The Far-Infrared Deep Extragalactic Legacy Survey (FIDEL)
Spitzer Legacy Data Products
Second Spitzer Data Release (DR2)

1.0 General information

2.0 Observations

2.1 Extended Chandra Deep Field South (ECDFS)
2.2 Extended Groth Strip (EGS)
2.3 GOODS-North


3.0 Data reduction

3.1 MIPS 70 and 160 micron data reduction

3.1.1 ECDFS and GOODS-N

3.1.1.1 BCD pipeline processing and filtering
3.1.1.2 Image combination, calibration, and astrometry

3.1.2 EGS

3.2 MIPS 24 micron data reduction

3.2.1 BCD pipeline processing
3.2.2 Frame-level post-BCD processing
3.2.3 Astrometry, image registration, and image combination


4.0 Data products

4.1 File names
4.2 World coordinates
4.3 Science images and flux units
4.4 Exposure maps
4.5 Standard deviation images
4.6 FITS headers


Tables:

Table 1: MIPS observations used in data release DR2
Table 2: Pixel scales and flux units for FIDEL DR2 MIPS data products


Other information:

List of FITS images in FIDEL data product release DR2
Example of FIDEL MIPS data product FITS header

Figures:

Figure 1: Extended Chandra Deep Field South FIDEL epoch 1+2 MIPS observations
Figure 2: Extended Groth Strip FIDEL epoch 1 + GTO MIPS observations
Figure 3: GOODS-North FIDEL MIPS 70 micron observations

1.0 General Information

This document describes the second release of data products from the Far-Infrared Deep Extragalactic Legacy Survey (FIDEL), a Spitzer Space Telescope Legacy Science program (PI: Mark Dickinson, NOAO).

This data release (DR2) consists of observations for all three FIDEL survey fields:

• Extended Chandra Deep Field South (ECDFS):
  Combined FIDEL epoch 1+2 images at 70 and 24 microns. This updates the epoch 1 (only) image released in DR1.

• Extended Groth Strip (EGS):
  FIDEL epoch 1 images at 24, 70 and 160 microns, coadded with data taken in the Spitzer guaranteed time observer program (henceforth, the "GTO data").

• Great Observatories Origins Deep Survey North field (GOODS-N):
  FIDEL observations at 70 microns.

The imaging data products are described in detail below, and consist of a science image mosaic at each wavelength plus associated exposure and noise maps.

2.0 Observations

FIDEL is Spitzer program ID (PID) number 30948, and was originally submitted with the title "A Deep-Wide Far-Infrared Survey of Cosmological Star Formation and AGN Activity." The program is obtaining data in three fields on the sky. Most of the data come from two fields, the Extended Chandra Deep Field South (ECDFS) and the Extended Groth Strip (EGS). Additional data are also being obtained in the GOODS-North area, in order to augment partial 70 micron coverage of that field from a GO-1 program (PID 3325, see Frayer et al. 2006b, ApJ, 647, L9). The observing strategies and data reduction are different for the three fields, and we discuss them each separately below.

Spitzer MIPS observations are now organized into "warm" and "cold" campaigns, in order to maximize the cryogenic lifetime of the telescope. "Cold" campaigns result in greater cryogen usage, and provide a lower background to allow 160 micron observations to be taken. The only cold campaign 160 micron data for FIDEL that have been reduced so far are those for EGS epoch 1, which are presented here.

The design of the AORs for the FIDEL MIPS observations is somewhat complex, and we will only give a high-level description here. Please refer to the Spitzer Observer's Manual for detailed explanations of the various observing modes and terminology. The target coordinates observing dates for the MIPS observations used in this data release are summarized in Table 1, which also provides links to detailed information about the AORs that make up each block of observations.

