J-PAS will cover at least 8000 deg2 in approximately 5 years, using an unprecedented system of 56 narrow band filters in the optical. The filter system was optimized to pursue three main scientific goals: first, to accurately measure photometric redshifts for galaxies up to z~1; second, to study stellar populations in nearby galaxies; and third, to resolve broad spectral features of objects such as AGNs and supernovae. The expected throughputs of the filters (including the CCD quantum efficiency, aluminum reflections and telluric lines) are shown in the figure below.

 

The main J-PAS instrument is a 2.5 m telescope with an effective field of view of 7 square degrees. That instrument has an étendue of about 26 m2 deg2, which is on a par with other state-of-the art instruments dedicated to wide-area astrophysical surveys.

We expect to measure positions and redshifts for more than 14 million red, early-type galaxies (LRGs) with L>L* and an apparent magnitude equivalent to iAB < 22.5. In the redshift interval 0.1 < z < 1.2, we expect a photo-z precision of σz ∼ 0.003(1+z).

The population of LRGs has a number density n > 10−3 Mpc-3  h3 galaxies within the ~10 Gpc3h−3 volume to be sampled by our survey. This high density ensures that the error in the determination of the BAO scale is not limited by shot noise. By itself, the J-PAS LRG survey will deliver precisions of order <4% in the dark-energy equation of state parameter w (if assumed constant), and can determine its time derivative when combined with future cosmic microwave background measurements. In addition, J-PAS will yield high-quality redshift and low-resolution spectroscopy for hundreds of millions of other galaxies, including a very significant high-redshift population. The data set produced by this survey will have a unique legacy value, allowing a wide range of astrophysical studies.
 

Survey Instruments

T250 Telescope

The main parameter to give the capability of a telescope for large area surveys is the étendue, defined as the product A­*f, where A is the aperture in m2 and f ­ is the field of view (FoV) in square degrees. Note that the final étendue value for larger telescopes is of the same order or smaller than that of some existing medium-size telescopes. Indeed, the combination of a large aperture and a wide FoV is a technological challenge. It is therefore necessary to look for new designs which optimize that parameter.

Taking into account the requirements of a survey like J-PAS, as well as technical aspects such as the vignetting of the primary by other elements or the filling factor of the focal plane, we have set the following first level requirements:

  • Effective aperture of 2.5m
  • FoV of 3 degrees, diameter
  • Scale of 22.67”/mm
  • Homogeneous throughput and optical quality along the field.
  • Robotic


Figures by AMOS       

T250 Camera

The camera and telescope are being designed in parallel. The camera contains: the filter trays, an optically neutral entrance window, the cryostat, cryo-cooling systems, the detector array, and the electronics and control system.

For the CCDs, we have converged on e2v's large-format chips, which are 9k × 9k pixels, 9μm/pixel.

The mosaic will be an arrangement of 14 units. The filters (one for each CCD) will be distributed in four different filter holder trays containing, 14 filters each. The filters will be placed as close as possible to the cryostat entrance window.

Considering a pixel to pixel separation between CCDs of c=15 mm we obtain a focal panel coverage efficiency of 71% using 14 CCDs.
 
 

T80 Telescope

This is a smaller telescope, whose main intended use is the photometric calibration of the main surveys with the T250. The T80 will also work as an open instrument.

The first level requirements are:

  • Effective aperture of 80cm:
  • FoV of 1.7 degrees, diameter, at least
  • Scale of 55.67”/mm
  • Homogeneous throughput and optical quality along the field
  • Robotic

 

T80 Camera

The pixel scale for this camera will be 0.501′′ /pixel. The large-format CCD then allows to cover up FoV(diameter) = 2.07 deg. The minimum required FoV is diameter = 1.7 deg. The Camera consists of a filter system of 12 filters, including the filter exchange mechanism (filter wheel, juke-box, etc.), the entrance window, cryostat, one 9k x 9k detector, and the corresponding electronics and control system. The T80 Camera communicates with the Telescope Control System and should be a benchmark for the T250 camera.

 

The Site: OAJ

OAJ - The Javalambre Astrophysical Observatory

Observatorio Astrofísico de Javalambre (OAJ) is located at El Pico del Buitre (Vulture's Peak) (40° 02' 28.67'' North, 01° 00' 59.10'' West), 1957 meters above the sea level, close to the village Arcos de las Salinas (Teruel). In the picture below, the site of the observatory is shown on the right corner.

The properties of the atmosphere and sky at the site of the OAJ are described in Moles et al. (2010), PASP 122: 363.

The site has an excellent seeing, very low artificial light contamination and is typically above the inversion layer. The picture below was taken from Pico del Buitre, and shows the cloud cover below the mountaintop (images from CEFCA).

Click here to visualize the geographical location of the site on Google Maps.

 

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