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astronomy is basically an observational science , and the development and advancement of the subject has relied heavily on surveys of the sky at optical wavelengths to expand our knowledge of the observable universe .
however despite the considerable advances in optical detector technology very little improvement has been made in large scale surveys beyond those available in the 1950 s when the palomar sky survey was carried out .
a photographic plate taken on a 1.2-m schmidt telescope is sky limited in about 1 hour but is only 1 - 2% efficient .
thus our current best wide field optical sky surveys are equivalent to no more that a @xmath060 second glance at the universe with modern ccds using a 1 m telescope . in spite of the inherent limitations of photographic plates ,
they are still used for major scientific programs . in recent years , this has been primarily due to the availability of online digital atlas images and catalogues based on these photographic material eg http://www.ast.cam.ac.uk/~apmcat , http://skyview.gsfc.nasa.gov/.    in an effort to rectify this apparently dismal situation , and to provide necessary underpinning imaging requirements for the 8 m telescope era , the 2.5 m isaac newton telescope on lapalma is being used to carry out a series of wide field imaging programs under the generic title of the int wide field survey(wfs ) project .
the wfs project consists of a series of independent survey programs with distinct aims as we outline below .
the wfs project takes into account both surveys like sdss ( gunn & weinberg 1995 ) which also uses a 2.5 m telescope and has an exposure time of @xmath060 seconds , and other ccd based surveys such as those that are being carried out by noao ( http://www.noao.edu/ ) and eso ( http://www.eso.org/ , nonino etal 1999 ) .
the unique elements of the int survey are : ( i ) optimal choice of fields so that most are easily visible from telescopes in both hemispheres ; ( ii ) inclusion of u band ; ( iii ) large area ( iv ) temporal information ; ( iv ) good overlapping coverage with existing deep radio surveys ie first , wenss ; ( v ) wide ra coverage optimised for efficient follow - up ; ( vi ) choice of sdss bandpasses for longevity .
this article briefly describes the wide field survey ( wfs ) program .
this is a peer reviewed survey program that aims to provide deep high quality ccd data to the community both quickly and in a convenient form .
the concept of the wfs originated in 1991 within the context of the science case for a ccd mosaic for the isaac newton telescope .
formal approval for the survey program began with a proposal to the ing board in october 1997 .
the primary goal was to exploit the excellent capabilities of a recently completed ccd based mosaic that effectively filled the unvignetted focal plane of the 2.5 m isaac newton telescope ( see figure  [ fig_wfc_geometry ] ) .
the immediate aim was to carry out a major ccd based multi - colour survey in a timely fashion over a period of 45 years and allow instant and easy access to the processed data to facilitate its rapid scientific exploitation    the wfs proposal was approved by the ing board in october 1997 with a subsequent ` announcement of opportunity ' closing in march 1998 .
conditions of solicitation included that the survey data is available to all uk and nl based astronomers in near real - time .
raw data is typically available as taken , whilst the pipeline reduced data is available after one month .
subsequently the raw and processed data is available to the rest of the astronomical community after one year .
pipeline processing of the data is the responsibility of the cambridge astronomical survey unit(http://www.ast.cam.ac.uk/ ) .
a wfs international review panel approved three main programmes in the first year , allocating five 
six ` dark / grey ' weeks per semester to the wfs . in june 1999 a first year review carried out by patt and the international review panel confirmed the continuation of the first year wfs programmes into 2000 .
the int wide field camera ( ives , tulloch & churchill 1996 , see also paper in these proceedings ) is mounted at the prime focus(f/3 ) of the 2.5 m isaac newton telescope on la palma , canary islands .
the camera consists of a close packed mosaic of 4 thinned eev42 2kx4k ccds .
the layout is shown in figure  [ fig_wfc_geometry ] .
the ccds have a pixel size of 13.5 microns corresponding to 0.33  /pixel . the edge to edge limit of the mosaic neglecting
the @xmath01 inter - chip spacing is 34.2. in normal survey mode we use a step size in ra and dec of 30 and 20 respectively .
this provides @xmath010% overlap on all edges and means that the partially vignetted chip is overlapped completely to aid photometric calibration .