2.1 Extended Chandra Deep Field South (ECDFS):

The bulk of the FIDEL ECDFS observations were taken in two epochs separated by approximately six months, in September 2006 and March 2007 (epochs 1 and 2, respectively). The Spitzer telescope orientation rotated by approximately 180 degrees between the two epochs, ensuring reasonably symmetric coverage in the MIPS 24 and 70 micron bandpasses. Data release DR2 presents the combined image from the first and second epoch FIDEL data at 24 and 70 microns. (An earlier reduction of the first-epoch data was released in DR1.) Additional MIPS scanning observations of the ECDFS were taken during cold campaigns in January and March 2007, and will provide 160 micron coverage for FIDEL, as well as additional depth at 24 and 70 microns. Those data will be included in the FIDEL 'v1.0' data products, planned for release in June 2008. The September 2006 observations were taken during a warm campaign, and therefore no 160 micron data were taken. The March 2007 observations were done cold, but using the photometry mode AOT (see below) optimized for 70 microns. They are therefore not ideal for the 160 micron parallel data, which are also spatially offset from the main target field. Those 160 micron data have not been reduced by the FIDEL team.

The observations were broken into a series of Astronomical Observation Requests (AORs), each several hours long, that were designed to enable efficient scheduling. For the first epoch of ECDFS observations, we had 27 AORs, falling into two basic types. The majority (24 out of 27) of the AORs were taken with the 70 micron array as the prime instrument (henceforth, "70-prime"), with the observing strategy optimized for that wavelength. These AORs have TARGET=ECDFS. Three other AORs were taken with the 24 micron array as the prime instrument ("24-prime"). These AORs have TARGET=ECDFS-mips24, and were used to fill gaps in the 24 micron areal coverage, for reasons described in more detail below. In the second epoch, there were 24 AORs, all taken with the 70 micron array as the prime instrument.

All ECDFS epoch 1 and 2 MIPS observations used the photometry-mode Astronomical Observing Templates (AOTs), which are described in more detail in section 8.2.1.2 of the Spitzer Observer's Manual. In this mode, the field of view is dithered using a combination of telescope and scan mirror offsets around a particular pointing position. The 70-prime AORs used the 70 micron compact source (small field) mode, while the 24-prime AORs used the 24 micron large source (large field) mode.

Generically, we consider the ECDFS to cover a square region approximately 30'x30' on the sky, oriented along the J2000 celestial axes. In the first epoch, the 70-prime AORs observed pointing positions that covered roughly the bottom two-thirds of the 30'x30' ECDFS. The parallel 24 micron data taken at the same time covered roughly the upper two thirds of the field. This situation was reversed by 180 degrees for the second epoch observations, providing roughly symmetric coverage of the field. The 70 micron pointings were carefully placed to avoid overlap with previous, deep 70 micron observations taken in a GO-2 program (PID 20147, PI: D. Frayer) which observed roughly the central 10'x10' of the GOODS-South field. This leads to the hole in 70 micron coverage seen in Figure 1, and also the "notch" on the southern portion of the 24 micron mosaic. These will be filled with the (currently proprietary) data from PID 20147, and a complete, combined image (also incorporating additional scanning mode data from FIDEL and from previous GTO programs) will be released as a "version 1.0" data product circa June 2008.

The MIPS observations were optimized to achieve roughly uniform exposure time at 70 microns in the combined epoch 1 + 2 observations. This, however, leads to non-uniformity in the depth of the parallel 24 micron observations, which are generally deepest in an E-W strip through the middle of the field, and somewhat shallower toward the northern and southern extremes. By avoiding the central field targeted by the Frayer 70 micron GO-2 program, we also introduce some gaps in the 24 micron coverage. These, as well as some "protruding corners", were filled by the additional 24-prime AORs, which were taken in epoch 1.

Figure 1 illustrates the layout of the ECDFS epoch 1+2 observations, showing the MIPS images and the relative exposure time maps at 24 and 70 microns. In the DR2 ECDFS 70 micron images, the mean exposure time over the majority of the field is approximately 4800s. This will be increased to approximately 7200s when the additional FIDEL scanning data and GTO observations are incorporated in a future data release. At 24 microns, the exposure time varies more. The deepest region in the central area has net exposure time approximately 24500s. Shallower regions have typical exposure times from 5800 to 11600s, while regions covered by the 24-prime "filler" AORs range from 2000s to 3400s. The median 24 micron exposure time within the nominal 30'x30' ECDFS area (excluding regions with no data) is approximately 8000 seconds in the DR2 images.


