VLT/ISAAC Ks-band imaging of the Hubble Ultra Deep Field (v1.0)

02 March 2010

Using the Infrared Spectrometer and Array Camera (ISAAC) mounted at the Antu Unit Telescope of the VLT at ESO's Cerro Paranal Observatory, Chile, deep near-infrared imaging observations of the Hubble Ultra Deep Field (HUDF) located within the Chandra Deep Field South region were carried out in Ks band. Based on public ESO archival data a single co-added image was created covering 5.40 arcmin2 of the HUDF region with a total integration time of 25.3 hours, an effective PSF of 0.36 arcsec FWHM, and a 5-sigma median depth for point sources of 25.60 mag (AB) which is 0.67 mag deeper than the respective image resulting from the VLT/ISAAC NIR imaging survey that was carried out to support the GOODS project. To this end, data of the 21-hours integration obtained under ESO program 73.A-0764, led by I. Labbe, were combined with the ca. 6-hours integration of this field obtained under program 168.A-0485, led by C. Cesarsky (ESO/GOODS). The data set presented here was described in Retzlaff et al. (2010) and utilized to characterize the completeness and contamination of the shallower GOODS survey tile F14 and to establish number counts of faint galaxies in Ks band.


Outline of the deep HUDF/ISAAC Ks image (yellow diamond) with respect to the ESO/GOODS tiling of the CDF-S region with 2.5 x 2.5 arcmin ISAAC fields (red and cyan) displayed over the HST/ACS GOODS z-band mosaic of Giavalisco et al. (2004). The pointing is centered on F14 whereas the instrument was rotated by ca. 45 degrees to optimally match the Hubble UDF observation

Release content


This data release consists of a single, astrometrically and photometrically calibrated image whose properties are tabulated below. (1) the field identifier; (2) the passband; (3) the total integration time in seconds; (4) the number of frames that make up the final image; (5) and (6) the period of observations; (7) the seeing measured as the average FWHM of stellar sources in arcsec; (8) the median depth defined as the total AB magnitude of a point source corresponding to the 5-sigma sky fluctuation of the flux in circular apertures for the deepest 50% of the total image area; (9) the total image area having a minimum depth of Ks=24.90; within an area of 5.40 arcmin2, which is 80% of the total area, a minimum depth of Ks=25.35 is reached.

  Attribute Value
(1) Field HUDF
(2) Band Ks
(3) Total integration time 91182.0 s
(4) No. of combined images 965
(5) Start of observations 2001-09-30T08:55:26.3
(6) End of observations 2004-10-03T06:48:23.9
(7) FWHM seeing 0.36 arcsec
(8) Median depth 25.60 (AB, 5 sigma)
(9) Total area 6.70 arcmin2
Preview of the HUDF/ISAAC Ks image using linear intensity scaling.
Click on the image to view in full-size.
Properties of the HUDF VLT/ISAAC Ks imag

Release Notes

Data processing

The ESO/MVM software, version 1.3.5, which was developed until Dec. 2006 as an extension of the original development within the ESO Imaging Survey project, has been used for image data reduction. ESO/MVM follows well established procedures for the reduction of NIR imaging observations taken in jitter mode by featuring a two-pass scheme to mask out sources prior to the final background estimation. The ESO/MVM software package is publicly available from this URL.

First, the raw calibration data, namely dark frames and twilight sky flats, were combined into master calibration files and used to correct the raw scientific images for the basic instrumental signatures. No attempt was made to correct for detector nonlinearity. However, the overall effect is expected to be rather small. The magnitude of possible nonlinearity effects on the final results is discussed in Retzlaff et al. 2010, Sect. 3.5.3.

Sky background images were computed from groups of 13 consecutive jitter images (Ks band) using sigma-clipped pixel-by-pixel image combination. Each science image was sky-subtracted using the linear interpolation in time of the two corresponding successive sky background images. Possible transparency variations from image to image were monitored by means of the signal-to-noise ratio (SNR) based on which outliers with exceptionally low SNR were automatically discarded. An individual rescaling of the images has not been done.

