The Spitzer Deep, Wide-Field Survey (SDWFS)
Updated First Data Release (DR1.1)

1. Introduction

This document describes the updated first data release (DR1.1) of the Spitzer Deep, Wide-Field Survey (SDWFS), a Cycle 4 Spitzer Legacy program (PI Daniel Stern, JPL/Caltech). Details of the survey design, processing, and inital science results are presented in Ashby et al. (2009).

SDWFS is a four-epoch survey of roughly 10 square degrees of the NOAO Deep, Wide-Field Survey field in Boötes. The first visit to the field occurred very early in the Spitzer mission, in 2004 January, as part of the IRAC Shallow Survey (Eisenhardt et al. 2004). Subsequent visits to the field as part of the SDWFS program reimaged the same area to the same depth (see Table 1).

This delivery consists of four-band IRAC mosaics, coverage maps, and catalogs corresponding to each of the SDWFS epochs — including the IRAC Shallow Survey, which is now considered epoch 1 of SDWFS. In total, there are 16 single-epoch mosaics and coverage maps, plus an additional set of four corresponding to the sum of all IRAC integrations of this field.

The data were all taken with the Infrared Array Camera (IRAC; Fazio et al. 2004). These "best-effort" data products have been delivered to the Spitzer Science Center (SSC) for public release in Spring 2009, and replace the inital data release (DR1) from Fall 2008. The modifications from DR1 are described below in Section 5.

2. SDWFS Observing Campaigns

Approximately 10 square degrees of Boötes were observed on each of four visits by Spitzer/IRAC. Those visits were arranged so as to survey the field with a range of time delays, both on short time-scales (≈ 2 hr) to facilitate identification of asteroids, and on month to year time-scales to facilitate searches for infrared variability. The cadencing was as follows:

Each visit consisted of IRAC mapping observations to 3 x 30 sec depth. Figure 1 shows the the total 3.6 μm SDWFS mosaic constructed using data from all four visits (e.g., nominally 12 x 30 sec depth) to illustrate the extent of the region covered by SDWFS. Coverage is similar in the other three IRAC bands. Details of the mapping strategy are presented in Ashby et al. (2009).

Figure 1: SDWFS IRAC 3.6 μm image of the Boötes field
This image co-adds all four epochs of 3.6 μm image observations, and detects ~700,000 sources.
Each square is approximately the size of the full moon.

3. IRAC Data Reduction

The data reduction was based on the IRAC Basic Calibrated Data (BCD). The BCD frames were object-masked and median-stacked on a per-AOR basis; the resulting stacked images were visually inspected and, when necessary, subtracted from all BCDs within an AOR to eliminate long-term residual images due to prior observations of bright sources. In general, this procedure was always necessary for the 3.6 μm images, but was not always necessary for the longer-wavelength data.

The first two epochs were processed with pipeline version 16.0.1. The 3.6 and 4.5 μm BCDs were examined by hand and modified using custom software routines to correct column pulldown and multiplexer bleed effects associated with bright sources. Much of this software is available from the Spitzer Science Center contributed software page and is based on algorithms designed and coded by SDWFS co-I's. The last two epochs were processed with pipeline version 17.0.4. For these, we used the so-called corrected BCD frames in which the BCD pipeline automatically applies multiplexer bleed and column pulldown corrections.

After these preliminaries, the data were organized and mosaicked into eight overlapping sub-fields, each slightly larger than a degree across, using IRACproc (Schuster et al. 2006) to augment the capabilities of the standard IRAC reduction software (MOPEX). The software was configured to automatically flag and reject cosmic ray hits based on pipeline-generated masks together with a sigma-clipping algorithm for spatially coincident pixels. IRACproc calculates the spatial derivative of each image and adjusts the clipping algorithm accordingly. Thus, pixels where the derivative is low (in the field) are clipped more agressively than are pixels where the spatial derivative is high (point sources). This avoids downward biasing of point source fluxes in the output mosaics.

Finally, a total coadd was constructed in the same manner as was done for the individual epochs, with the minor change that only temporal outlier rejection was performed.

Tiles for all five datasets (e.g., each of the four epochs individually as well as the full data stack) were mosaiced into a single images for each band and epoch/stack using the Montage toolkit (ver. 3.0). The coverage maps in overlapping regions were used as a means of preventing the low-coverage 'crust' on the periphery of each tile — with its relatively high prevalence of cosmic ray hits — from degrading the final full-field mosaics.

This procedure has been iterated several times by the SDWFS team to improve the image quality. The images and coverage maps described in this document are designated version 3.4 (version name: 'obama-obama'). Based on Monte Carlo analysis of the measured surface brightness distributions in randomly located positions, we derived the 5σ depths in 4 arcsec diameter apertures listed in Table 2. The depths have been aperture corrected, assuming an unresolved source.

4. Contents of the Data Delivery

The outcome of the reduction process is a suite of 20 mosaics and 20 coverage maps: four for each epoch, plus another four for the total SDWFS 12 x 30 sec coadds. All of these are included in this data delivery. The images are resampled to 0.84 arsec per pixel, so that each mosaic pixel subtends half the area of the native IRAC pixel.

With this delivery, the SDWFS team is also supplying four band-matched catalogs corresponding to each epoch and the total SDWFS coadd: 20 catalogs in all. The catalogs are ASCII text files created with SExtractor; each of the four was generated using a different IRAC band as the selection image. The coordinates of cataloged SDWFS sources correspond to 2MASS catalog positions within 0.2 arcsec. Note that SExtractor substantially underestimates the true magnitude uncertainties since it only accounts for statistical uncertainties and does not account for systematic uncertainties inherent to IRAC observations. To somewhat account for this discrepancy, the tabulated uncertainties double the SExtractor derived uncertainties (in flux density, flux per unit frequency) space. The 20 catalogs are as follows:

The columns definitions are the same for each of the catalogs:

5. Changes from DR1

Since DR1, we have made several modifications to the SDWFS data processing, extraction, and catalogs which significantly improve the quality of the released catalogs. DR1.1 should be considered to replace DR1.

The changes are:

• We have modified some of the mosaicking parameters in order to improve cosmic ray rejection.

• We have modified the SExtractor parameters to improve detection of faint sources.

• We have improved SExtractor modeling of celestial backgrounds near the edges of the IRAC coverage in the single epoch mosaics. In DR1, sources within 512 mosaic pixels of the coverage edges had poor background estimates, resulting in systematic biases in the photometry, particularly at 5.8 μm. This has now been fixed.

• We now use aperture corrections derived from isolated, bright stars in our own data rather than using the IRAC Data Handbook aperture corrections. The aperture corrections are listed in Ashby et al. (2009).

• We now provide 4 and 6 arcsec diameter aperture-corrected aperture magnitudes rather than DR1 which provided the same for 3 and 6 arcsec diameter apertures.

We encourage use of these data. Please reference Ashby et al. (2009) for the survey description and initial data release. Please also notify D. Stern (daniel.k.stern AT jpl.nasa.gov) of submitted papers making use of these data.