Figure 1: Extended Chandra Deep Field South FIDEL epoch 1+2 MIPS observations
The 24 and 70 micron images are shown at left, and their corresponding exposure maps are shown at right.
The red box is the same in all panels, and has dimensions 30'x30'.

2.2 Extended Groth Strip (EGS):

The Extended Groth Strip (EGS) has extensive multiwavelength observations with many ground- and space-based observatories. Together, these comprise the (All-Wavelength Extended Groth strip International Survey (AEGIS)), of which FIDEL is one component. Previous Spitzer observations of the EGS include IRAC and MIPS data taken with guaranteed time invested by the Instrument Teams. The complete two-epoch FIDEL MIPS observations of the EGS will have a typical exposure time approximately 12 times longer than the GTO MIPS observations of this field. DR2 provides data products from the first epoch of FIDEL EGS observations, and thus has roughly half of the final planned exposure time. The second epoch observations have just been obtained and are now being reduced.

The EGS is a long, narrow strip on the sky, oriented at a position angle of approximately 40 degrees (E of N). The elongated geometry favors the use of scan mode AOTs (see section 8.2.1.1 of the Spitzer Observer's Manual), in which Spitzer scans back and forth along a given direction, with cross-scan offsets. The EGS observations were also taken in two epochs, in January and July 2007, with the telescope orientation reversed by 180 degrees. The observations required very tight timing constraints on the observations, which were met quite precisely by the SSC schedulers. The first epoch EGS observations (released here) consisted of 32 AORs.

The goal for the FIDEL EGS observations was to obtain full depth exposures at three MIPS wavelengths (24, 70 and 160 microns) for an overlap region approximately 90 arcmin long and 10 arcmin wide. The separation on the sky of the fields of view for the three MIPS channels leads to displaced coverage at the three wavelengths in a single epoch, with the pattern reversed in the second epoch observations. The final, coadded two-epoch data set will have roughly symmetric coverage with full 3-band depth over the 10'x90' primary field, and shallower "overscan" regions extending beyond it.

The EGS was previously observed with MIPS as part of the guaranteed time observer program (PID 8, PI: Fazio). These earlier, shallower data have been used in a variety of scientific investigations, including papers published in the AEGIS Special Issue of the ApJ Letters. As part of a general public release of AEGIS data sets, we have combined the GTO observations taken in December 2003 and June 2004 with the FIDEL data for this data release. The December 2003 MIPS data were taken very early during Spitzer's operations, and the telescope entered safe-mode partially through the observations, causing them to abort; 4 AORs and an incomplete 5th were obtained. The observations were rescheduled for June 2004, when 12 AORs were executed. Both of these GTO observations are somewhat misaligned with the axis of the FIDEL data. The GTO scans also were longer than the FIDEL scans, covering 120 arcmin (plus overscans). This provides coverage of additional area, primarily at the southwest end of the EGS.

Figure 2 illustrates the combined FIDEL epoch 1 + GTO observations, showing the MIPS images and the relative exposure time maps at 24, 70 and 160 microns. The FIDEL area can be distinguished in the exposure maps as the region with deeper coverage. In the DR2 EGS 70 micron images, the mean exposure time in the deeper FIDEL area (including the GTO data used here) is approximately 3800 seconds. This will be increased to approximately 7200s when the second epoch FIDEL observations are incorporated in a future data release. The corresponding exposure time at 160 microns is about 700 seconds, while at 24 microns it is about 7200 seconds, both of which will be roughly doubled in the final data products.


Figure 2: Extended Groth Strip FIDEL epoch 1 + GTO MIPS observations
The 24, 70 and 160 micron images are shown in the top row, and their corresponding exposure maps are shown below.
The deeper area covered by the new FIDEL observations is easily seen in the exposure maps.

2.3 GOODS-North:

Approximately 100 square arcmin in the northern field of the Great Observatories Origins Deep Survey (GOODS)) was observed with MIPS at 70 microns in a cycle 1 guest observer program (PID 3325, see Frayer et al. 2006b, ApJ, 647, L9). Those were the first very deep field observations taken with MIPS, with a typical exposure time of approximately 3 hours per position, covering approximately the central 10 arcmin x 10 arcmin region of the GOODS-N field. As part of FIDEL, we have obtained additional 70 micron observations with similar net exposure time to extend the coverage to the northeast and southwest, in order to cover the complete 10'x16' GOODS-N area.

