Background

The ESO Imaging Survey has emerged from the recognition of the urgent need for producing samples suitable for VLT observations prior to its scientific operation and the long debate within the ESO community for the long term need of imaging capabilities in support of VLT scientific programs.

The most recent attempt to address this problem was made inn the summer of 1995 when a small Panel was organized at the ESO headquarters to investigate and make recommendations about the scientific needs and the technical requirements for wide-field imaging capabilities at ESO in support to the VLT. The Panel was composed by ESO staff, visiting scientists and scientists in the Garching area, including: T. Broadhurst. S. Cristiani, L. da Costa, R. Gilmozzi, B. Leibundgut, R. Méndez, G. Monnet, A. Renzini (chair), P. Schneider and J. Villumsen. The Panel first identified a set of topics that required extensive imaging observations either on a stand-alone basis or as preparatory work for VLT programs. Among the topics considered were: search for primeval galaxies; high redshift QSOs; distant clusters; low surface brightness galaxies; weak gravitational lensing by galaxies, clusters, and large scale structures; galaxy inventory of low redshift clusters; search for high redshift supernovae; search and study of individual objects in nearby galaxies (globular clusters, planetary nebulae, massive stars, HII regions, etc.); stellar population studies in the Milky Way, the Magellanic Clouds and the dwarf spheroidal galaxies; and finally the search for exo-planets by gravitational microlensing. The Panel reviewed all these science cases thoroughly as well as all the possible ESO options for wide-field imaging. The Panel concluded that to ensure a competitive use of the VLT in several research areas required both a very wide field imager at a 2-4m class telescope, and a moderately wide field imager at the VLT.

Since then several positive developments have taken place thanks to the continuous effort of ESO as well as the initiative of individual institutes in the ESO Community. The Max-Planck-Institut fuer Astronomie (Heidelberg, MPIA) has offered financial and technical support to equip the ESO-MPIA 2.2m telescope with a wide field camera (~35'x35'), while the Osservatorio Astronomico di Capodimonte (Napoli) has offered financial and technical support for the development of the 8Kx8K CCD detector for the camera. While this initiative is now well under way, the Osservatorio di Capodimonte has also offered to ESO to construct a 2.5m telescope optimized for wide field imaging, a proposal now under evaluation at ESO. In addition, the STC has recently approved the construction of two new, highly competitive instruments for the VLT, the optical and near infrared multiobject spectrographs (VMOS and NIRMOS, respectively), each with imaging cameras with a field of view 14'x14'.

Despite these important new developments none will start operation before the first light of the VLT. The recognition of the long timescale for more suitable solutions motivated the formation of a group led by L. da Costa and including A. Baker, J. Beletic, D. Clements, S. Coté, W. Freudling, E. Huizinga, R. Méndez, and J. Ronnback (all from ESO) to find a short term solution for the problem of creating statistical samples by early 1998, that is, before the first period of VLT proposals. with the available infrastructure. During several months the Group worked hard exploring in great detail the possibility of conducting an imaging survey at the NTT during the second semester of 1997, about one year before the beginning of the VLT scientific operations, making the data available to the ESO community before the proposal deadline. On 12.05.96, the Group released its final document, a 77 pages feasibility study, that included a careful analysis of the science drivers of the survey, along with its technical, observational, and operational aspects. The document also included contributions on specific topics by H. Boringer, S. Cristiani, P. Schneider, and J. Villumsen, most of them members of the original Imaging Panel.

On 31.05.96 the ESO Director General illustrated to the OPC the benefits that an imaging survey would have for the early VLT observations. The OPC fully endorsed the notion, and recommended the formation of a Working Group (WG) that would be responsible for elaborating a formal proposal for the imaging survey by 31.10.96, to be discussed at the following meeting of the OPC on 27/29.1196. Being recognized as an integral part of the VLT Programme, the ESO Imaging Survey was placed under the scientific responsibility of the VLT Programme Scientist. who was also responsible for organizing the Working Group and of coordinating its activities. On 10/11.07.96 the WG had its first meeting in Garching with the presence of G. Chincarini, S. Cristiani, J. Krautter, K. Kuijken, Y. Mellier, D. Mera, H. Röttgering and P. Schneider. From ESO, besides the authors of the present article, several other astronomers attended the meeting, including J. Bergeron, S. D'Odorico and R. Gilmozzi. The WG members had received in advance the da Costa et al. document, and adopted it as the starting point of the discussion. The WG soon endorsed the concept, goals, and general design of the proposed survey. This was followed by a very productive discussion about the various trade offs, such as depth versus sky coverage, length versus width and filter selection, among others. As a result of the discussion important improvements were made to the original design, and it was requested to the ESO astronomers to prepare a revised and more concise proposal to be circulated in advance of the next meeting.

