EIS Definitions and Conventions

30 August 2004

Introduction

The EIS survey system is designed to administer and document (via integration with a robotically maintained WEB service) multiple surveys and strategies in their entirety, from survey definition to release of science-grade products by nominally operating in unsupervised mode. Achieving this requires the introduction of well-defined procedures to enable adaptive, automatic and sequential addressing of information. Occasionaly, these may be need to be properly explained, which will be done in this document when necessary.

In addition, the system also produces a large variety of survey products requiring clarification on the different data types, FITS header keywords and conventions.

The purpose of this document is to serve as a quick reference document for users of data products produced by the EIS survey system. This is a ``live'' document which will evolve time as required.

Pointing Nomenclature

For some surveys involving external collaborators, inconsistencies may be present between adopted conventions which, while inconvenient, are unavoidable. Below some of these differences are described and their origin explained.

Ideally, the standard adopted by EIS to name a field is to use the survey name and/or region as the root, followed by a numerical suffix generated by the particular algorithm used in the construction of the "Observation Block" (OB) and the associated survey strategy (as defined by a set of pointings, instrument and passband). Typically, this numerical suffix is assigned in an increasing sequence based on the R.A. and Dec. defining the pointings. This should be done when a survey is first defined. In this way, all possible identifiers of a field utilized by the EIS system (both internally and externally, e.g. OB name, object name, target name, field name) and their graphical representations on the EIS WEB pages form a homogeneous set.

Historically, however, the conventions have evolved in time due to several factors:

At their outset, prior to the introduction of a systematic global approach, several surveys were defined case-by-case and in isolation.
Over time, some pre-existing and on-going surveys had their geometries changed; methods for addressing positions were reviewed; surveys were extended, amalgamated or otherwise re-defined using modified algorithms.
In the absence of pre-determined standards, collaborative surveys co-administered by independent groups have led to different notations. The interface to a common scheme (when this requirement is inevitably identified) becomes particularly difficult leading unavoidable to inconsistencies.

Hence, in practice, the desired ideal case is not always attainable and requires some retroactive adaptations. The following noteworthy discrepancies between these different conventions are:

There exist surveys for which addressing information was assigned in isolation at the survey outset and which were also assigned a sequential value to be incorporated into, and referenced by, the EIS system at a later date. For these surveys, while the multiple nomenclatures persist simultaneously, sequentially numbered fields and/or regions referred to in EIS indices do not correspond to numerical values which may appear in region and/or field names assigned prior to introduction of the EIS system. For example, in the GOODS survey, the field indexed by the EIS system (including the web service) as "1" does not necessarily correspond to the field name originally defined to be "F01" in the GOODS survey
The current EIS terminology prefaces fields with root names derived from the particular survey name whereas the same fields may be refer to by a subjective name (e.g.XMM-06 = SHARC-2). This applies to the OB names as well as the OBJECT keyword in image FITS header. Thus, the use of this keyword to rename the distributed images is discouraged since it may lead to confusion.
Sequential TILEID numbers appearing in the FITS header of an EIS product are automatically assigned by the EIS system when binding mosaic images and their contributing tiles into a set of related products for a given release. The binding depends on the geometry and completeness of the contributing reduced data. Therefore TILEID numbers do not necessarily correspond to other indices such as the field identifiers used by the Survey teams, or the field identifiers employed by EIS algorithms.