In DR2, we provide mosaics of the FIDEL 70 micron data for GOODS-N, illustrated in Figure 3. The typical exposure time at 70 microns is approximately 9000 seconds. 24 micron observations were taken in parallel with the 70 micron data, but are offset beyond the area of the nominal GOODS-N field, and have not yet been reduced. (24 micron observations of the GOODS-N field proper are available as GOODS Legacy data products.) The FIDEL GOODS-N observations were taken in photometry mode during a warm Spitzer campaign, and no 160 micron data were taken. The version 1 FIDEL data products, to be released in June 2008, will incorporate all MIPS data that are available for the GOODS-N area, including the Frayer GO data and observations from the MIPS GTO program.


Figure 3: GOODS-North FIDEL MIPS 70 micron observations
The 70 micron FIDEL data are shown at left, and the corresponding exposure map is shown at right.
The outline of the GOODS-N ACS mosaics is indicated schematically.
In a future FIDEL data release, the central region of GOODS-N will be filled
with 70 micron data from Frayer et al. GO-1 program PID 3325

DR2 consists of "best effort" early reductions of the FIDEL data sets. We expect improvements in future data products as we have time to experiment with variant methods and techniques, and as additional data are incorporated. In this release, we have not aimed for complete consistency between the reductions for the different FIDEL survey fields, as the reductions were done at different times (as the data came in) by different subsets of the FIDEL team.

3.1 MIPS 70 and 160 micron data reduction

Data reduction for the Ge:Ga arrays at 70 and 160 microns requires careful treatment of a variety of time-dependent effects, as described in detail in Gordon et al. 2005 and in the MIPS Data Handbook. For the products in this data release (DR2), the ECDFS and GOODS-N 70 micron data were reduced by FIDEL team members at the Spitzer Science Center, whereas the EGS 70 and 160 micron data were reduced at the University of Arizona, using different software. In many respects, the data reduction methods used for the different data sets are quite similar, but the details may differ, and the exact form of the final data products (e.g., pixel scales, flux units) differ between the fields. We note these differences in the documentation that follows.

3.1.1 ECDFS and GOODS-N
3.1.1.1 BCD pipeline processing and filtering

The filtered BCD (fbcd) products for the ECDFS and GOODS-N fields were downloaded from the Spitzer archive to make a quick-look image and an initial source list. The raw data were then re-reduced from scratch using the Ge Reprocessing Tools (GeRT, version 060415 [2006 April 15]). The GeRT BCD processing was done with the best set of optimized pipeline parameters derived previously from deep 70 micron photometry data of GOODS-North (see Frayer et al. 2006b).

Filtering is crucial for deep 70 micron photometry data. The calibration stimulator (stim) flashes are used every 6 data collection events (DCEs), and latents due to these stim flashes accumulate over time. These latents correlate with column. To remove stim flash latents, we used a median column filter. After column-filtering, the residual drifts of the detectors as a function of time were removed by the application of a median high-pass time filter per detector (with a filter width of 16 DCEs). This combination of filters has been shown to give the best sensitivity for deep photometry data. The positions of bright sources in the BCDs were flagged so that the filtering corrections were not biased by the presence of sources. This has been shown to maintain point-source photometry while removing the "side-lobe" artifacts seen in the on-line filtered products (see Frayer et al. 2006a, AJ, 131, 250). The median filtering techniques yield small offsets from zero in the average level of the filtered-BCDs. These offsets correlate with the DCE position within the stim cycle and were removed by subtracting the median level from each fbcd. With offline re-processing of the data, we have improved the sensitivity of the products by more than 20% over the on-line filtered products.

