This section describes the basic data products the user will receive from the Spitzer Archive.
The data delivery consists of a directory hierarchy with a name unique to that AOR. In this hierarchy are the Level 1 (Basic Calibrated Data, BCD), as well as a number of subdirectories containing the Level 0 (raw data), the calibration files, log files, and the Level 2 (post-BCD) data. The exact contents of the data delivery vary according to what the user has requested from the Spitzer Heritage Archive.
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, post-BCD).
6.1 Level 0, or Raw Data
Raw data are wholly unprocessed, except for those steps necessary to render them into a readable FITS format, i.e., depacketization and decompression. They are, however, supplied in the event that users wish to reprocess their data in a different manner from the pipeline processing. 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.
6.2 Level 1, or Basic Calibrated Data (BCD)
BCDs are exposure-level data after having passed through the pipelines. Instrumental signatures have been mostly removed, and the BCDs have been absolutely calibrated into physical units (i.e., MJy/sr = 10-17 erg s-1 cm-2 Hz-1 sr-1). 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 that was 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 adversely by a known effect. 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.
6.3 Calibration Files
For each BCD, the pipeline calibration server generated several estimates of the current detector characteristics. These calibration files are supplied to the user. Users are also able to request from the Archive the files that were used to generate, e.g., 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.
6.4 Level 2, or Extended Pipeline Products (Post-BCD)
Pipeline processing of Spitzer data by the SSC also included more advanced processing of many individual data frames together to form more “reduced” data products. Known by the generic title of “post-BCD” processing, this extended pipeline refined the telescope pointing, produces mosaicked images and combined spectra.
6.5 Calibrated Data Units
The BCD product, which is the primary data provided to users after pipeline processing, consists of individual frames where the pixel values are in units of MJy/sr. Jansky is a flux density unit defined as:
1 Jansky = 1 Jy = 10-26 W m-2 Hz-1 = Fν (6.1)
The conversion between Jansky and flux density in W m-2 per unit wavelength is accomplished via
Fν x 10-26 x c/λ2= Fλ (6.2)
where the wavelength bin-width is specified in the same length units as λ and c. For example, if c is taken as 3x1014 μm s-1 and λ is specified in microns, the above equation results in Fν being in units of W m-2 μm -1.