The standard data processing (SDP) of Chandra data is divided into several stages or "Levels" (see the level ahelp page). At the end of each level, a well-defined set of data products are made and are used as the basis for the next level of processing.

L0

takes raw Chandra spacecraft telemetry and splits it into convenient FITS files.

L0.5

divides the telemetry on observation boundaries.

L1

applies instrument corrections to produce "event files" with aspect corrected positions for each event.

L1.5

(only for grating data) uses the position of the zeroth-order image to produce corrected positions for the dispersed counts (which can only be identified at this level). This removes blurring due to rotation.

L2

filters the event file to good time intervals (GTI), produces a candidate source list, and - if a grating observation - extracts a dispersed spectrum.

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.