The Data Preparation includes all the EXSAS utilities that are necessary to produce data in a form ready for subsequent scientific analysis, starting from photon event lists and auxiliary satellite and calibration data files usually provided with the ROSAT Observation Dataset (ROD).
Photon filtering and photon binning -- hereafter indicated as Data Projection -- as well as data correction and background subtraction, are therefore typical tasks of this package.
Any photon filtering within specific time intervals, energy ranges and sky or detector portions can be easily performed via command lines or execution of command files. Logical operations among different photon lists can be performed as well.
Photons can be binned, depending on the available parameters, into light curves, spectra, profiles or images. Many options are available, such as irregular rebinning of spectra, physical suppression of time gaps in light curves, production of energy and true color images.
Binned data can be corrected for different instrumental effects. The correction procedures access appropriate instrument tables (such as the effective area as a function of energy and detector position) which are delivered with the ROD. Optionally the user may select different ones.
To allow fast access to the photons of a specific source, all photons in the original ROD delivered Photon Event Table are ordered in a scheme of small sky maps. Direct access of the relevant sky maps substantially reduces the photon filtering time. This becomes especially important when a Photon Event Table is very large (this file may contain up to millions of photons).
Also, any sequence of filters and logical operations on photon sub-lists is always applicable without reading the input photon list more than once.
For more complex handling of photon lists it is possible to prepare (ASCII) command files with the advantage that the execution of that command file implies a single reading of the input Photon Event Table.
The EXSAS Data Preparation package has been designed in a way that intermediate results are not necessarily written to disk: with a single sequence of commands (a Projection Pipeline) it is always possible to select different sub-lists of photons, to bin them according to the specified parameters, to correct them and to output the results into appropriate files. Moreover, it is possible to output more than one dataset into the same file (e.g., many spectra or light curves into different columns of the same table). Any spatial filtering and/or merging of photon lists, regardless of how complex, is always performed as a sequence of elementary selections concatenated by logical operators. The EXSAS headers always keep the history of the performed operations thus allowing subsequent tasks to use this information. Remember that only EXSAS commands read and update this header!
It is up to the user to exploit all the options which are available in the Data Preparation package. The next sections are meant to make this fundamental step in the data handling within EXSAS as straightforward and feasible as possible.