The Great Observatories Origins Deep Survey (GOODS)
Spitzer Legacy Data Products
Second Data Release

Document version: 11 May 2005
Minor updates: 29 December 2005


1.0 General information

2.0 Observations

2.1 Fields and program IDs
2.2 IRAC observing strategy and AORs

3.0 Data reduction

3.1 SSC pipeline processing
3.2 Frame-level post-BCD processing
3.3 Astrometry and image registration
3.4 Image combination
3.5 Exposure and weight map scaling
3.6 Flag maps

4.0 Data products

4.1 File names
4.2 World coordinate system
4.3 Science images
4.4 Exposure maps
4.5 Weight (inverse variance) maps
4.6 Flag maps
4.7 FITS headers


Table 1: Parameters for GOODS IRAC superdeep epoch 2 data
Table 2a: Chronological AOR summary, IRAC GOODS-S epoch 2
Table 2b: Chronological AOR summary, IRAC GOODS-N epoch 2
Table 3: Flux and magnitude conversion factors
Table 4: GOODS IRAC flag map values

Other information:

Complete list of FITS images in the first data product release
Example of GOODS data product FITS header


Figure 1: IRAC color composite of a portion of GOODS-N.

1.0 General Information

This document describes the second release of data products from the Great Observatories Origins Deep Survey (GOODS) Spitzer Space Telescope Legacy Science program.

This second data release (DR2) consists of "best-effort" reductions of data taken with the Infrared Array Camera (IRAC, Fazio et al. 2004) on-board Spitzer. These are images from the second epoch (out of two) of the "superdeep" IRAC observations for each of the two GOODS fields. The superdeep program is sometimes also described as the "deep" program in GOODS literature, and is to be distinguished from the "ultradeep" program that covers a small portion of the GOODS-N field.

The data products are described in detail below, and consist of mosaiced, co-aligned images in all four IRAC channels on both fields, plus associated exposure, weight and flag maps.

The GOODS team is writing a paper which will describe the observations and data (Dickinson et al., in preparation). Please reference this paper when using these data products in published research.

For the most part, the observations, data reduction, and data products are very similar to those from the first GOODS data release (DR1). These are described in more detail in the documentation for GOODS DR1, and we will not repeat most of that information here. Instead, we will concentrate on documenting features that are unique to the DR2 data or data products.

2.0 Observations

Here, we provide a brief description of the IRAC superdeep epoch 2 observations.

2.1 Fields and program IDs

The two GOODS fields are described in the documentation for GOODS DR1, section 2.1. Coordinates, position angles, and other important parameters for the epoch 2 IRAC observations of the GOODS fields are summarized in Table 1. An IRAC color composite image of the historical HDF-N area is shown in Figure 1.

Figure 1: IRAC color composite (channels 1, 2 and 4) of a portion of GOODS-N, highlighting the area of the historical WFPC2 HDF-N.

IRSA Image

2.2 IRAC observing strategy and AORs

The second-epoch IRAC superdeep observations were executede during IRAC campaign 12 for GOODS-S and campaign 16 for GOODS-N. The design of the observations and their Astronomical Observation Requests (AORs) are very similar to those described in the documentation for GOODS DR1, section 2.2. The AORs for the epoch 2 observations were generally somewhat longer than those for the epoch 1 observations, which enabled improvements in scheduling efficiency. In addition, the total exposure times were slightly shorter than those from epoch 1. In all, 30 superdeep AORs were executed per field for the epoch 2 observations. The more efficient scheduling of the AORs enabled them to be scheduled within a shorter total time interval, and hence over a smaller range of telescope roll angles. The mean position angle of the epoch 2 images, and the range of roll angles over which they were taken, is given in Table 1.

Table 1 - Parameters for GOODS IRAC superdeep epoch 2 data

Spitzer program ID 194 169
Target name CDF-S HDF-N-CXO
RA (J2000) 03:32:30.37 12:36:54.87
Dec (J2000) -27:48:16.8 +62:14:19.2
Mean position angle -14 deg +52 deg
Range of PAs 4.5 deg 4.2 deg
Start date/time 2004-08-12 01:22:26 2004-11-20 12:18:20
End date/time 2004-08-18 16:58:28 2004-11-25 17:15:11

Table 2 - Chronological AOR summary, IRAC superdeep observations

The second-epoch GOODS observations were taken approximately six months after the first epoch (for each field), when the telescope orientation has rotated by 180 degrees. In this way, the areas that were imaged with IRAC channels 1 and 3 in the first epoch were imaged with IRAC channels 2 and 4 in the second epoch, and vice versa, completing four-band coverage of each GOODS field. The exposure time per channel per sky pointing is approximately 23 hours per epoch. and is approximately double that in the overlap strip if data from the two epochs are coadded.