Glossary

FIELD: An individual pointing of an instrument defines a field. Fields are thus instrument specific and if exposures at the same coordinates are taken with different instruments, they constitute different fields. Each field belongs to exactly one region.
REGION: Region is the area covered contiguously by fields belonging to the same strategy. Each region belongs to exactly one survey.
STRATEGY: An observing strategy is a combination of telescope, instrument, filter, exposure time and expected limiting magnitude. Strategies are grouped by wavelenght domain (optical/infrared) and by limiting magnitude (deep/shallow). Each of these groups is also referred to as Strategy.
SURVEY: A survey is a group of strategies covering one or more regions.
FILTERS: All filters listed on the EIS WWW pages have the format ''passband''+''ESO Filter ID''. Details about these filters can be found on the telescope pages which are directly linked from the filter names.
OBSERVATION BLOCK (OB): as defined in the ESO Data Flow System (DFS).
REDUCTION BLOCK (RB): one RB is a group of raw images obtained in the same night which share the same position, passband and form a sequence in time. The content of one RB is usually identical to an executed OB. However, the system also allows RBs to be formed by concatenation of similar sequentially executed OBs. In addition individual exposures can be discarded based on configurable constraints on their properties (e.g. background counts, DIMM seeing values, integration time, etc.)
REDUCED IMAGE: A "reduced image" is the "nightly" product of a reduction process of a so-called "Reduction Block" (RB).
STACKED IMAGE: A "stacked image" is the product of an advanced process in which "reduced images" sharing the same instrument, passband and position on the sky are grouped into so-called "Stack Blocks" (SB). In the assembly of SBs, individual "reduced images" can be discarded according to configurable constraints on their properties (e.g. measured seeing, amplitude of PSF distortion, visual grade, OB completeness). The images of the thus assembled SB are re-scaled and accumulated in a weighted co-addition procedure to generate a single final output product.
MOSAIC IMAGE: A "mosaic image" (or simply "mosaic") is an image formed from several contiguous (reduced or stacked) "images" produced by the EIS survey system, which have been subsequently re-scaled to share a common zero point and co-adjoined as "tiles" to form a single, large-scale product.
MOSAIC TILE: A "mosaic tile" (or simply "tile") is an image (reduced or stacked) produced by the EIS survey system which has been subsequently re-scaled such that it shares a zero point common to other tiles and a large-scale "mosaic image" (or simply "mosaic") of which it forms a component.
VERSION: A release "version" represents a wholesale, global change: either introducing a new set of products, affecting or superseding the quality or properties of more than 5 per cent of previously release products.
REVISION: A release "revision" represents localized changes which may affect individual products with a released data set, while leaving others unaffected. Individual products found to be anomalous may be removed or replaced. New products may be added. The total revisions must not alter more than 5 per cent of a pre-existing set of products.
ZP: FITS header keyword ZP contains the value of the zero point (magnitudes) computed from night solutions. In a reduced image the ZP in the header is given by ZP = ZP' + KX. where ZP' is the zeropoint at zero airmass (determined from the linear fit), K is the extinction coefficient and X is the observed airmass. Therefore, the Vega magnitude is given by mag (Vega) = -2.5*log(flux) + ZP. In the case of a stack the ZP is the zeropoint at zero airmass where the flux is the number of counts directly measurable on the images (note that the reduced images are normalized to 1 sec). It can also be set to a default value in which case the associated error is set to a negative value.
ZPERR: FITS header keyword ZPERR contains the value of the uncertainty (magnitudes) associated with the determination of ZP. This is estimated to be the scatter relative to the best linear fit to the flux measurements of standard stars as a function of the airmass. It can also take the values of (-1) for nights with poor fits; (-2) for nights without standards; (-3) for nights in which the zeropoint deviates significantly from a ``typical'' (normally median) value.
ZPMOSAIC: FITS header keyword ZPMOSAIC contains the value of the zero point (magnitudes) applied to the processed tiles in order to refer instrumental magnitudes to the common photometric system of the mosaic, MOSAICID. The value for ZPMOSAIC is determined by a re-scaling process in which individual reduced images are harmonized to become tiles sharing a common zero point for the entire mosaic.
ZPMOSERR: FITS header keyword ZPMOSERR contains the value of the uncertainty (magnitudes) associated with the determination of ZPMOSAIC.
ZPMOSSYS: FITS header keyword ZPMOSSYS contains the photometric system in which ZPMOSAIC and ZPMOSERR are expressed.
MOSAICID: FITS header keyword MOSAICID contains the the unique identity of the mosaic to which an individual tile belongs to.
MOSTILES: FITS header keyword MOSTILES contains the total number of individual tiles contributing to the larger mosaic, MOSAICID.
TILEID : FITS header keyword TILEID contains the unique identity of the individual tile within the larger mosaic, MOSAICID. TILEID does not necessarily map directly to any other FITS keyword value (e.g. OBJECT).

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EIS Definitions and Conventions

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