.nominal photometric limits in 1 arcsec seeing for int wide angle survey [ cols="^,^,^,^,^,^ " , ]     notes : the exposure times per band vary between these field see the wfs www page for further details .
also , in some fields the was program has added wavebands to those obtained by the original pis . @xmath2 the actual filters may be different eg kpno b or harris v etc .
the main science programmes chosen include a ` wide shallow ' programme , a smaller deep area programme , and a programme to address temporal variability .
the specific programs are described briefly below :      this is the largest approved programme approved and includes sub - projects ranging from determination of cosmological parameters ( via sn type ia ) to searches for solar system objects .
the underlying philosophy of the was survey is encompassed in table  [ table_was_science_themes ] where we summarise the time requirements of over 20 topical scientific programs .
if all these programs were carried out under the normal pi s based time allocation procedures the total on - sky time required is almost 600 nights .
however , if the programs are combined they can be executed in around 100 nights . by merging the requirements of the various programs we end up with a highly efficient observing strategy .
an important aspect of the reduced time requirements is that the projects will also be executed quickly .
the limiting magnitudes and wavebands being used are summarised in table  [ table_was_nominal_limits ] .
figure  [ fig_int_ao_gal_mags ] shows how these limits transform onto the observational plane for extragalactic studies .
the main survey region are listed in table  [ table_was_major_survey_regions ] .
a number of smaller regions are also being surveyed as determined by calibration requirements and the observing schedule .
in addition , we are adding bands to other programs so that we can increase the areal coverage of multi - colour data at low cost .
these fields are listed in table  [ table_was_supplemental_survey_regions ] .
the was program is the umbrella programme for the wfs project and leads the coordination efforts with the other programmes on , for instance , field and filter selection , to maximise scientific return of the wfs project .
all programs remain autonomous during this procedure so that the peer reviewed science goals are protected .
some of the science goals of the was are outlined below :    @xmath3 * galactic studies : * including both halo and disk white dwarf luminosity function which are relevant to both dm models and to independent calibrations of the hubble time ; stellar density distributions towards the ngp , to improve extant k@xmath4 determinations of the local dm ; stellar counts towards the anti - centre and other widely spaced directions , to determine the stellar warp and refine models of galactic structure    @xmath3 * clusters of galaxies : * the aim is to determine the space density and cluster - cluster correlation function over the range 0.5@xmath5z@xmath51.0 .
galaxy clusters are the largest gravitationally - bound structures in the universe , and the study of their abundance and evolutionary history with look - back - time places strong constraints on cosmological parameters and the primordial power spectrum that gave rise to the observed large scale structure .
@xmath3 * radio sources & radio galaxies : * deep optical identification of radio sources allows : accurate counts of different types of host along mjy tracks in the p - z plane , studies of radio source luminosity evolution ; multi - band investigation of giant - e standard candles ; the largest known sample of low - luminosity rgs with good photometry ; large - scale structure from photometric redshifts and cell counts in redshift slices ; accurate optical positions of first sources for wyffos/2df followup .
@xmath3 * intermediate redshift type 1a supernovae : * whilst dramatic progress has been made in the determination of the fundamental cosmological parameters ( @xmath6 , @xmath7 ) in the last two years , the analysis is now limited by systematic errors .
identifying @xmath020 type 1a supernovae in the critical range 0.1@xmath5z@xmath50.4 will allow a detailed treatment of these systematic errors .
the was also incorporates two independent distinct science programmes in the spring semester centred on virgo and the north galactic pole .
in fact , in the proposal submission procedure many co - i s of the was program submitted discrete proposals .    * * a multicolour survey of the virgo cluster : pi , davies * this aims to obtain the galaxy luminosity function ( lf ) of the virgo cluster as a function of colour and position in the cluster . * * the millennium galaxy catalogue ( mgc ) : pi , driver * the mgc will provide a complete and local galaxy catalogue .
