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IRAC: Data Products |
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This section describes the basic data products the observer will receive
from the SSC. More details are contained in (a) the IRAC Pipeline Description Document,
which describes the algorithms implemented in the IRAC pipeline; and (b)
the IRAC Data Handbook, which provides a
detailed guide to the data products and how to use them, both of which are
available on the SSC website. The data delivery consists of a directory
hierarchy with a name unique to that AOR. In this hierarchy are the BCD
data, as well as a number of subdirectories containing the raw data, the
calibration files, log files, and the post-BCD data. The exact contents
of the data delivery vary according to what the observer has requested
from the SSC data archive via Leopard. IRAC data are supplied as standard FITS
files. Each file consists of a single data collection event (i.e., a single exposure), and contains one image corresponding to one of the four IRAC arrays (the exception being post-BCD products, described below). The FITS headers are populated with keywords including (but not limited to) physical sky coordinates and dimensions, a photometric solution, details of the instrument and spacecraft including telemetry when the data were taken, and the steps taken during pipeline processing. There are three primary image data types that are supplied for each AOR (raw, BCD, mosaic).
Raw DataRaw data are wholly unprocessed except for those steps necessary to render them into a readable FITS format, i.e., depacketization and decompression. This is the form in which data enters the IRAC pipeline. Most observers are unlikely to use these data beyond sanity checking of the pipeline data products. They are, however, supplied in the event that observers wish to reprocess their IRAC data in a different manner from the SSC pipeline. By comparison to ground-based astronomy, these are the raw data one gets from a camera and writes to disk while observing at the telescope. The raw data consist of 16-bit integer FITS files. The headers are populated with all of the ancillary science telemetry keywords.
Basic Calibrated Data (BCD)BCDs are exposure-level data after having passed through the IRAC pipeline. There is one BCD for each integration taken by IRAC. Instrumental signatures have been removed, and the BCDs are absolutely calibrated into physical units (i.e., MJy/sr=10^(-17) erg s^(-1) cm^(-2) Hz^(-1) sr^(-1)). In addition, the pointing refinement (see next section) based upon 2MASS catalogs will be supplied in the BCD header. Continuing the analogy with ground-based observing, the BCDs are data that have been reduced, but not yet combined into a final image. This is the primary science data product produced by the SSC.Ancillary files are supplied with each BCD. These ancillary files contain several types of information regarding each pixel in each image. An image containing the uncertainty for each pixel is supplied. A mask image contains status bits indicating the probability that any given pixel has been affected by a radiation hit; whether or not the linearity solution could be applied; if a pixel is saturated, dead, hot (always on), or abnormally noisy. Pixels with strong residual images are flagged in the mask file. Log files are also supplied, and from these and the header keywords the entire pedigree of every data product can be derived. All of the data and ancillary files are in FITS format, containing a header with keywords and their values followed by a binary image, except certain log files that are in simple ASCII format. The standard FITS header keywords are all present so that essentially any FITS file reader is able to read the files, and all images have 256x256 pixels. Observers should expect to receive a data volume approximately 3.5 Mbytes per frame as their BCD, including ancillary files. Information about the data that observers receive is posted here. Calibration FilesFor each BCD, the pipeline calibration server generates several estimates of the current detector characteristics. These include a map of the pixel-to-pixel response variation (flat field) and an estimate of the dark current consisting of a laboratory dark and a sky dark. These calibration files are supplied to the observer. Observers are also able to request from the Archive the files that were used to generate the sky darks, sky flats, and absolute calibration. The photometric calibrators are not included with each science observation, but they are available via a separate request to the Archive.
Extended Pipeline Products (Post-BCD Pipeline)Pipeline processing of IRAC data by the SSC also includes more advanced processing of many individual IRAC frames together to form more "reduced" data products. Known by the generic title of "post-BCD" processing, this extended pipeline refines the telescope pointing, attempts to correct for residual bias and produces mosaiced images. We do not attempt to improve (relative to the BCD) the point source or extended emission flux calibration by automatically comparing to a reference source catalog. The mosaic only includes data from a single AOR, so that observers who break their map into multiple AORs (for example those whose maps cannot be completed within the 8 hour time limit of an IRAC AOR, or those who are making multiple-epoch observations) need to recombine them to obtain their ultimate images.All IRAC BCD images contain a pointing estimate based on the output of the Spitzer pointing control system (startracker and gyros, the boresight pointing history file). This initial pointing estimate is accurate to about 0.5". The post-BCD pipeline performs additional pointing refinement for all IRAC frames. This is achieved by running the SSC point source detector on the channel 1 and 2 frames and comparing the resultant list of point sources to the 2MASS catalog. The results are then averaged, and the known focal plane offsets between all four channels applied to produce a "superboresight" pointing history file which is then applied to the data during end-of-campaign reprocessing. This improves the pointing accuracy of the frame to better than ~0.3". This refined RA, Dec appears in the header as the CRVAL1, CRVAL2 keyword values. The SSC pipeline mosaicker produces a single image (1 per band) from many input images. First, the BCDs are corrected for overlap consistency. The parts of the images that overlap are forced to have the same background value via addition of an offset. Then a "fiducial frame" is derived. This is the definition for the output frame in terms of its physical size, projection, and orientation. Because IRAC has such a large field of view, projection effects are non-negligible, and the mosaicking and coadding process must reproject the data. The fiducial frame finder seeks to minimize the amount of "blank" area in the output mosaic by rotating the output projection such that it is aligned with the map axes. This is useful for long thin maps, where potentially the output mosaic could be very large, but with a great deal of empty space. The mosaicker then reprojects all of the input data onto the output projection. It reads the SSC WCS, which contains the field pointing center, rotation, scale, and instrument distortion, and reprojects this onto a standard TAN FITS projection. In the process, the data are undistorted. The reprojected images are interpolated onto the fiducial image frame with outlier rejection, rejecting radiation hits that happen in overlapping observations. The outlier rejection scheme is specifically designed to work well in the case of intermediate coverage, and may not be adequate for all observations and science programs. In addition to a sky map (in units of surface brightness), a noise image and coverage map are also produced. The post-BCD pipeline modules have been made available for general public use by the SSC. They consist of a number of C-modules connected via PERL wrapper scripts. Namelists are used for input. In most cases their operation simply consists of supplying the software with a list of input image files; by default they read and understand the IRAC image headers.
Graphical representation of data pipeline and products
For more information, see the IRAC chapter of the SOMGo back to IRAC Data Products page
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help@spitzer.caltech.edu http://ssc.spitzer.caltech.edu/irac/products/overview.html
