30 September 2005

As part of the Great Observatories Origins Deep Survey (GOODS), near-infrared imaging observations of the Chandra Deep Field South (CDF-S) were carried out in J, H, Ks bands, using the ISAAC instrument mounted at the Antu Unit Telescope of the VLT at ESO's Cerro Paranal Observatory, Chile.

These data were obtained as part of the ESO Large Programme 168.A-0485 (PI: C. Cesarsky). Data covering four ISAAC fields in J and Ks bands were also drawn from the ESO programmes 64.O-0643, 66.A-0572 and 68.A-0544 (PI: E.Giallongo), which were part of the previous data releases.

Since ISAAC observations are still ongoing, this release consists of J, H and Ks images of fields that have already been completed and are available in the ESO Science Archive Facility  up to the end of ESO observing period 73 (effectively March 2005).

This data release includes 24 fully reduced VLT/ISAAC fields in J and Ks bands, covering 159.1 and 159.7 arcmin² of the GOODS/CDFS region, respectively, and -- new in this release -- 19 fields in H band covering 126.7 arcmin², as well as three mosaics in the J, H and Ks bands. The data were reduced and prepared for release by the Advanced Data Products group in the Virtual Observatory Systems department.

• Overview and field layout
• Release content
• Release notes
• Data format
• Data retrieval
• Acknowledgements

Overview and field layout

The mosaic of ISAAC tiles included in this release is overlayed on the HST/ACS GOODS z-band mosaic: 21 tiles in the J, H and Ks bands (cyan, thick), 5 additional tiles in  the J and Ks bands (cyan, thin). Each ISAAC field is 2.5 arcmin across. Further observations in the red tiles are scheduled for 2005 (P75). Get full size image.

Mosaic image for each band (J, H, and Ks) and the respective weightmap with field layouts [click to enlarge]

Release content

Summary of reduced ISAAC fields

The images being released are tabulated to the right, consisting of 69 images in J (24), H (19) and Ks (26) bands, astrometrically and photometrically calibrated. The table gives: in column (1) the field number; in column (2) the passband; in column (3) the total integration time in seconds; in column (4) the seeing measurement in arcsec; in column (5) the RMS zero point error; in column (6) the number of frames that make up the final image; in column (7) the period of observations; in column (8) the 1-sigma sky background limit (AB) measured in circular apertures of 0.7 arcsec diameter. The same table is also available in ASCII format.
In addition, we release for convenience the full mosaic of all ISAAC tiles in J, H and Ks bands (see figures above) with an accurate astrometric solution and uniform photometric zero point.

Outlines of all ISAAC tiles can be overlayed onto any image with WCS information using the DS9 region files for J and Ks band and H band, respectively.

Comparison to previous releases

This release is the third of this survey. It revises and significantly extends the previous v1.0 release of April 30, 2004. The first release consisted of 8 fields in J, H and Ks (Fields 09, 10, 11, 14, 15, 16, 20, 21). The previous release consists of a new reduction of these same 8 fields, with the addition of 13 new fields in J-band and 15 new fields in Ks. The present release contains 3 new fields in J and Ks bands (Fields 06, 18, and 29) and 19 new fields in H band. Besides these 25 new images this release also includes the re-reduced data of all the previously released fields. With respect to the previous release (V1.0) the exposure times of the re-reduced images are slightly smaller in many cases due to a more stringent screening and rejection of poor quality reduction blocks.

Note the suffix 'n' (new) for the fields 25n and 26n is due to a re-arrangement of the ISAAC mosaic in the initial stage of survey to ensure maximum coverage of the GOODS/CDFS area within the time allocation. Reduced observations  in the original field positions (as part of programme ID 168.A-0485(E)) are also included in this release (~5 hours in Ks band only in fields named F25,26).

The main areas of improvements in this release are:

• ESO-MVM v1.9 was used which includes the following improvements over previous versions: more robust selection criteria for building reduction blocks, bug fix for the sigma-clipping procedure removing cosmic rays etc., selection of reduced reduction blocks based on homogeneous sky transparency and seeing. These improvements in the end led to a slightly better image quality.
  
• The method used for photometric calibration is similar to the previous release (see below), here however we also provide  an estimate for the zeropoint uncertainty which reflects the variation of photometric quality among images in the survey region.
  

The present release superseeds the previous ESO/GOODS data releases version 0.5 and version 1.0.

Anticipated future releases will focus on the remaining ISAAC/GOODS fields (01, 02, 12, 17, 28, 32) for the J and Ks band, the completion of the H band coverage, and the ZP evaluation from observations of photometric standard stars.

Release Notes

Data reduction method

The ISAAC data were reduced using an improved version of the ESO/MVM image processing pipeline (v1.9), which was originally developed within the EIS project. This software package is publicly available in an earlier version (this URL), we anticipate the release of the current version in Q4 2005.

Each field is the co-addition of a number of sky-subtracted frames (NCOMBINE in the image header) grouped in a number of Reduction Blocks. Images are registered using an accurate astrometric grid defined by an R-band ESO Wide Field Imager (WFI) image of the CDFS.

Astrometric Calibration

The astrometric calibration was derived using a reference catalog generated from a deep R-band WFI image which was astrometrically calibrated using the Guide Star Catalog GSC2.3.  The astrometry has been compared by the GOODS team with calibrated data from the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS). These comparisons yield rms scatter in astrometry of 0.1'' across the entire area.

