Deriving HRC-S Degap for XRCF Settings

Flat Field Data

Flat field illumination of the HRC-S was performed in the HRC lab on 1997 May and June (after the XRCF calibration). During the period May 26 to June 1 the HRC-S was operated with the MCP HV and trigger thresholds set at the levels used during the XRCF calibrations. The data were captured using the flight electronics and a spacecraft interface simulator that would allow data rates ten times greater than in flight. The raw data files contain records consisting only of packed MCP event data. Only a single segment of the HRC-S could be illuminated at any one time.

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 fileSegmentX-ray lineX-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.

Projection: HRC-S
U-axis Undegapped Flat-Fields at XRCF Settings

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.

Projection: HRC-S
V-axis Undegapped Flat-Fields at XRCF Settings

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.

Quadratic Degap Coefficients

Quadratic degap coefficients will not only stretch the data to fill in the gap but will stretch by different fractional amounts relative to the center of the tap to redistribute any peaking in the raw distribution. The two figures below show the result of applying a fixed set of quadratic degap coefficients (U-axis: a = 1.18, b = -0.16, V-axis: a = 1.11, b = -0.10). These coefficients have been used as an interim set for the processing of XRCF data.

HRC-S U-axis Quad-coefficient Degapped Flat-Fields at XRCF Settings

HRC-S V-axis Quad-coefficient Degapped Flat-Fields at XRCF Settings

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.

HRC-S U-axis Quadratic Degapped Flat-Fields at XRCF Settings

HRC-S V-axis Quadratic Degapped Flat-Fields at XRCF Settings

Back to "HRC Degapping Corrections"

Dr. Michael Juda
Harvard-Smithsonian Center for Astrophysics
60 Garden Street, Mail Stop 70
Cambridge, MA 02138, USA
Ph.: (617) 495-7062
Fax: (617) 495-7356