Then, for each OB, the individual background-subtracted images were astrometrically registered to each other with sub-pixel accuracy and co-added (see below) to form a preliminary version of the OB images. The purpose of these first-pass images is the creation of masks that mask the astronomical sources in order to improve the sky background computation in the second pass. To this end, all sources exceeding ~3 times the local RMS noise were detected on the PSF-convolved image and -- to also exclude the wings of the source profile -- each source's region was artificially enlarged by a factor of 2 (linearly) before creating its mask. Thus, in the second pass, the sky background images were re-computed exclusively from image pixels that belong to background regions whereas the rest of the procedure was repeated unaltered. Furthermore, in order to improve the background subtraction for very faint sources, we applied a third pass in the OB processing using a source mask generated from the co-added OBs that resulted from the second pass.

Astrometric Calibration

The astrometric calibration is based on a dense reference catalog which was generated by the GOODS team from a deep R-band image of the CDF-S and was also used for the production of the GOODS/ACS image mosaics (Giavalisco et al. 2004, ApJ, 600, L93). The image was obtained with the Wide Field Imager mounted at the 2.2-m MPG/ESO telescope at La Silla, and astrometrically calibrated using the Guide Star Catalog (GSC2). Each OB image was astrometrically registered using the reference catalog whereas image distortions were modeled with a 3rd order polynomial. The resulting internal astrometric accuracy between 0.05 and 0.06 arcsec RMS as inferred from the GOODS/ISAAC survey is representative for the HUDF Ks image.

For the astrometric grid of the OB images the same projection as for the GOODS/ACS images has been adopted and a pixel size of 0.15 arcsec has been chosen so that one ISAAC/HUDF pixel subtends exactly a block of 5 x 5 GOODS/ACS pixels. The individual jitter images were resampled to this grid using the Lanczos-3 interpolation kernel. During the process of image co-addition, bad-pixel masks were taken into account and each contribution was recorded pixel-by-pixel to build up the respective weight map.

Photometric calibration

The photometric zero points (ZP) for the HUDF Ks data were bootstrapped from the calibrated F14 Ks image of the GOODS VLT/ISAAC imaging survey, implying similar photometric uncertainties (cf. Retzlaff et al. 2010, Sects. 3.5.3 and 4.3). An estimate of the overall error is given by linearly adding up the individual contributions, that is the internal accuracy of the photometric solution (≤0.017 mag), the residual flat field error (0.025 mag, 1 sigma), and the possible nonlinearity bias (about <0.01 mag) for objects not brighter than 17 mag (AB), resulting in the total photometric error of up to 0.05 mag (1 sigma). The imaging data was re-scaled so that the final image has a ZP of exactly 26.0 mag (AB), which in fact is quite close to the typical instrumental ZP. Note that a correction of Ks,AB=Ks,Vega+1.895 was used to establish the photometric calibration in the AB system.

Corresponding keywords in the fits headers were set accordingly, PHOTZP=26.0, PHOTSYS="AB" and PHOTZPER=0.05The exposure time is normalized to unity EXPTIME=1 so that AB magnitudes can be obtained as


Data format

This data release contains two files, the science image and the associated weight map, each one as a separate FITS file. The weight map is an inverse variance map (e.g. it should be used with SExtractor using the parameters: WEIGHT_IMAGE weight_map.fits -WEIGHT_TYPE MAP_WEIGHTAll relevant parameters are included in the FITS headers in compliance with ESO Advanced Data Products; the OBJECT keyword in the FITS header was set to "HUDF".

The file names for the science image and the associated weight map are ADP.ISAAC_HUDF_KS_V1.0.fits and ADP.ISAAC_HUDF_KS_V1.0.WEIGHT.fits respectively.

Data retrieval

Please click on the package name below to request the data set from the ESO science archive.

Package Description Size
VLT/ISAAC HUDF Ks imaging 2 FITS files 17.2 MB


When using data products provided in this release, we request reference to the publication Retzlaff et al. (2010). In addition, please use the following statement in your articles when using these data:

Based on observations made with ESO Telescopes at the La Silla or Paranal Observatories under programmes 73.A-0764 and 168.A-0485.