3.1.1.2 Image combination and astrometry

The data were coadded onto a sky grid with a scale of 4.0 arcsec/pixel using the MOPEX mosaicing and source extraction software (version 030106). The astrometry for the 70 micron data uses the default pointing solutions from telescope telemetry without further correction. Typical uncertainties in the Spitzer pointing solution from AOR to AOR are of order 1 to 1.5 arcsec, which is small compared to the 70 micron beam size and pixel scales. We did refine the 24 micron pointing solution for the ECDFS (see section 3.2.3), and may apply these refinements to the 70 micron data as well in future re-reductions. The data values in the science image mosaics have units of MJy per steradian, as for the SSC BCD data products. Surface brightness is preserved when the mosaics are reprojected to a final pixel scale. The data units are discussed in more detail in section 4.3.

3.1.2 EGS

The EGS observations taken with the Ge:Ga arrays at 70 and 160 microns were reduced and mosaiced using the Data Analysis Tool (DAT) developed by the MIPS Instrument Team. The procedures are described in detail in Gordon et al. 2005, and are generally very similar to those described above for the reductions of the ECDFS and GOODS-N data, with a few differences in detail:

• In addition to processing the FIDEL first-epoch observations, we have incorporated data taken during the GTO observations of the EGS, as discussed in section 2.2.

• The EGS 70 micron mosaic was projected at a scale of 4.925 arcsec/pixel, compared to 4.0 arcsec/pixel for the ECDFS and GOODS-N data products. The 160 micron EGS mosaics have a scale of 8.0 arcsec/pixel.

• The MIPS team DAT produces image mosaics whose data values have units of data numbers (DN) per second. The data values per pixel were conserved when the images were reprojected for mosaicing, i.e., surface brightness (not total flux) is conserved. We have converted the data values from DN/s to MJy/sr, as used in SSC BCD products and the DR2 ECDFS and GOODS-N mosaics, by multiplying by the standard flux conversion factors (FLUXCONV) for each MIPS wavelength taken from the the MIPS Data Handbook section 3.7.2 (see also Gordon et al. 2007, PASP, in press). The data units are discussed in more detail in section 4.3.

3.2 MIPS 24 micron data reduction

The 24 micron data for the ECDFS and EGS were reduced by the FIDEL group at NOAO, using very similar procedures for both fields, which we describe below. The GOODS team has previously released 24 micron images of GOODS-N, which were reduced using similar procedures, although with significant differences as well. See the documentation for the GOODS Legacy data products for details.

3.2.1 BCD pipeline processing

Reduction of the 24 micron data started with the products generated by the SSC Basic Calibrated Data (BCD) pipeline. The BCD versions used for each data set were: ECDFS epoch 1 (S14.4.0), ECDFS epoch 2 (S15.3.0), EGS epoch 1 (S15.0.5), GOODS-N (S15.0.5), and EGS GTO epochs 1 and 2 (S14.4.0).

As was described in section 2.1, for ECDFS epoch 1 there are two distinct types of 24 micron observations: 10 second exposures taken when the 70 micron array was the prime camera, and 30 second exposures taken with the 24 micron array as the prime camera. Each has its own dither pattern, and the reductions were treated separately where necessary. For the ECDFS epoch 2 and for all EGS observations, the 24 micron data were taken with 10 second exposure times.

3.2.2 Frame-level post-BCD processing

In post-processing of the individual BCD frames, we applied the following steps:

Create masks for latent images: Bright sources observed with MIPS can produce latent images that persist into subsequent exposures. This often happens when Spitzer observes relatively bright stellar targets before or in between FIDEL AORs. We took the median of all 24 micron exposures per AOR and compared them to one another in the time order of their observing sequence. Time-varying latent images, typically at the 0.5% level, were apparent in some AORs. We produced masks to set affected pixels to zero weight when combining the data into image mosaics.

Scan mirror delta-flatfields: A median of all data show that the BCD frames have residual flatfielding artifacts at the 1% level. These artifacts include some variability with scan mirror position, suggesting that the pipeline spot-map corrections are not quite perfect. Additionally, the default scan mirror position (CSM=2007.) shows large scale features (in the 70-prime data only) which are not evident in either the other four scan mirror position medians or in any of the 24-prime data from the epoch 1 ECDFS observations.

