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Flanking Fields Data Reduction / Technical Issues

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Information is available about the following topics:

STIS 50CCD imaging

Description of data products

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   

-----  -------        -------- 
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

Data Reduction/Technical Issues

The flanking fields were processed in very closely the same manner as described in the STIS Imaging section. These notes point out the exceptions.

WFPC2 imaging

Description of data products

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.

Combined Images

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

Correlated Noise

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

This page is under construction

Check here for information on the status of the NICMOS Flanking Field observations.

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This page was last updated on December 23, 1998.
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