The following table lists the HRC-S flat-field illumination data files. The illuminated HRC-S segment is indicated The segements are labeled "+1", "0", and "-1" based on their relative positions in spacecraft coordinates. The +1 segment covers coarse positions 0-63, the "0" segment covers 64-127, and the "-1" segment covers 128-192 (see the HRC focal plane layout).
|Raw data file||Segment||X-ray line||X-ray energy|
Below is a plot of the U-axis projection of these combined flat-fields in undegapped coordinates. The active part of the detector spans coarse positions from 2 to 13. The area from the low coarse position edge to the center of coarse position 4 was occulted so that the high-energy suppression filter would not be illuminated (reflections from it would have made the flat-field data more confusing). The gradient in the histogram from the center of coarse position 4 to the gap between 6 and 7 is caused by the relative misalignment between the occulting plate and the detector. The sharp discontinuity that occurs on coarse position 10 is due the the change in the UV/ion shield transmission in the low-energy suppression filter region.
It is clear from this projection that the assumption of flat illumination/response is not valid. The strong peaking of the distributions toward the center oef the coarse position cannot be corrected with a linear degap coefficient.
The following figure shows the V-axis projection in undegapped coordinates. The individual HRC-S MCP segments are plotted in the three panes of the figure. The step and spike discontinuities at the low coarse position end of segment +1 and the high coarse position end of segment -1 are due patterning of the photocathode, similar to that around the perimeter of the HRC-I.
The V-axis distributions also tend to peak toward the center of the coarse position. This peaking cannot be smoothed out with linear degap corrections.
Even a cursory inspection of these projections reveals the need for improvement in the coefficients. No set of fixed coefficients will perform adequately; position dependent degap coefficients are needed.
A set of position dependent degap coefficients can be derived from the raw (undegapped) projections. The HRC UNIX Tools Data Analysis System tool nmkdegap takes a projection and generates a file with quadratic degap coefficients. The figures below show the result of applying the quadratic degap coefficients derived this way to the HRC-S "XRCF" flat-field data set. The degap coefficients used are given here.
Back to "HRC Degapping Corrections"
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