Each AOR consists of a series of dithered exposures taken in a 2x2 mapping pattern. Most of the epoch 2 AORs for both GOODS-S and GOODS-N used 14 dithers per map position, with 56 independent sky pointings respectively over the whole map per AOR. A few AORs had only 8 dithers per map position and 34 pointings overall per AOR, varying the AOR duration in order to provide flexibility for telescope scheduling.

3.0 Data Reduction

The reduction of the epoch 2 IRAC data followed the same general procedures outlined in the documentation for GOODS DR1, section 3.0. Here, we note any significant differences from those procedures.

3.1 SSC pipeline processing

The GOODS team started reductions of the data using products generated by the Spitzer Science Center (SSC) Basic Calibrated Data (BCD) pipeline. The CDF-S IRAC epoch 2 BCD data were processed with pipeline version S10.5.0, while the HDF-N IRAC epoch 2 BCD data were processed with pipeline version S11.0.1. A very small number of frames were not correctly processed by the SSC pipeline, and thus no BCD products were available. We hope to recover those frames for future data products.

3.2 Frame-level post-BCD processing

One new processing step was applied to the GOODS-N epoch 2 images for channel 4 (only) which has not been used for other GOODS data products to date. In channels 1 through 3 the GOODS IRAC observations use 200 second frame times. During that time, channel 4 takes four 50 second exposures at the same telescope pointing. For the GOODS-N epoch 2 superdeep data, we combined those four exposures, allowing for small, constant additive offsets from image to image (which we attribute mainly to small variations in the bias level), and with outlier rejection to eliminate cosmic rays. Thereafter, the resulting four-frame "bundled" average channel 4 images were treated as single exposures with 200 second effective exposure times, and were processed along with the data in the other channels. This "bundling" improves the signal-to-noise ratio for source detection at each channel 4 dither position, while eliminating most cosmic rays in the channel 4 data. This facilitates the source detection and centroiding that is used in the image astrometry and registration procedures discussed below.

In post-processing of the individual BCD frames (and the "bundled" channel 4 images for GOODS-N), we applied the following steps:

These are described in more detail in the documentation for GOODS DR1, section 3.2.

3.3 Astrometry and image registration

We followed the astrometry and alignment procedures described in the documentation for GOODS DR1, section 3.3, with the exception that for GOODS-N channel 4, the "bundled" sets of four exposures taken at a common dither position were treated as a unit rather than as individual frames.

3.4 Image combination

Image combination followed the procedures described in the documentation for GOODS DR1, section 3.4.

3.5 Exposure and weight map scaling

The scaling of the exposure and weight maps is described in the documentation for GOODS DR1, section 3.5. The exposure maps approximate the number of seconds of on-sky data retained at each pixel position in the final coadded images. The weight maps are proportional to the exposure maps, and are normalized to equal the measured inverse variance of the pixel shot noise at the level of the background. This was calibrated by using split image procedure described in the DR1 documentation.

3.6 Flag maps

The flag maps follow the conventions described in the documentation for GOODS DR1, section 3.6. Flag values and their meanings are listed in Table 4.

4.0 Data Products

GOODS DR2 consists of FITS images of the second epoch IRAC superdeep data for both GOODS fields. Our understanding of IRAC instrument behavior and data processing is continuing to evolve, as are the software pipelines and the calibration of the instrument. The current release provides "best-effort" data products available at this time, and will eventually be superseded by reprocessed versions in a future data release. The version numbers for these data products, based on GOODS internal nomenclature, are v0.30 for both GOODS-S and GOODS-N in channels 1 through 3. For channel 4 in GOODS-S, the version number is 0.31, reflecting a small change that was made to the astrometric alignment procedures. For channel 4 in GOODS-N, the version number is 0.32, reflecting the use of "bundled" channel 4 frames, as discussed in section 3.2.

4.1 File names

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

1) GOODS field ("n", "s")
2) Instrument (here, always "irac")
3) Channel (here, "1", "2", "3", "4")
4) Data set and epoch (here, always "s2" for "superdeep epoch 2")
5) Release version
6) Image type ("sci" = science image; "exp" = exposure map; "wht" = weight, "flg" = flag map)

As an example, the GOODS-S IRAC channel 1 superdeep epoch 2 science image (version 0.30) is named "s_irac_1_s2_v0.30_sci.fits".

4.2 World coordinate system

All GOODS imaging data products are generated using a common scheme for world coordinates and pixel projection, which we briefly describe here. The images are projected on a tangent plane, with a the tangent point (CRVAL1,2) selected to be near the center of each field (GOODS-N, GOODS-S). They are aligned with north up (+y) and east left (-x). The pixel scales for GOODS imaging data products from different telescopes and instruments are always chosen to be integer multiples of one another. For the IRAC GOODS images, this scale is 0".600/pixel, which is approximately (but not exactly) half the native IRAC pixel scale. (Other scales for GOODS public-release data sets include 0".15/pixel for the ESO/VLT ISAAC CDF-S data, and 0".03/pixel for the HST/ACS Treasury Program images.) The pixel position (CRPIX1,2) that corresponds to the tangent point (CRVAL1,2) is always set to be a half-integer value. In this way, GOODS imaging data products from different telescopes and instruments can always be mapped to one another by simple integer rebinning, if desired.