this survey is being carried out in the b band and lies in a region of sky covered by the 2df redshift survey
.      this programme is carrying out deep imaging of 10 deg@xmath1 to a limiting magnitude of b=26 and i=24.5 .
it will enable the study of the evolution of galaxy clustering as a function of colour at faint magnitudes and provide a catalogue of rich galaxy clusters at intermediate red shifts .
this programme is searching an area of @xmath010 deg@xmath1 , studying photometric and astrometric variability on scales of one hour to a year to a magnitude of v=25 .
example areas of investigation include : the evolution of specific galactic populations ( e.g. cv s , rr lyraes , halo agb stars , brown & white dwarfs , kuiper - edgeworth belt objects , sdb stars ) , the structure of the galactic halo , statistics of optical transients related to @xmath8-ray bursts , and deep proper motion studies .
in order to maximise the scientific value of the wfs data the was survey is concentrating on fields that are equatorial and hence follow - up can be carried out from telescopes on both hemispheres .
this simple consideration doubles the scientific return of the survey .
we also deliberately centred some of the fields on landolt photometric calibration fields ie sa95 and sa114 .
the choice of photometric wavebands was relatively straight forward .
we decided to use bands similar to the sdss bands(fukugita etal , 1996 ) .
note our u and z bands are not identical to the sdss bands .
see the wfs www pages for further details .
the choice of the sdss bands means that the int surveys will be directly comparable with work carried out as part of the sdss .
interestingly , the sdss g band is very close to the ukst @xmath9 band .
however , manufacturing delays have meant that we had to start the survey using the standard int filter set .
survey data is being obtained on a monthly basis and thus a summary of the data obtained will soon be out of date .
a complete summary of observations obtained is kept on - line at http://www.ast.cam.ac.uk/~wfcsur/status .
the situation at the end of may 1999 was that @xmath060 deg@xmath1 had been observed in the first ten months of the survey .
the data products currently available for access include :    * observing logs built from the fits headers * a sybase www user interface to access the raw and processed data * library bias frames , flatfield frames , defringing frames and non - linearity corrections * colour equations for all filters * processed 2d image maps , with a full record of processing steps in the fits headers * astrometric calibration , with the world coordinate system in the fits headers * photometric calibration  zero points and extinction    in the coming months the data products provided will be expanded after some quality control to include :    * object catalogues , generated using apm based routines ( irwin , 1985 ) and sextractor ( bertin & arnouts 1996 ) .
further information about the int wide field imaging survey can be obtained at the isaac newton groups www page ( www.ing.iac.es/wfs ) or the uk mirror(www.ast.cam.ac.uk / ing / wfs ) .
in addition , the wide angle survey has as a www page at www.ast.cam.ac.uk/~rgm / int_sur/. further details of the pipeline processing are contained in a paper by irwin and lewis(these proceedings ) .
the authors would like to acknowledge the unsung builders of the int wide field camera , the various peer review committee members who have guided the project since it inception in 1991 and the encouragement of their many colleagues during the long gestation period of the int wide field survey project .
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the 2.5 m isaac newton telescope(int ) is currently being used to carry out a major multi - colour , multi - epoch , ccd based wide field survey over an area of @xmath0100deg@xmath1 . 
 the survey parameters have been chosen to maximise scientific return over a wide range of scientific areas and to complement other surveys being carried out elsewhere . 
 unique aspects of the survey is that it concentrates on regions of sky that are easily accessible from telescopes in both northern and southern terrestrial hemispheres and that it the first public survey to use filters similar to that being used by the sloan digital sky survey . 
 a major aim of the the int wide field survey program is to bridge the gap between the all - sky photographic 2 and 3 band surveys such as the palomar and uk schmidt sky surveys and the ultra - deep keyhole surveys such as the hubble deep field(williams et al , 1996 ) . 
 apart for the science that can be derived directly from the optical data , the datasets will provide ideal targets lists for multi - object followup with fibre and slit based systems(eg gmos , 2df , wyyfos , fmos ) based systems on 4 m and 8 m class telescopes .    
 surveys , archives , databases