Cosmic evolution can be defined as the variation with z of a given property once the physical dispersion of that property at every z-value, i. e., the cosmic variance, has been taken into account. Given the inhomogeneity of the observed Universe up to scales of several tens of Mpc, the study of Cosmic Evolution implies the analysis of large numbers of similar objects and the use of large enough volumes to properly average and smooth out the local inhomogeneities. Taking into account the present knowledge about the distribution of galaxies and structures, we have considered that a field corresponding to several times 50 Mpc should be covered.
To get the optimum compromise between large area and depth, good spectral resolution and coverage, we have defined the Advanced Large, Homogeneous Area Medium Band Redshift Astronomical, ALHAMBRA-Survey. It is a photometric survey primarily intended for cosmic evolution studies. We propose to cover a large-area (4 square degrees) with 20 contiguous, equal width, medium band optical filters from 3500 A to 9700 A, plus the three standard broad bands, JHK, in the NIR. Thus, it is placed halfway in between the traditional imaging and spectroscopic surveys. It will make possible the study of many different astronomical problems in a self-contained way.
ALHAMBRA-survey is a photometric survey. The survey covers a total area of 4 square degress in the sky distributed in 8 different fields (with half a degree per field) with 20 contiguous, equal width, medium band photometric filters from 3500 A to 9700 A, plus the standard broad bands JHKs in the Near Infrared.
The basic, material goal of the ALHAMBRA-Survey is the building of a CATALOGUE containing the multifilter photometry of all the detected objects, and the deduced SED and redshift values. The ALHAMBRA-Survey is designed to be sensitive, within a well defined limit magnitude, to any kind of object irrespectively of its nature and redshift. This initially unbiased nature of the Survey makes it potentially valuable for many different fields in Astrophysics, from galactic astronomy to the world of Galaxies and Cosmology.
The main scientific goal of the ALHAMBRA-Survey to provide the community with a set of data appropriate for the systematic study of Cosmic Evolution. The hypothesis of homogeneity and isotropy implies the existence of maximally symmetric subspaces and the existence of a 1-to-1 relation between redshift and time. This is a model independent prediction, prior to any consideration about the value of the cosmological parameters. Precisely, the ALHAMBRA project intends to constitute a foliation of the space-time, producing narrow slices in the z-direction whereas the spatial sections are large enough to be cosmologically representative.
Although the primary goal of ALHAMBRA is the study of the cosmic evolution, the survey covers several astrophysical subjects, including clusters of galaxies, galaxy morphology and environment, stellar population studies, Milky Way structure, and stellar studies. All these studies can be archived since ALHAMBRA provides both images and a low-resolution espectra energy distribution of different objects with a broad wavelenght coverage (ALHAMBRA-spectra). ALHAMBRA will detect more than 6 x 105 object that will be analized in a self-consistent way.
We use LAICA, the new Large Area Imager for Calar Alto, to obtain deep imaging in all 20 optical contiguous filters covering from 3500 Å to 9700 Å. LAICA is installed at the 3.5m telescope Prime Focus using the 3-lens corrector K3. It has 4 CCDs each with 4096x4096 pixels. The image scale is 0.225 arcsec per pixel. The read out of the CCDs is low-noise (about 8 electrons) and fast (less than 1 minute). The filter module is similar to a slide projector. Each set of 6 filters (4 for the science CCDs plus 2 for the guider CCDs) is mounted in a filter holder; 20 filter holders can be stored in the magazine. The selected filter is moved from the magazine into the beam by a grabber.
Two exposures are needed in order to get two strips of 1 o×0.25o each, for a total of 0.5 square degrees. A detailed knowledge of the image distortion and flatfielding is required. The image degradation in the corners of the field should be minimal due to the high optical quality of the corrector optics. Detailed information on LAICA and the observing procedures can be found at
The project was designed having in mind all the subtleties of the techniques to get photometric redshifts, to be able to use them in the most advantageous way. Our goal was to determine the number and width of the of filters to get, for a fixed total amount of observing time, accurate SED and z determination for the largest possible number of objects, and to be sensitive to relatively faint emission lines. Given the performance of the instruments to be used, the total exposure time in the optical domain was fixed to 100 ksec. Since we will use the standard filters in the NIR, we concentrate in the following on the characterization of the optical filter set. The ALHAMBRA optical photometric system was eventually designed to include 20 contiguous, medium-band, FWHM = 310 Å, square-like shaped filters with minimal overlapping in lambda, covering the complete optical range from 3500 to 9700 Å. With this configuration it is possible not only to accurately determine the SED and z even for faint objects, but also to detect rather faint emission lines.
We have calculated for the selected filters the system efficiency (filter+CCD) taking into account the quantum efficiency for LAICA's CCDs:
