LASCO Processing Levels

TELEMETRY

QUICK LOOK

The output from the instrument is merged into the SOHO telemetry stream, transmitted to the DSN and then delivered to the LASCO work stations (IWS) at the EOF. For about 12 hours per day, SOHO data are received by the IWS in near real time, and for the remaining time, the data are received with a few hours' delay. Every day the previous day's real-time and playback raw data are ftp'd from the EOF. Both of these data sets are designated quick look.

LEVEL 0 ("LZ")

The various ground stations of the DSN record the data onto tape and mail the tapes to the IPD at GSFC. These tapes are processed by NASA /IPD and then sent to the LASCO team several months after the data were collected. The Level Zero ("LZ") data have fewer missing data packets (blocks) than the Quick Look data.

LEVEL 0.5 (FITS)

Raw data (quick look or LZ) are processed to separate the telemetry stream into compressed image files. These files are then decompressed, oriented so that solar north is at the top of the image, and then written out as a FITS file. They have been "rectified" to put solar north at the top of the image. This means that rows and columns may have been transposed, depending upon the readout port and the orientation of the telescope on the spacecraft. Calibration images (calibration lamp images, dark current images, or pseudo-continous readout images) are not rectified. Note that SOHO is maintained so that the vertical axis is perpendicular to the solar equator, however there is a slight roll angle offset of each of the LASCO telescopes from solar north. Level 0.5 includes no corrections for instrument response, stray light, etc., and are in units of DN (digital number or counts).

LEVEL 1 (FITS)

Level 0.5 images may be calibrated into physical units (e.g. mean solar brightness for C1 and C2). The calibration involves corrections for the flat field response of the detector, radiometric sensitivity, stray light, geometric distortion, and vignetting (see the LASCO Calibration Page). Level 1 data are suitable for quantitative scientific analysis. For every Level 1 image there is a single Level 0.5 image. While the Level 1 calibration can be applied to Level 0.5 derived from quick look data, it is generally applied only to the LZ-derived FITS files. The calibration is still not final; however, it is possible to obtain calibrated images using reduce_level_1.pro in the LASCO section of the SolarSoft IDL library. Please contact us if you wish to use calibrated LASCO data.

LEVEL 2 (FITS)

Processing involving multiple images, such as a polarization, color analysis or on/offline combinations, are considered Level 2 processing. Such processing may involve grouping images by time, color filter, polarizers or by wavelength. The Level 2 products are produced directly from Level 0.5 FITS files for a subset of images.