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The CXC performs SDP, or "pipeline" processing, on all Chandra science data. This processing has several levels or stages. All data products from SDP undergo V&V.

Level 0 (L0):

takes raw Chandra spacecraft telemetry and splits it into convenient FITS files. The telemetry is then divided along the observation boundaries. These products should not be used for user-level analysis; if you feel there is a reason to revert to the L0 products, please email the Helpdesk for guidance.

Level 1 (L1):

takes L0 output and applies instrument-dependent corrections, such as the aspect solution. The L1 outputs have not had anything irreversible done to the data (e.g. no photon event rejection). These products are sent to the observer, but they are generally only needed when reprocessing the data manually.

Level 1.5 (L1.5; only applies to grating data):

determines the zero-order centroids for sources in the field. Those positions, along with the spectral region definitions, are then used to calculate diffraction coordinates for the dispersed photons (which can only be identified at this level.) The resulting products are a source list, an augmented event list, and a spatial extraction region; see the Analysis Guide for Chandra High Resolution Spectroscopy for specifics on grating processing.

Level 2 (L2):

takes L1 outputs (or L1.5 for grating data) and applies standard, but irreversible, corrections. (By "irreversible" we mean that information has been lost that cannot be regained from the L2 products alone.) This includes filtering the event file on the good time intervals (GTIs), cosmic ray rejection, and position transformation to celestial coordinates (RA, Dec). A candidate source list and "finished" event file are produced, as well as a dispersed spectrum for grating data.

Level 3 (L3):

derives higher level information from the L2 outputs. This includes more precise source detection and characterization (fluxes, morphology), plus cross-correlation with other catalogs.

The information from L3 processing is available to the public as the Chandra Source Catalog.

Once an observation has made it successfully through the pipeline, it is passed to the Verification and Validation (V&V) team. The products are checked by CXC scientists to ensure data quality and to investigate the cause of any exposure losses. Only after the data passes V&V is it released to the observer; the V&V report is included in the data distribution so that the user is aware of any potential data issues.

The processing version is recorded in the header keyword "ASCDSVER". CIAO is designed for use with data with a processing version of R4CU5UPD8 or higher. Note that the version naming convention changed after version R4CU5UPD14 to the "DS" system, starting with DS 6.0.0.

This FAQ entry shows you how you can use CIAO to find out the processing version of your data.

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