory, etc.

The ESO DICB issues and maintains a generic dictionary that serves as a non-binding template for other dictionaries; the purpose being to illustrate the minimum set of keywords suggested for each context and to give guidelines in the usage of them.

The format of the ESO Data Dictionaries is given in section 8 . The ESO VLT Generic dictionary is available at http://archive.eso.org/DICB

In addition to data dictionaries, the DICB also releases and maintains data products specifications that describe the layout of FITS frames and other file structures used by the observatory.

Examples in this document have been included for explanatory purposes only. The authoritative reference for keyword specifications are the ESO Data Dictionaries.

The on-line version of this document, the Data Dictionaries and other DICB information is located on the Science Archive server at http://archive.eso.org/DICB. Facilities for searching and selective display of keywords are also available.

Requests for changes or additions to this document or any of the ESO Data Dictionaries must be submitted to the DICB for consideration. Please refer to appendix A for details.

This document and the DICB data dictionaries supersede the older Archive specification, Data Interface Requirements, Ref No. ARC-SPE-ESO-00000-1/1.4 which becomes hereby obsolete.

1.4 Glossary

Calibration Frame A frame used in the process of data reduction to remove instrument signature from observations.
CD-ROM/DVD-ROM Storage media in use by the computer industry to store data and programs. The CD-ROM and DVD-ROM support directory based file systems and structure.
Data Interface Set of definitions that describe the contents of data products (see section 2 for a detailed discussion).
(VLT) Data Flow System The system that handles the flow of scientific data and information for the ESO VLT. It includes subsystems for proposal handling, observation handling, science archiving, data pipeline and quality control (see [10] and [11] ).
Data Preview A highly compressed view of an observation, mostly obtained by reducing observation frames via a suitable pipeline procedure.
Data Products Data files delivered to PI/Co-Is as result of the execution of ESO observing programmes or setup information submitted to ESO in relation to observing programmes. Data products include observation frames, observatory calibrations, meteorological and seeing measurements relevant to primary observations, etc.
Data Organizer A tool to classify frames and associate suitable calibration data to observation frames.
Flexible Image Transport System (FITS) A standard data format widely used in the astronomical community. FITS is defined in [1] . FITS files are divided in a header part and a data part. The header part consists of keyword = value records that describe each a parameter. The FITS header describes the structure of the data part and includes, in ESO data products, also the description of the observation performed.
FITS Keyword A word of maximum 8 alphanumeric characters used in FITS headers to encode parameter information related to the data formatted in the FITS file.
Graphical User Interface (GUI) A user interface based on the presentation of data and command options via graphical panels and user selection via mouse and keyboard data entry.
Log File A computer readable file containing log records. Log files are written by handlers that receive log requests from distributed applications running in the on-line environment. Typically, log handlers will record major normal operations as well as unforeseen events and errors. The format of log files is defined in section 5.1 .
Observation Block The smallest schedulable observational unit for the ESO VLT. An observation block contains a sequence of high level operations, called templates that need to be performed sequentially and without interruption in order to ensure the scientific usefulness of an observation. Observation blocks may include only one target acquisition. Observation blocks also contain scheduling and pipeline reductions requirements and collect all status information of operations as they pass through the data flow system from definition, execution, reduction to archiving.
Observation (raw) Frame The data product containing the result of an observation. Different instrument modes produce in general different observation frames.
Observation Summary Table (OST) A table created and managed by the Data Organizer. The table summarizes the characteristics of a set of frames, most probably from an observing run.
Observing Programme A list of observation descriptions and targets to be observed to achieve a scientific aim. Observing programmes are proposed by PI and are granted observing time by a time allocation committee (e.g. the ESO OPC). For the VLT, observing programmes will be formulated during Phase 2 Proposal Preparation in terms of Observation Blocks .
Phase 2 Proposal Preparation Detailed preparation of observations. This phase is used by astronomers who have been granted observing time in order to give detailed observation setup for each target within their Observing Programme . For the VLT and the NTT, PI/Co-I's will describe their observation requirements through building Observation Blocks for each target.
Reduced Frame The result of data reduction applied to either raw or calibration frames. Reduced Frames are used to either generate data previews or for further calibration.
Setup File A computer readable file containing configuration information for either telescope, instrument, detector, etc.
Template High level VLT operation. Templates provide the means to group commonly used procedures in a well defined and standardized unit. Templates have input parameters described by a template signature , and produce results that can serve as input to other templates. As an example, an Acquisition Template takes target coordinates and produces through an interactive procedure the precise positions used later, e.g. to place the slit.
Translation/Alias Table A table containing alternative names for ESO standard keywords. This table is used by data delivery tools or control software to translate short names into ESO standard parameter keywords. The Science Archive can deliver FITS files with non-ESO keyword headers by translating the ESO standard into an external specification defined through a translation table.
VLT Control Software (VCS) The software tools and systems that are directly involved in the control of VLT instruments, telescopes and related hardware. It enables and performs the acquisition of scientific data.

1.6 Conventions used in this document

The following conventions are used throughout this document:

Keywords appear in monotype font (e.g. NAXIS ).
Keyword data types are given in the tables of FITS keywords (e.g. table 1 ) in the leftmost column with the following codes:

(L)	boolean/logical
(I)	integer
(S)	character or string
(R)	double precision

Character strings in keyword values are left justified, and trailing spaces are not significant.
Angles are measured in degrees, the convention for optical elements is summarized below:

Grism Angle The angle of grisms is defined as the angle between the groves and the alignment pin on the front face of the instrument. The alignment pin is duplicated on the rotator and the instrument.
Slit Angle The angle of a slit is defined as the angle between the slit and the alignment pin on the front face of the instrument. The alignment pin is duplicated on the rotator and the instrument.

Angles that relate to the projected sky along the light path are measured with a right-hand orientation as shown in figure 1 . The position angle g is measured East of North. Two tilt angles are needed to describe elements that are not perpendicular to the optical axis: a and b . They give respectively the tilt against the plane perpendicular to the optical axis along the celestial East-West axis and along the celestial North-South axis.
Other angles measured following the conventions given in [12] .

Conventions for angles related to the projected sky plane.

1.8 Release notes

The current release of this document is issued to document the features implemented in the NOV97 release of the DFS and VCS software. Issues that have been discussed and agreed upon by DICB but are not yet fully implemented in the software, or issues that will be decided upon in the next six months have been left out for a later version 2.0 of this document. Among them there are:

ObservationBlock parameters;
a definition of the Data Package to be delivered to PI's of VLT observations;
data product definitions for each instrument template;
keywords to describe multi-chip, multi-port detector readouts that are stored in FITS image extensions;
a naming convention for FITS frames delivered to ESO users;
an object classification scheme for science targets;
a new category OCS to describe OS specific setup keywords.

Many other data structures are not yet well defined. At the time of release, when the VLT and its instruments are under construction and the VLT Data Flow System is under development, it was considered propitious to nevertheless have a document like this one as a means to pave the way towards a complete data interface specification.

When applicable, areas that need further definition are mentioned in the corresponding sections. The most important are listed here for convenience:

Interferometer parameters;
adaptive optics parameters;
active optics parameters.

Suggestions and comments to this document will be very much appreciated, please send them to the DICB chairman (M. Albrecht, malbrech@eso.org ).

Data Interface Control Document

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1 Introduction

1.1 Purpose and scope

1.2 Applicable documents

1.3 Reference documents

1.4 Glossary

1.5 Abbreviations and Acronyms

1.6 Conventions used in this document

Conventions for angles related to the projected sky plane.

1.7 Acknowledgments

1.8 Release notes

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