WFCAM observing strategy
Some issues relevant to the WFCAM Science Archive
Date: June 9, 2003
This short discussion document summarises some issues concerning WFCAM observing
strategy that are relevant to the WFCAM Science Archive (WSA). Archive
developments and design to date are fully documented at the WSA
homepage . The purpose of this document is to inform dicussions
concerning WFCAM observing strategy at the workshop meeting at Edinburgh on
12/13 June, 2003.
The following are issues that have arisen over the course of the analysis and
design phase of the WSA (all are not necessarily applicable to every UKIDSS
- Survey design:
- Organisation of observations into sub-regions and fields: it is assumed
that each of the UKIDSS wide-area survey will divide their surveyed regions
into chunks, each chunk being divided into a predefined system of fields,
where one field corresponds to one WFCAM device `paw print' on the sky.
- For ease of housekeeping within the WSA (eg. identification of associated
sets of detections in single passbands into merged multi-colour source
records) it will be most advantageous to have fixed lists of uniquely
identifiable fields (analogous, for example, to the field numbering systems of
the Schmidt photographic surveys). That is not to say anything about how
any particular survey uses paw-prints and/or tiles to cover any given surveyed
region; all that is needed is a set of defined paw-print centres and associated
unique (within any given UKIDSS subsurvey LAS, GPS, ...) numeric identifiers.
- Calibration (see also the calibration discussion document )
- if any of the following issues are relevant, then the details need to be
folded into the WSA design (eg. ways of identifying associated calibration
fields as such, and not part of the main target fields, need to be considered):
Are there any oustanding issues (eg. colour terms of higher order than is
currently specified in  and ?
- local, secondary standard fields
- bootstrap overlapping fields (eg. current proposal is to make some
use of non-photometric conditions and later recalibrate using overlapping
frames that provide a greater overlap than the natural detector overlap)
- observations for instrumental characterisation (eg. mesostepping for
photometric field-dependency measures)
- systems and units: AB/Vega/luptidues can be accomodated in the archive
design, but a decision on which are required would be helpful
- Organisation of catalogue data within the WSA -
it is important to ensure the
current relational design  does not preclude any desired observing
strategy. The current relational model for catalogue data uses the same basic
template for each of the UKIDSS surveys: each survey image in each
passband gives rise to many detections; each detection is merged into
a multi-colour, multi-epoch source (it is anticipated that, at least
in relatively uncrowded regions, this source catalogue will be the first
port of call for science exploitation); this source list is used to
drive source re-measurement in a uniform way across all passbands
observed for a given survey; each multi-band source re-measurement
consists of single passband re-measurements analogous to the original
detections, and linking back to the images:
- survey design: choice of paasbands and the requirements for the `curation'
procedure outlined above (merging, source re-measurement, etc.) should be
finalised as soon as possible to enable archive implementation. Are all
survey requirements covered in the current design as detailed in ?
- re-observations: WSA ingestion of sub-optimal observations and their
subsequent replacement by re-observations is allowed for in the current
design, but the exact mechanism for propagating the neceassary housekeeping
information from survey design, through the dataflow system, and into
the archive is presently unclear.
- Finally, the nested surveys within UKIDSS give rise to a complex series
of requirements on the science archive which has resulted in a correspondingly
complex, but flexible, WSA relational design. Housekeeping and curation of
these surveys will only be successful if as much descriptive information as
possible is specified and propagated into the science archive. The sooner
observing strategies, and particularly survey designs, are specified the better.
Procedures can be put in place to insert and/or propagate metadata into the
science archive and it's design.
WFCAM/VISTA Science Archive Development,
Photometric Calibration for WFCAM data, Simon Hodgkin & the WFCAM Calibration
WG, 5/6/2003, Version 1.
WFCAM Science Archive database design document;
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