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ASCA Guest Observer Facility

SIS Imaging Mode Explanation

FAINT: The energy of each event in a FAINT mode datafile, is characterised by a 3 x 3 pixel grid centered on the brightest pixel of the event. As such, FAINT mode data offer the most complete description of events and the potential for realizing the maximum spectral resolution. FAINT data have 4096 channels.

BRIGHT: Data taken in FAINT mode occupy a lot of telemetry. In fact, it is often the case that FAINT mode cannot be telemetered without saturation. BRIGHT mode is derived from FAINT mode and is more compact. Instead of using nine numbers to characterize each event, BRIGHT mode uses two: a single PHA value and a grade to describe the shape of the event. In addition, BRIGHT mode data have 2048 channels rather than 4096.

BRIGHT2: In a sense, all SIS imaging data start off as FAINT mode, and whether they are telemetered as FAINT or BRIGHT depends on telemetry constraints. However, in order to perform conventional spectral analysis, each event must have a single PHA value. This is already the case for BRIGHT mode data, but FAINT mode data have to be converted somehow. Users have two options. They may either use BRIGHT data which have been converted from FAINT data using the identical algorithm as used on the spacecraft, or they may use BRIGHT2 data which have been converted from FAINT data in a different way. Both conversions are performed by the FAINT FTOOL, and are part of the current version of standard processing. There are advantages and disadvantages to both kinds. If FAINT data are converted to BRIGHT, they can be combined with true BRIGHT mode data (i.e., data originally telemetered in BRIGHT mode). For a typical observation done in a mixture of FAINT and BRIGHT modes this provides the largest homogeneous dataset to work with. The conversion to BRIGHT2 mode, on the other hand, makes use of information in the original FAINT data (and not present in BRIGHT data) to perform two small corrections: the event-by-event echo correction and the DFE correction. The resulting BRIGHT2 data have a more accurate energy scale but cannot be combined with "true" BRIGHT data because they require different responses. BRIGHT2 data have 4096 channels.

DFE: The Dark Frame is the "zero-level" of the energy scale with respect to which PHA values are measured. The Dark Frame Error (DFE) is the offset in this level. It is variable: during satellite night it is typically ~3 ADU, corresponding to ~10 eV, but in daytime it can rise to ~20 eV. For FAINT mode datafiles, the FTOOL FAINTDFE calculates the DFE which is then used by the FAINT FTOOL in the conversion from FAINT mode to BRIGHT2 mode. Typically, the DFE is much smaller than the statistical uncertainties in the data. Users can therefore perform their SIS analyses entirely on BRIGHT mode data without loss of accuracy. However, once you have derived a result by fitting a BRIGHT spectrum, it is prudent to make a corresponding spectrum from BRIGHT2 data to test whether your result is sensitive to DFE.

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This file was last modified on Monday, 13-Sep-1999 15:37:08 EDT

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