The Copernicus data sets are stored using the Flexible Image Transport System (FITS) file format. Each of the 3 different data sets described below contain a standard primary header followed by a binary table extension with no intervening primary array. As an example, the raw data set c084.pep has the structure:
Primary Array header: 1 record with 31 keywords data: 0 records Binary Table Extension header: 4 records with 139 keywords data: 1447 records
All the descriptive keywords such as coordinates, spectral type, magnitude, etc., as well as general comments and documentation, are contained in the primary header. The extension header contains only those keywords needed to define the structure of the binary table. Note however that each data set is different in that the raw files use the variable length array facility, the coadded scans contain fixed length vectors, and the spectral atlas binary tables contains only scalar values.
Raw Data Sets:Each of the 551 raw data files contain all the spectral scans for a given target. Information on one scan is stored as one row in the FITS binary table extension. Each row contains 40 fields of various data types and has a total length of 124 bytes. The length of the binary tables vary from 5 rows to more than 23,000 rows.
Because individual spectral scans vary in length and standard binary tables require fixed-length vectors, the raw data sets were created using the "variable length array facility". This format is described in the original binary table proposal and appendix B of the NOST FITS Standard, but has not yet been officially approved as a standard FITS format. Basically each row contains 4 variable-length vectors (i.e., counts, background, time, and background status) which are stored in a heap at the end of the binary table. The number of points and the byte locations in the heap are stored as scalar numbers within the binary table. The Raw Data Field Descriptions contains more information on the contents of the binary table.
The file names for the raw data sets are in the form "Cnnn.PEP" where nnn is the original Copernicus target number. As an example, C084.PEP contains all the scans for target number 84 (i.e., Gamma Cass).
Coadded Scans:U1 scans taken close in time and U2 scans taken close in time were coadded using the programs STACK and PSTACK. The scans were coadded primarily to improve S/N. The resulting coadded scans are stored in 1-row binary table files. The primary header contains the keywords from the raw data files plus additional information including wavelength range, number of scans coadded, start and end scan numbers, start and end exposure times, number of data points in coadded scans, and a listing of the precautionary comments associated with each of the individual scans.
The 1-row binary table contains 6 vector fields of various data types and are defined in the Coadded Scan Field Descriptions. The length of the rows vary between files.
The coadded scan file names are in the form Cnnn-### where nnn is the original raw data target number mentioned above, and ### is the coadded scan number for a given target stored sequentially with time. The extension is either .U2 for U2 scans, and .UU1 or .BU1 for unblocked or blocked U1 scans. (Blocking refers to the use of the U2 detector to block stray light that might enter the U1 vent hole. Blocked and unblocked scans were not coadded.)
Spectral Atlas Files:
Spectral atlases were produced for several targets and the data was published and stored at the National Space Science Data Center (NSSDC). These files were dearchived and reformatted as binary table FITS files. Each row of the spectral atlas file binary tables contains information for one wavelength. Therefore all fields contain only scalar data. See the Copernicus spectral atlas page for more information.