We created delta flats for each of the five scan mirror positions and divided these into the data. For ECDFS epoch 1, this was done separately for the 24-prime and 70-prime images. The frames affected by the latent artifacts are masked before creating the delta flats, and all low-number data collection events (DCEs) visibly affected by the bias boost (see below) are excluded from the delta-flat medianing. The corrections are applied to the unmasked frames, including the low-number DCEs.

Remove cross-scan background gradient from low-number DCEs: Bias boosts occur at the start of every sequence of data collection events (DCEs) indicated by a unique EXPID number. We created median images from all observations grouped by DCE number (DCENUM) to look for any residual structure correlated with time since the bias boost. The first frame in each sequence (DCENUM=0000) shows large-scale residual features after division by the scan mirror delta-flatfields. While most of these features have disappeared by DCENUM=0001, the mean background level of the low-number DCEs (with DCENUM approximately less than 0010) remains depressed relative to the high-number DCEs (with DCENUM > 0015), with a 0.2-0.3% gradient in the cross-scan direction across the chip. These effects are more obvious in the shorter, 10-second 70-prime frames, and tend to fade as the DCE number increases.

For the DCENUM=0000 frames, we used the median image as a tertiary flat-field correction. In the 70-prime data, for 0001 <= DCENUM <= 0013, we fit a low-order function along the row axis of the median images, and divided that shape into the individual frames with DCENUM=0001 to 0013. For the 24-prime data from ECDFS epoch 1, the individual DCEs have longer exposure times (30s instead of 10s), and these DCE-dependent effects are greatly reduced. There, we applied corrections only to the DCE=0000 and 0001 frames.

Remove AOR-to-AOR flatfield differences: In some data sets, after the above steps were performed, large scale, low-level patterns were still seen, varying from AOR to AOR. These were prominent in some observations and not in others. We interpreted these as larger-scale latent image patterns, which depend on the nature of targets imaged by MIPS prior to the FIDEL observations. In order to remove these, we constructed new median images of the dithered data within each AOR, and then subtracted these from each individual image within that AOR. For uniformity, we applied this step to all 24 micron data taken in 70-prime mode, regardless of whether a clear residual pattern was seen. This correction was only applied to the 70-um prime photometry-mode data; the relative small number of independent dither points per AOR in the 24-prime observations left us with insufficient spatial sampling to construct reliable median images per AOR without effects of source contamination which might subsequently affect photometry.

Jailbar patterns: Low level jailbar patterns (vertical striping associated with small bias offsets in the four interleaved readout amplifiers) could be seen in the DCE=0000 images. However, these were effectively removed by the multiplicative corrections described above. (The jailbar correction should probably be additive, not multiplicative, but the amplitude is so small (0.1-0.2%) that this should have very little impact.) Residual jailbar patterns were visible in only a small number of data sets, mainly those very strong cosmic ray hits. These were discarded from the final combinations.

Sky subtraction: Cosmic rays and bad pixels were masked in all frames during the MOPEX combination step (see below). After a first pass run of MOPEX, these pixel defect masks were saved, and then used to discard pixels when calculating a median sky background level (iteratively sigma-clipped) for each individual science image. These median values were subtracted from the data prior to (re)mosaicing.

3.2.3 Astrometry, image registration, and image combination

Image combination for the FIDEL MIPS 24 micron data was carried out using the MOPEX software (version 030106) provided by the Spitzer Science Center.

First, we created preliminary 24 micron mosaics for each AOR. MOPEX aligns the images on the assumption that the world coordinate information in the BCD image headers, which are generated based on telescope telemetry, are correct. Cosmic rays and other image defects were rejected as outliers in the stack of repeated observations at a given sky position. MOPEX generates masks of these outliers, which were checked to ensure reasonable behavior.

For each AOR-mosaic, we generated a catalog of 24 micron sources and matched this to a catalog of IRAC source positions in the same fields. For the ECDFS, we matched the MIPS data to a preliminary version (v1.0) of the IRAC data from SIMPLE, a GO-2 Spitzer Legacy program (PID 20708; PI: P. van Dokkum). (The images and catalogs from SIMPLE Legacy data release 1 are version 1.1, and thus somewhat updated from the ones used here for astrometric reference, but should be largely consistent.) External checks indicate that the SIMPLE astrometry (which is tied to that from the MUSYC project) agrees with that for the GOODS Spitzer and HST data within approx. 0.1 arcsec on average, although there may be local residuals that are somewhat larger. For the EGS, we used the GTO IRAC data from PID 8, as reduced and cataloged by the IRAC instrument team (Barmby et al., in preparation). The EGS IRAC astrometry was tied to 2MASS.