Since the release of the GOODS HST/ACS v1.0 data products on 29 August 2003, we have found that the absolute astrometry for the GOODS-N field is slightly offset in declination from the reference frame defined by VLA 20 cm source positions from Richards (2000). This difference is approximately

dec(VLA) - dec(GOODS-N) = -0.38 arcsec

with negligible offset in right ascension. We have not yet adjusted the coordinate reference frame of the GOODS-N IRAC data products to account for this, in order to keep the Spitzer and Hubble GOODS data products on the same world coordinate system and pixel grid. We intend to adjust the coordinate reference systems for both the Spitzer and Hubble GOODS data products in future releases. There is currently no indication of any similar offset for the GOODS-South data products, although the radio data in the south are not as useful for this purpose as are those for the HDF-N.

4.3 Science images

The pixel intensities for GOODS IRAC data products are given in units of DN per second, derived from the original SSC BCD products (which have units of MJy/sr) using the FLUXCONV BCD header keyword. The IRAC Data Handbook, version 1.0, Table 5.2, provides the best current determination of the flux conversion factors for each channel as derived by the SSC. We summarize this information here in Table 3, providing the flux densities in micro-Janskys and the AB magnitudes that correspond to a count rate of 1 DN/sec. This information is also recorded in the image headers in the keywords FLUXCONV and MAGZERO (see section 4.7). For reference, we also list the detector gain (electrons/DN) in each channel; note that the effective gain for the GOODS mosaics (which are normalized to DN/sec) varies with the exposure time as a function of position.

Although the IRAC point spread functions have smaller FWHM, and hence better angular resolution, than had been anticipated before launch, they nevertheless place a substantial fraction of light at large radii away from the center of a source. Careful attention to aperture corrections is therefore needed in order to properly measure source fluxes from any IRAC data set, particularly for extremely deep, crowded images like those from GOODS. In a future data release, we will provide object catalogs and discuss source photometry in detail.

Table 3 - Flux and magnitude conversion factors

Channel Wavelength FLUXCONV
AB for 1 DN/s
Detector gain
1 3.6 microns 3.922 22.416 3.3
2 4.5 microns 4.808 22.195 3.71
3 5.8 microns 20.833 20.603 3.8
4 8.0 microns 7.042 21.781 3.8

4.4 Exposure maps

The exposure maps represent the IRAC integration time in seconds at each position on the sky in the co-added image mosaics, after rejection and masking of outlier pixels (e.g., cosmic rays, pixel defects, muxbleed, etc.). Fine-scale granularity from pixel to pixel in the exposure maps is a consequence of the process of drizzling the images onto a sub-sampled pixel grid using the point kernel.

4.5 Weight (inverse variance) maps

The weight maps represent the inverse square of the RMS pixel-to-pixel noise (in DN/s) at the background level of the images. The construction of these maps is described in detail in the documentation for GOODS DR1, section 3.5 . They represent the shot noise component due to the sky background and instrument noise only, and does not include Poisson noise from sources, nor any measure of photometric uncertainty due to source crowding or confusion.

4.6 Flag maps

The flag maps identify regions of the images with and without data in a given channel, with reduced exposure time, or where there may be residual muxbleed that could affect source detection or photometry. The flag images are bit maps, i.e., integers that represent the sum of bit values, each of which indicates a different flag conditions.

Table 4 describes the flag values, where the "bit number" starts at 0, and the "flag value" is the equivalent integer value for that bit setting. Bits not described in the table are currently unused for flag settings. These bit values will often appear in combination. For example, regions with < 20% of the modal exposure time (bit 1, flag value 2) also have < 50% of the modal exposure time (but 0, flag value 1). Therefore, those pixels will have flag values of 2 + 1 = 3. Regions with no data will have flag values 64 + 2 + 1 = 67. Regions with residual muxbleed (flag = 16) and also < 50% modal exposure time (flag = 1) will have flag = 16 + 1 = 17.

Note that the regions flagged for muxbleed were defined by eye, and should not be regarded as definitive or complete, but may serve as a useful warning in regions where muxbleed could affect object catalogs. We hope to improve muxbleed removal and flagging in future data releases.

Table 4 - GOODS IRAC flag map values

Bit number Flag value Condition
0 0 >50% of the modal exposure time
0 1 <50% of the modal exposure time
1 2 <20% of the modal exposure time
4 16 Region with significant residual muxbleed (Ch1 & Ch2 only)
6 64 No data (zero retained exposure time)

4.7 FITS headers

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

Example of GOODS data product FITS header