The draft of the EIS proposal was distributed to the WG members in 09.96, and on 04.10.96 the WG met for the second time, this time with the presence of two additional members S. Charlot and R. Saglia, and thoroughly discussed the proposal. While the drift-scan part of the EIS proposal was unanimously endorsed by all the WG members, a few among them expressed some reservation concerning the pointed observations. Although fully acknowledging the outstanding scientific value of this part of the proposal, some argued that individual teams from the community could have submitted a regular proposal on their own to conduct the corresponding observations, while maintaining proprietary data rights for some time. The WG finally endorsed both parts of the survey, and issued the following recommendations to the OPC:

- The WG recognizes the urgency of an imaging survey in preparation of VLT observations. Taking into consideration the VLT timetable this survey must be carried out in 1997.

- The WG recommends ESO to consider EIS as an integral part of the VLT project and that the needs of the survey should be addressed in a timely fashion.

- The WG recognizes that the NTT is currently the only available telescope to carry out the survey.

- The WG endorses the scientific goals, the design and the number of hours requested.

- The WG recognizes that great utility of this survey as a pilot program for future surveys. In particular, the experience gained with EIS should allow a more efficient implementation of the surveys with the new wide field camera at the ESO/MPI 2.2m telescope, to start late in 1998.

- The WG recommends the preparation of a WEB site describing this survey as well as possible future surveys to keep the community informed and to get feedback from it. Periodic progress reports would be included as well as answers to most frequently asked questions The members of the WG agree to collaborate with the ESO staff to answer questions on their areas of expertise. Similarly, the survey should be announced in the Messenger.

- The WG recommends that regular NTT programs be prioritized for both in the Period 59 and 60, and telescope time be allocated on conditional basis depending on the time available.

- The WG encourages ESO to seek help for the preparation of the survey and the subsequent data analysis by establishing collaborations with other European centers and by recruiting post-docs and senior scientific personnel that could provide expertise in the analysis.

An additional and independent endorsement to EIS came from the ESO VLT Key Program Working Group that stated in its final report: "The working group strongly endorses the OPC policy to reserve substantioal amounts of NTT-time for programs that are preparatory to VLT programs. In particular, the working group expresses very strong support for the important initiative of the ESO Imaging Survey on the NTT as a vital preparation for the first few years of the VLT".

In the letter to the OPC accompanying the EIS proposal the VLT Programme Scientist after some general considerations included the following additional point: "The ESO scientists appearing as PI and Co-I of the EIS proposal commit themselves to avoid any personal use of the survey data before they become public. If appropriate, the time limit of this commitment can be extended at the discretion of the OPC. A similar commitment will be asked to all those members of the community that may come to ESO to handle the data."

The EIS proposal was formally submitted to the OPC on 31.10/96 as requested, with the VLT Programme Scientist as PI, and L. da Costa, W. Freudling, S. D'Odorico, P. Quinn, J. Spyromilio, and J. Beletic as Co-Is. Since the proposal was not intended to provide data rights to the team, the team composition reflected exclusively the functional aspects of the Survey. While LdC had played a key role in designing EIS, all the others represented specific functional responsibilities which are essential for the completion of the Survey, (WF as manager of the Workpackage especially set up in support of EIS, SDO as Instrument Scientist of both EMMI and SUSI2, PQ as Head of the Data Management Division, JS as Head of the NTT Team, and JB as Head of the Optical Detector Team). In practice, a much larger number of people has been and will be directly or indirectly involved in the EIS project.

On 29.11.96 the OPC reviewed the EIS proposal and concluded (quote from the minutes of the OPC):

``This preparation was recognized of crucial importance for an early scientific impact of the VLT. The proposal had been discussed during the [OPC] Panel meetings and their chairs brought forward that it had been generally agreed that this project should be carried out. (Omissis) OPC considered this survey to be an ESO initiative for the benefit of the community to which everybody should be invited to participate, however, it should not exclude any other initiative in the same direction. It was agreed that a new proposal including the details to be presented to the OPC should be re-submitted in Spring for final approval."

As of January 1997, the drift-scan section of the survey has already been tentatively scheduled in the period 07-09.97. Funds have been allocated to the project and a visitors program has been established to bring those interested in working in EIS to spend from several months up to a year at the ESO headquarters. The survey has also received high-priority in the Optical Detector Division, responsible for the EMMI upgrade, and in the Data Management Division, responsible for the computer support and archiving of the data.