IRAC Ultra Deep Field (IUDF)

Data release of the science images, exposure time maps, and PSF maps

Data reduction v0.9
Data release version 1 (April 2015)
Readme version: 17 April 2015
Survey Paper: Labbe et al. 2015, ApJS

1. Summary:

The IRAC Ultradeep Field (IUDF) combines all ultradeep data ever taken with the
Spitzer Infrared Array Camera (IRAC) at 3.6μm and 4.5μm over GOODS-South and
the HUDF (+ two HUDF parallel fields). The deepest observations come from the IRAC
Ultra Deep Field (IUDF, ID 70145, PI Labbé) and IRAC Legacy over GOODS (IGOODS,
PID 10076, PI Oesch) programs, combined with a number of archival IRAC legacy
programs. The data release includes reduced image maps, PSF maps, and reduced
maps of all individual AORs.


2. General information:

Instrument:     Spitzer IRAC
Filters:        Channel 1 (3.6μm) and 2 (4.5μm).
Field location: 03:32:29.460 -27:48:18.32
Area:           23.1 x 28.4 arcmin
Contributing Programs:
PID             Title       PI
70145           IUDF        I. Labbé
10076           IGOODS      P. Oesch
194             GOODS       M. Dickinson
70204           ERS         G. Fazio
80217           S-CANDELS   G. Fazio
60022           SEDS        G. Fazio
30866           UDF2        R. Bouwens


3. Data reduction

The data reduction is described in Labbe et al. (2015) and follows the
procedures used in Damen et al. 2011 (SIMPLE survey).

Briefly, the starting point for the reduction are the BCD frames produced
by SSC pipeline. Subsequently, the data are post-processed by subtracting
a median sky, correcting for column pull down and muxbleed and muxstriping,
masking pixels affected by persistence caused by bright sources, and
rejecting cosmic rays.

The astrometry is calibrated by minimizing the position residuals of
sources in common with the deep WFC3 catalogs of 3D-HST (Skelton et al.
2014). The rms residuals are < 0.1 arcsec.

The post-processed frames of each AOR of all programs are drizzled
(Fruchter & Hook 2002) onto a reference grid and averaged. The reference grid
is a tangent plane defined by the CANDELS (Koekemoer, A. M. et al., 2011, ApJS,
197, 36), using the same tangent point (orientation: north up, east left), with
a pixel scale of 0.3 arcsec/pixel. In addition to a full stack of all data,
the release also includes separate stacks for each AOR (on the same grid). These
are useful to study the reliability or variability of sources.

The image quality is a constant FWHM=1.49" over the field.

The units of the final science images are counts/sec, where a count rate of
1 DN/sec corresponds to a flux density 34.04 microJy or 20.07 AB magnitudes.

The units of the exposure time maps are seconds. The exposure time maps are created
by multiplying, at each position, the number of bcd-frames that contributed
to each pixel by the integration time of the frames. The exposure map thus reflects
the exposure time in seconds on that position of the sky, not the average exposure
time per final output pixel.

To facilitate prior-based WFC3+IRAC photometry, this release contains high
signal-to-noise PSFs, created from the data, and the information needed to
reconstruct the spatial variation of the PSF due to survey geometry. First, a
constant "super PSF" is construct from bright stars in all the AORs. This serves
as a very high SNR template created from the data on the same pixel scale as the
science image. Second, a map is created which records the weight and rotation
angle of all AORs that contributed to the final image. The effective PSF
at each position of the final image can then be reconstructed by rotating and
weighting the super PSF according to that map.


4. Data products

The IUDF data release consists of three main data products: 1) the
combined science images and exposure time maps, 2) spatially
varying PSF maps, 3) reduced images of all individual AORs.

IUDF_CH1_v0.9_sci.fits 	- science image at 3.6μm [AB = 20.07]
IUDF_CH1_v0.9_exp.fits  - exposure time map at 3.6μm [sec]
IUDF_CH2_v0.9_sci.fits 	- science image at 4.5μm [AB = 20.07]
IUDF_CH2_v0.9_exp.fits  - exposure time map at 4.5μm [sec]

The PSF maps are supplied in a multi-extension fits file. The first extension is
the "super PSF". The second extension contains a 4-dimensional array, containing
the weights and rotation angle of all AORs recorded on a a rectangular grid in
RA and Dec.

IUDF_CH1_v0.9_psf.fits - PSF map at 3.6μm
IUDF_CH2_v0.9_psf.fits - PSF map at 4.5μm

Reduced images of all individual 272 AORs, are registered and drizzled onto the
same grid as the full stack science image. The naming convention is:

r[XXX]_CH[Y]_v0.9_[ZZZ].fits.gz

where XXX is the aor key, Y is the channel number (1 for 3.6μm and 2 for 4.5μm), and
ZZZ is sci (for science image) or exp (for exposure time map).

A download script is provided which uses the command line tool curl to download the
entire dataset and which can be edited to download only the files needed.