We computed an iteratively sigma-clipped mean astrometric offset between the MIPS 24 micron positions and the IRAC positions. We also visually inspected vector fields of the astrometric residuals looking for other systematic trends, and found none of significance. The astrometric offsets were applied to the world coordinate information (CRVAL1,2) in the headers of each image within that AOR. For this version of the reduction, only simple shifts in RA and Dec were applied. The process was repeated for each 24 micron AOR.

The astrometrically corrected images were then re-mosaiced, combining data from all AORs. For the ECDFS epoch 1 observations, the 70-prime and 24-prime observations were combined separately with MOPEX because they have different exposure times per image. However, the same "fiducial image frame" was adopted for the output mosaics to ensure that they would both be projected onto the same pixel grid. MOPEX produces "coverage maps" showing the number of input images that contribute to a given pixel position in the combined mosaic. We rescaled these to approximate exposure maps by multiplying by the exposure time per DCE (10 and 30 seconds, respectively, for the 70-prime and 24-prime data sets), and then combined the 70-prime and 24-prime sub-mosaics, weighting by their respective exposure maps.

The final image mosaics were produced on a tangent plane projection, with a scale of 1.200 arcsec/pixel. For the ECDFS, we aligned the pixel axes with the J2000 celestial coordinate axes. Due to the elongated geometry of the EGS, we chose to orient that final mosaics [CHECK AND DESCRIBE]

4.1 File names

File names for these FIDEL data products include the following components, separated by underscores ("_"):

1) "fidel" prefix
2) Field (here, "ecdfs", "egs", or "goodsn")
2) Bandpass ("24", "70", or "160")
3) Data set and epoch (described below)
4) Data version number
5) Image type ("sci" = science image; "exp" = exposure map; "std" = standard deviation image)


For item 3 (data set and epoch), we use the following notation: "p1" = photometry mode, epoch 1 (for GOODS-N in DR2), "p12" = photometry mode, combined epochs 1+2 (for ECDFS in DR2); "s1plus" = scanning mode, epoch 1, plus additional data, in this case, from GTO program PID 8 (for EGS in DR2). As an example, the FIDEL ECDFS 70 micron epoch 1+2 (photometry mode) version 0.50 science image is named "fidel_ecdfs_70_p12_v0.50_sci.fits". Similarly, the exposure map for the EGS 160 micron epoch 1 (scanning mode) plus GTO data are named "fidel_egs_160_s1plus_v0.5_sci.fits". The version numbers are based on FIDEL internal nomenclature, and have no particular significance, other than to indicate that these are best-effort preliminary releases (i.e., version < 1.0).

• List of FITS images in FIDEL data release 2.

4.2 World coordinates

FIDEL image data products images use a tangent plane sky projection. The ECDFS and GOODS-N images are aligned with the J2000 celestial axes, while the EGS images are oriented at a different angle due to the elongated geometry of the field. The pixel scales for the various images are given in Table 2. Note that in the DR2 data products, the EGS pixel scale at 70 microns is different than that for the ECDFS and GOODS-N, since the data reduction on different fields was done independently. For a given field, the WCS tangent points (CRVAL1,2) of the images at different wavelengths are generally different in the DR2 images. Therefore, a transformation (beyond simple linear translation and resampling) would be needed in order to achieve accurate pixel alignment from 24 to 70 microns. However, the world coordinates of the two images should agree within the typical measurement uncertainties for source positions.