All images have a pixel scale of 0.15", which is exactly a factor of five larger than the pixels in the GOODS/ACS images.

Photometric calibration

The photometric calibration of the present data is based on the image mosaics in J, Ks and H constructed from a mosaic of photometrically calibrated SOFI images of the EIS-DEEP and DPS infrared surveys conducted over the same region (Vandame et al. 2001). The SOFI images (5'x5') encompass a number of tiles of the ISAAC mosaic (2.5'x2.5' each), yielding more robust relative calibration across the entire surveyed area. A ZP for each ISAAC field was determined from the SOFI images using a sample of ~400 ``stars'' which have been identified in the HST/ACS images based on SExtractors stellarity index and flux ratio. All ISAAC images were first PSF-matched to the value of the corresponding SOFI image (~0.9 arcsec) employing Gaussian convolution, then aperture magnitudes of these stars were used to determine a ZP for each individual tile in the AB system. Between 3 and 12 sources per tile have been used.

This procedure yielded ZPs with RMS scatters ranging between 0.01 and 0.06 mag in J-band, 0.01 and 0.08 mag in Ks-band and up to 0.17 mag in H-band.  These numbers also account for systematic uncertainties inherent to the calibration procedure due to e.g. residual non-uniformities in the images (either ISAAC or SOFI).

For those tile where less than three isolated stars were available for accurate photometry all high signal-to-noise sources which appear pointlike in the ISAAC images based on a FWHM criterion were used for the calibration.

The resulting RMS zeropoint errors are given for each image of the three bands J, H, and Ks, respectively.

However, note that to provide a homogeneous photometric ZP across the entire GOODS field, we have rescaled all images to the same ZP of 26.0 in the AB system. Corresponding keywords in the fits headers have been set accordingly, PHOTZP=26.0, PHOTSYS=AB, and PHOTZPER. The exposure times are also normalized to unity (EXPTIME=1), so that AB magnitudes in all released fields, including the mosaics, can be obtained as:  mag(AB)=-2.5*Log(flux)+ZP. The following AB corrections have been used: J_AB=J_Vega+0.90, H_AB=H_Vega+1.38, and K_s,AB=K_s,Vega+1.86.

To determine systematic errors in this calibration procedure, independent/crossed photometric calibrations are needed on the SOFI and ISAAC data. The SOFI ZPs were found consistent (within a few percent) with  2MASS photometry of a limited number of stars in the field.

A comparison of the present release data with photometric catalogs from Saracco et al. 2001 (J and Ks band), Cimatti et al. 2002 (Ks band) and Moy et al. 2003 (H band) did not reveal any systematic difference taking into account the given error margins.

The evaluation of ZPs from observations of photometric standard stars (available in the ESO Archive) is planned in future releases. We encourage the user community to communicate to the GOODS/EIS team any relevant information on this topic.

Image Mosaics: in addition to the individual image tiles, this release also includes mosaics of the co-adjoined tiles as single fits files in J, H and Ks bands, as well as corresponding weight-maps. The WCS information and accuracy of the individual tiles is preserved in these mosaics. A uniform ZP of 26.0 can be used (e.g. with SExtractor) across the entire field, however, it is important to note that the PSF varies from tile to tile within each mosaic (see table below).  In the absence of proper aperture corrections or PSF matching procedures, this would impact efforts to create multi-color catalogs. The J, H and Ks mosaics have slightly different dimensions, however they can be readily registered using the intrinsic WCS information.

Image Mosaics

In addition to the individual image tiles, this release also includes mosaics of the co-adjoined tiles as single fits files in J, H and Ks bands, as well as corresponding weight-maps. The WCS information and accuracy of the individual tiles is preserved in these mosaics. A uniform ZP of 26.0 can be used (e.g. with SExtractor) across the entire field, however, it is important to note that the PSF varies from tile to tile within each mosaic (see table below).  In the absence of proper aperture corrections or PSF matching procedures, this would impact efforts to create multi-color catalogs. The J, H and Ks mosaics have slightly different dimensions, however they can be readily registered using the intrinsic WCS information.

Data format

The data files in this release come in pairs: one is the science frame, the second one is the corresponding weight map (*.weight.fits) defined as a variance map (e.g. it should be used with SExtractor using the parameters: -WEIGHT_IMAGE weight_map.fits -WEIGHT_TYPE MAP_WEIGHT). All relevant parameters are included in the FITS headers (see a sample here) which have been changed with respect to the previous release in order to be consistent with future VO compliant releases of ESO Advanced Data Products.

The following file naming convention has been adopted:

   GOODS_ISAAC_xx_b_v1.5.fits
   GOODS_ISAAC_xx_b_v1.5.weight.fits

for the field number <xx> in the band <b> and the respective weight map;

   GOODS_ISAAC_mosaic_b_v1.5.fits
   GOODS_ISAAC_mosaic_b_v1.5.weight.fits

for t`he mosaic image in the band <b> and the respective weight map.

Additionally, field names can be found in the OBJECT keywords in the FITS headers.

Data retrieval

Please make your selection on the page linked to below.

Acknowledgements

When using data products provided in this release, we request acknowledgement of the ESO/GOODS project and referring to the related publication Vandame et al. (2005), to be submitted by end of Oct 2005 and posted on this web site. Please also use the following statement in your articles when using these data "Observations have been carried out using the Very Large Telescope at the ESO Paranal Observatory under Program ID(s): LP168.A-0485".