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2.4. Calibration and Processing of the HST Observations

2.4.1. Standard Processing

Standard WFPC2 CCD processing following Windhorst et al. (1994b, 1994c, 1998a), Driver et al. (1995a), Neuschaefer & Windhorst (1995), and Odewahn et al. (1996) included bias and dark-subtraction, and flat-fielding. Photometric calibration was done using the STSDAS On-The-Fly-Reduction (OTFR) routines as available since summer 2000. Custom calibration, in general, does not significantly improve upon the STScI WFPC2 pipeline, owing to the significant work that went into building and improving that pipeline. The OTFR takes into account the latest improvements in knowledge of the instrument every time one retrieves data from the HST Archive.

Because the mid-UV images have extremely low sky-background levels, the background subtraction is limited by the quality of the bias and dark current removal. It is therefore important that the very best possible biases and latest dark-current and hot-pixel maps are used. We paid close attention to whether the correct dark-frames were used when observations were taken near the monthly warm-up of WFPC2 (to decontaminate the optics and anneal many of the new hot-pixels). The OTFR uses the best available super-dark taken after the relevant science images, but before the next decontamination. Hence, it is conservative in nature - repairing more pixels than needed, but never too few. We re-ran the OTFR on all data one month after the last images for this project were taken (in April 2001) to incorporate the latest knowledge on the WFPC2 data. The difference between this second run and the first was very small, but went in the direction of removing a more appropriate (smaller) number of pixels deemed "hot".

We co-added all images in the same filter after registration using integer pixel shifts. Our in-house IDL routine STCombine (Pascarelle et al. 1998; Cohen et al. 2002) was used to optimally remove the signal induced by the many Cosmic-Ray (CR) hits. This routine, optimized for the low signal domain, applies a one-sided 2-sigma rejection in creating the final stacked image (following Windhorst et al. 1994a).

2.4.2. Achieved S/N in the WFPC2 Images

The Zodiacal sky-background at the North Ecliptic Pole is ~ 24.0 mag arcsec - 2 in F300W (Windhorst et al. 1994b, 1998a) and ~ 24.7 mag arcsec - 2 in F255W (Cornett et al. 1994), and is as low in the sunlit part of the orbit as it is in the occulted part (unless the angle to the Earth's limb becomes small). Since our WFPC2 mid-UV images are read-noise limited, the resulting 1-orbit 1-sigma SB-sensitivity is 25.1 ± 0.15 mag arcsec - 2 in F300W and 23.0 ± 0.15 mag arcsec - 2 in F255W on a per pixel basis. The relation between the detected SB-level and the S/N in a pixel is given by: S/N = 10 - 0.4 (µF300W - 25.1) and S/N = 10 - 0.4 (µF255W - 23.0) for F300W and F255W, respectively. These mid-UV SB-limits are consistent with the values expected from the Cycle 5-6 images in F410M and F450W (Pascarelle et al. 1996, Odewahn et al. 1996, Windhorst et al. 1998a) and the relatively red color of the zodiacal sky-background. Taking into the account the (1 + z)4 SB-dimming and the typical (U - I) color of galaxies at z appeq 1-2, the SB-sensitivity reached in the present data matches that achieved for typical faint I ltapprox 26 galaxies seen in deep HST images.

The 1-orbit 3-sigma point source sensitivity is 26.4 ± 0.15 mag in F300W and ~ 24.5 mag in F255W. Hence, many of the galaxies in our sample are resolved into their brightest star-forming regions and - most likely - into their OB associations and young star clusters. This is not true, however, for most of the merging/interacting galaxies that were selected into our sample from the sample of Hibbard et al. (2002). Because these systems are relatively rare, they tend to be 2-3 times more distant than the bulk of our sample (see Table 1, Col. 14), and so are not resolved into stars.

2.4.3. Red-leak

The F300W filter has a significant red-leak, causing a fraction of an object's flux longward of 4000Å to be detected in this mid-UV filter. Fig. 3.10 of the WFPC2 Handbook (Biretta et al. 2001) shows that red-leak portion of the QE × T curve of the F300W filter resembles the throughput curve of the F814W filter which transmits mostly photons in the 7000-9000Å range. Table 3.13 of the WFPC2 handbook suggests that the red-leak is generally no more than 5% of the total F300W flux for stellar populations dominated by stars of spectral type K3V or earlier, although it can be as much as 10-50% of the total F300W flux for stellar populations dominated by M0-M8V stars. Hence, even for elliptical galaxies with K-star spectra, the red-leak is expected to be relatively small, and for late-type galaxies dominated by young hot stellar populations it should be almost negligible.

For realistic galaxy SEDs, Eskridge et al. (2002a) find that the red-leak is typically 5-7% of the total F300W flux, and never exceeds 10% of the F300W flux, not even in the reddest galaxy bulges. We verified this for several red galaxies in our sample by subtracting a fraction of the F814W images from the F300W images, after appropriately rescaling with the relative exposure time, and making sure both images were registered the same way. This fraction of the subtracted F814W image amounted to 7% of the total F300W flux in the brightest region of the galaxy bulge that is presumed to be dominated by G8-K3 stars (following the red-leak as modeled by Eskridge et al. 2002a).

We found that for the redder stellar populations in those images no noticeable additional structure was introduced in our F300W images at the locations of the brightest F814W flux. To illustrate this, a very red star is seen just above the center of both edge-on galaxies ESO033-G22 and IC 4394 in the F814W images of Fig. 3.19 and 3.20. For IC 4394, Fig. 4.20 shows how red this star is, where it is seen just South of the galaxy center. These stars are saturated in the F814W images of both galaxies, and at the corresponding locations in the (non-red-leak corrected) F300W images of Fig. 3.19-3.20, only a very faint red-leak flux is seen. These worst case examples show that the apparent F300W morphology of any of our galaxies would not be significantly affected by the red-leak in a few of the very reddest and brightest galaxy areas in the F814W images. Such areas would have to be significantly saturated in our F814W exposures to generate significant red-leak in the F300W images, and none of our targeted galaxies were saturated anywhere in the F814W images. Hence, for the current qualitative presentation of the mid-UV images, and given that our sample is biased toward the bluer galaxies, we have thus not corrected the images presented in the mid-UV atlas of Section 3 for the small contamination by red-leak in the brightest and reddest areas.

For accurate quantitative measurements of galaxy properties we will subtract the red-leak in future papers where necessary (e.g., Eskridge et al. 2002a). This will be done together with the determination of pixel-to-pixel SEDs for each galaxy, in case there are subtle dependencies of the red-leak on the red SED. However, to first order there should be no such dependency, since galaxies of all spectral types show remarkably little change in their relative SED's between 7000-9000Å.

2.4.4. Data Archiving

As we expect these data to be of use beyond the scope of our immediate science goals, we will make all images available to the community in digital form when this paper goes to press. We do this, even though the photometric zero-points for part of the ground-based images are not yet established. We will update the FITS headers as new photometry becomes available (see also Taylor et al. 2002). Hence, the FITS headers in the public data base will override any values currently listed in Table 2. The images will be made public via ADS and also at the following public Web-Site at ASU:

Both the HST and ground-based data will be made publicly available at this site. The raw WFPC2 data can be obtained from the HST Archive. Additional information regarding this survey and its planning, the observations, and reduction procedures can be found at:

and at:

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