Table 2 - Pixel scales and flux units for FIDEL DR2 MIPS data products



Field	Band (micron)	Pixel scale (arcsec)	Science data units (BUNIT)	mJy/data unit (FLUXZERO)
ECDFS	24	1.200	MJy/sr	0.03385
ECDFS	70	4.000	MJy/sr	0.37607
GOODS-N	70	4.000	MJy/sr	0.37607
EGS	24	1.200	MJy/sr	0.03385
EGS	70	4.925	MJy/sr	0.57011
EGS	160	8.000	MJy/sr	1.50429

4.3 Science images and flux units

The FIDEL DR2 science and standard deviation images have intensity units of MJy per steradian, like the SSC BCD products. Because the pixel scales are different for different images, so is the conversion from total data units integrated over a source to the source flux density. The flux units are summarized in Table 2. The last column of that table gives the multiplicative factor (FLUXZERO) that should be used to convert summed counts within some aperture to a source flux density in milliJanskys, i.e.,

flux density (mJy) = FLUXZERO * data units

These are computed using the pixel scales (also given in Table 2) and standard calibration conversion factors (FLUXCONV) from instrumental units to MJy/sr taken from the the MIPS Data Handbook section 3.7.2 (see also Gordon et al. 2007, PASP, in press). The absolute uncertainties on the calibration factors are 4%, 7%, and 12% at 24, 70 and 160 microns, respectively.

No color-corrections have been applied to the flux values. The MIPS flux conversion factors are calibrated for 10000K stellar spectral energy distributions. Galaxies, which can have significantly different spectral energy distributions, may require a multiplicative color correction term. This is discussed in the MIPS Data Handbook section 3.7.4 (version 3.2.1), where color correction factors are tabulated. At 70 microns, typical color correction factors for galaxies are of order 1.07 to 1.09, depending on the shape of the SED. The correction factors are generally smaller at 24 microns.

Pixels in areas where there are no data are set to zero.

[Note that the units for the FIDEL 24 micron science image data products are different than those adopted for the GOODS 24 micron data reductions, e.g. for the GOODS-S field, which is embedded within the ECDFS. The GOODS team converted the MIPS BCD data units back to DN/sec.]

Due to the interpolation used when subsampling the MIPS pixels to produce the mosaiced images, data values in adjacent pixels of the science images are strongly correlated with one another. This correlation leads to an apparent suppression of the background noise, effectively a local smoothing. Therefore (as in many astronomical data sets), one should use caution when interpreting the measured RMS noise of the sky background. The standard deviation images (see section 4.5) provide an estimate of the local image noise, but one that should also be considered cautiously, as discussed below.

4.4 Exposure maps

The exposure maps (*_exp.fits) represent the approximate total MIPS integration time in seconds at each position on the sky in the co-added image mosaics. The MOPEX and DAT mosaicing software produce "coverage" maps, which represent the number of individual exposures which contribute to each pixel in the output image mosaic, and we have multiplied these by the approximate mean exposure time per data collection event (DCE) to produce the exposure maps. The precise exposure time per pixel is likely to be slightly different (probably smaller), due to rejection of outlying values such as cosmic rays, array defects, etc.

4.5 Standard deviation images

We provide standard deviation maps (*_std.fits), which are an output product from the MOPEX and DAT mosaicing software. At each pixel position in the output mosaic, the standard deviation values are computed from the RMS of the individual pixel values from images that contribute to that mosaic position, divided by the square root of the number of exposures taken at that position. The units of these images are the same as for the science images (see Table 2. Regions of the images where there are no data are set to zero in the standard deviation images.

We caution that these standard deviation maps should be regarded as indicative only, and regarded with some skepticism! They assume a certain noise model, namely, that the shot noise for N exposures decreases as 1/sqrt(N), and that no other systematics contribute to the image-to-image standard deviation that is measured. Although careful studies of noise properties in MIPS data have shown that indeed the noise does reduce roughly as 1/sqrt(t) (see, e.g., Frayer et al. 2006b), this scaling is not precise and in particular has not been validated for these particular FIDEL data. The actual image noise that affects photometry may also have contributions from source blending and confusion, which is not taken into account in these standard deviation maps. Finally, sources can appear as "rings" of enhanced RMS in the maps, primarily due to small variations in image alignment or due to pixel undersampling, such that regions of strong image gradients (i.e., near the FWHM point of a point source) lead to enhanced RMS.

4.6 FITS headers

The FITS headers of the FIDEL data product images incorporate various useful and relevant information about the images.

• Example of FIDEL MIPS data product FITS header