Flanking Fields Data Reduction / Technical Issues
Check the page with warnings and
advisories before making any use of the data.
Information is available about the following topics:
STIS 50CCD imaging
Follow this link for a legend of
the file names for the final reprocessed data.
The analysis and reduction of the STIS flanking fields proceeded in
similar fashion as for the imaging of the Main STIS Field. Please read
first the description of those
observations, with particular emphasis on the description of the
Flanking Field
observations.
For Each of nine fields, STIS observation were made in parallel mode
operation to the primary instrument WFPC2. After Guide Star
Acquisition a "CR-SPLIT" 1200s exposure was followed by a 1300s
"CR-SPLIT" exposure at a new dither (shifted) position, but at same
roll angle. A second 'orbit' of observations immediately followed
with two more dithered "CR-SPLIT" exposures of 1300s apiece.
Mode Cent Wave FWHM Det. FOV Tot FOV Tot Exp
---- --------- ----- -------- ------- -------
50CCD 5850A 4410A 52"x52" 60" 5100s
FIELD RA_TARG DEC_TARG
----- ------- --------
POS1 338.448683577 -60.5389133361
POS2 338.363228572 -60.5755798231
POS3 338.482371926 -60.5811353514
POS4 338.524469698 -60.5619687787
POS5 338.438601453 -60.5991908183
POS6 338.394829563 -60.6169685088
POS7 338.56782685 -60.5444688644
POS7 338.600629493 -60.5772464816
POS9 338.390813099 -60.6439128206
The flanking fields were processed in very closely the same manner as
described in the STIS Imaging section. These notes point out the
exceptions.
- 1. The DARK calibration reference file used in the CALSTIS
data reduction steps was constructed using a 'delta-dark'
image appropriate for the epoch of the flanking field
observations.
- 2. No cosmic ray rejection masks were used except those constructed
within the XDITHERII DRIZ_CR processing.
- 3. There were no differences in the commanded rolls to the spacecraft
between the dithered exposures. No significant roll offset was
found in the processing. The only rotation applied in the
combining process was the +5.4 degree rotation to North.
- 4. DRIZZLE drop-size used for the flanking fields was a "pixfrac" of
0.8.
WFPC2 imaging
The final data products for all of the WFPC2 Flanking Field
observations are now available. The current version 1.0 (11/25/98 and
12/21/98) represents our best shot at a first combination of the WFPC2
data taken in the Hubble Deep Field South Flanking Fields.
Follow this link for a legend of
the file names for the final reprocessed data.
For positions 1-9, images were taken in the F814W filter. Longer (9
orbit) flanking field observations were also taken when STIS was
observing the NICMOS main field. These observations consist of both
F606W and F814W images. Note that both data and weight images are
presented in a single mosaic, with all four WFPC2 detectors combined
onto the same image plane. The weight images represent the inverse of
the variance predicted from a noise model at the mean sky level of the
observations, taking into account masking of cosmic rays and hot
pixels.
The analysis and reduction of the WFPC2 flanking fields proceeded in
similar fashion as for the imaging of the Main WFPC2 Field. For
information on the data reduction, please read first the technical information for the Main WFPC2
Field.
The combined images are the result of combining all individual
exposures, optimally weighted for the background signal, and resampled
to a pixel scale of 0.0498" using the "Drizzle" package developed by
Fruchter and Hook and available in IRAF/STSDAS. The images are
expressed in counts per second at a gain of 7. They have been rotated
to have North approximately up (about 0.5 degrees from vertical). The
coordinates of the images are set in the header WCS parameters, and
can be retrieved for example via the IRAF task xy2rd. The absolute
astrometry of the fields have not yet been converted to a standard
astrometric system more precise than the HST Guide Star Catalog, and
hence are uncertain by 1-2 arcsec. Work is underway to tie the fields
to astrometrically measured stars kindly provided by the Naval
Observatory. This work should be complete by the end of 1998.
The depth and coverage for each filter varies across the field of
view, due to the variety of pointings that were combined together.
The image depth wanes when approaching the edges of the area covered,
as well as in a near-vertical seam between detectors which received
much lower coverage than the central region of each chip. This
variable depth is reflected in the image weights, described below.
As a consequence of the decreased image quality, the outer regions of
each image are less reliable, especially in terms of cosmic ray
rejection. Any oddly-shaped objects appearing in only one filter near
the edge of the images presented here has a good probability of being
a piece of an unrejected cosmic ray.
Images have been trimmed (set to zero) where the weight per pixel was
zero. However there are a number of low-weight pixels around the edge
that still have residual cosmic rays.
WFPC2 detectors differ slightly in their sensitivity. The individual
input images have been scaled to the response of the WF3 detector,
thus the zero points determined for WF3 (at gain 7) apply. For
reference, the most recent determination of the zero point for
infinite aperture in the VEGAMAG system is:
F814W 21.66
Those planning to run detection and photometry software on these data
should be aware that the noise is strongly correlated between adjacent
pixels in the final drizzled images. Consult the main HDF-S web page
for more details.
NICMOS imaging
Check here for information
on the status of the NICMOS Flanking Field observations.
Send us your feedback or questions.
Visit the Hubble Deep Field South main
page.
Visit the Space Telescope Science Institute home page.
This page was last updated on December 23, 1998.
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