GOODS/CDF-S CTIO 4m U-band Imaging: Version 0.5 data release
On UT 2001 September 14-18, we used the
MOSAIC II imager
on the CTIO 4m Blanco telescope to obtain deep U-band observations
of the CDF-S/GOODS field. The observers were Daniel Stern
(JPL/Caltech), Claudia Winge (Gemini), and Mike Ledlow (Gemini).
Here we decribe the observations and the data processing,
and make available the initial, version 0.5 ("best effort") release
of the reduced image. An improved re-reduction (version 1) will
be available here in summer 2002.
The first four nights of the observing run were photometric,
while the final night was essentially lost to thick cirrus.
Seeing at u' was variable and relatively poor. The observations
were all taken using 20 min dithered exposures. A log of the
Table 1. Log of Observations
||Total exposure time
||Mean seeing FWHM
| 2001 Sept 14
|| 5 hr.
| 2001 Sept 15
|| 4 hr. 40 min.
| 2001 Sept 16
|| 3 hr. 40 min.
| 2001 Sept 17
|| 4 hr. 20 min.
| 2001 Sept 18
|| 1 hr.
|| thick cirrus
The observations were taken through CTIO filter c6021,
the 5".75x5".75 SDSS u' filter. This filter was chosen
in order to minimize bandpass overlap with the standard
B-band and the ACS F435W filter, in hope of providing
better photometric redshift and Lyman break constraints.
The CTIO SDSS u' filter was later shipped to the Tucson
NOAO offices for a subsequent observing run at KPNO (March 2002).
While in Tucson, the filter transmission function was scanned
in the Tucson facilities. There, it was found to have a red
leak, as well as lower-than-expected peak throughput.
This red leak may affect the photometry for objects in this
data set, especially those with red colors.
We will present further analysis of this with the
version 1.0 data release.
Data reduction, version 0.5 release
A preliminary, "best effort" data reduction (V0.5) has been done
by Daniel Stern, using the MSCRED reduction package in IRAF and
MOSAIC reduction cookbook scripted by Buell Jannuzi. The final
image represents 17 hr. 20 min. of integration time, with a plate scale
of 0.268 arcsec per pixel, and approximate seeing FWHM ~ 1.6".
For this v0.5 release, we have not yet derived photometric zeropoints
using the standard star observations taken at the telescope.
For a preliminary photomeric calibration, we have
matched a catalog of objects from our CTIO u' image to the
ESO Imaging Survey (EIS)
2.2m/WFI U' image of the same field
Arnouts et al. 2001),
using photometry apertures with diameter equal to three times the
This exercise leads to an AB magnitude zeropoint:
AB(u',CTIO) = 30.95 - 2.5 log10 (N_ADU)
where N_ADU is the number of counts in an object in the
reduced image. The reduced image reaches an approximate
depth of AB=25.8 mag (S/N=5 in a 3".2 diameter aperture).
Astrometry is calibrated to the USNO-A2 catalog automatically as
part of the MSCRED pipeline - a necessary step before co-adding
the data because of distortions over such a large field of view.
The astrometric accuracy has not yet been carefully validated,
but should be ~0".5 or better. It is encoded in the image
header using IRAF WCS conventions.
The release image has been oriented so that north is up and east is left.
Limitations of the v0.5 "best effort" data reduction
In order to achieve rapid turnaround of this data set for public
release, many shortcuts were taken for the v0.5 data reduction.
- No screening/vetting of bad-seeing data was applied.
- No checks were done on the accuracy of the default chip-to-chip
gains. Inaccuracies would compromise the photometry.
- Problems with the stacking led to striping in the final mosaic,
associated with the gaps between CCDs. These were removed in an
ad-hoc manner by fitting and subtracting each row and column of the
image with a low-order polynomial.
- Photometry was done by comparison with EIS data rather than using
the copious photometric standard calibrations obtained during the
- For the final stack, data were simply summed with no
weighting by (inverse) airmass or PSF quality.
Anticipated improvements for future (v1.0) data release
For the next release (v1.0) of the data, we are making the
following improvements in the data reduction process:
- More careful attention is being applied to the extensive
calibration data obtained. In particular, a bright spectrophotometric
standard star was observed in all 16 of the amplifier read-out areas
of the CCDs (2 amplifiers per CCD * 8 CCDs) during a photometric
night. We will use this to assess the accuracy of the relative gains.
- We are checking how successful the crosstalk corrections are, possibly
opting to mask out crosstalk contaminated areas rather than attempting
to correct them.
- We will screen out the bad-seeing data before creating the final
mosaic. This should result in a significant improvement to the
image quality of the final stack, and probably to the magnitude limit
for most faint sources.
- Stacking will be done with proper weighting of the data to
account for the relative transparency between frames.
- Proper stacking should also remove the striping associated with
the CCD gaps.
The version 0.5 release of the GOODS CTIO 4m u' image of the CDF-S
is available as a FITS image.
This image is quite large, 9393 x 9502 pixels in extent,
with a file size of 357 Mbytes. It should not be downloaded
We also provide a 2x2 rebinned version of the image which
is a bit more manageable. Given the PSF FWHM, this should
be adequate for most purposes.
ESO Imaging Survey (EIS) data
Data for the CDF-S region from the
ESO Imaging Survey (EIS)
as part of their
Deep Public Survey (Deep-2c) can be obtained worldwide from
the ESO Archive. These include EIS U and U' images taken at the
MPG/ESO 2.2m telescope at La Silla with the Wide Field Imager (WFI).
These data are described in
Arnouts et al. 2001. The EIS CDF-S U-band images
have comparable sensitivity to the CTIO data, but with better
Note that the U' filter used for the 2.2m/WFI observations
is relatively similar to the SDSS u' filter
used for the CTIO observations (except, hopefully, for the
red leak). By comparison, the U filter used for the 2.2m/WFI
obserations is a narrower, redder bandpass, with effective
wavelength 3710A and FWHM 325A.
Information about the WFI filters can be found at the La Silla
observatory web pages.
The ESO CDF-S observations from GOODS and EIS
can be obtained from the
ESO/GOODS or the
EIS/GOODS web pages.
Copyright © 2002 The Association of Universities for
Research in Astronomy, Inc. All Rights Reserved.