Residual dark distribution (RDD) is the error in ASCA's onboard correction for CCD dark current. RDD can broaden spectral lines, push low energy events below the detection threshold, and cause the wrong grade to be assigned to an event.
The Processing Team now produces monthly RDD maps which can be used with the correctrdd FTOOL to adjust FAINT mode data for RDD. These maps are available by anonymous FTP in the directory /asca/data/trend/rev2/rddmap on legacy.gsfc.nasa.gov.
For more on RDD and RDD correction see: The ASCA Data Reduction (ABC) Guide and the Ascanews #5 article by Dotani, et. al.
Values for 4 CCD mode are shown in blue, 2 CCD mode in green and 1 CCD mode are shown in red. Also, points for chip 0 are connected by a solid line, chip 1 by a dotted line, chip2 by a dashed line, and chip 3 by a dot-dashed line.
RDD is dependent on the temperature of the CCD. This plot only shows data taken while the CCDs were between -62 and -61 C. Nevertheless, many of the small "bumps" in the plot are due to temperature variations within this restricted range.
Note that RDD is largest for 4 CCD mode, and is generally considered unimportant for 1 CCD mode. Also, RDD tends to be greater for SIS1 than for SIS0. Radiation damage is causing RDD to worsen with time.
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