# Deriving HRC-I Degap for XRCF Settings

## Linear Degap Coefficients

Linear coefficients will shift all events away from the coarse
position by a constant fraction. These can be selected by generating
projections for a series of linear degap coefficients and then
selecting for each gap the coefficient that removes the gap. The
figure below shows the result of doing this for the HRC-I "XRCF"
flat-field data set. The degap coefficients are given
here. While the coefficients vary
from tap to tap, the average coefficient for the U-axis is 1.054 and
for the V-axis it is 1.037.

The resulting histograms show little evidence for residual
gaps. This set of coefficients have been used by the ASCDS for all
reprocessing of HRC-I XRCF data since August 1997. Expanded views of
a few sections of the histograms, comparing the raw (undegapped) to
the linear degapped data, are shown below (click on image to get full
view).

These expanded views show how the gaps have, in general, been
removed. However, these views also show that there are taps for which
the distribution is not flat. In some cases, notably the V-axis
projection from 25 - 35, the distribution peaks toward the center of
each tap. This would be improved by adopting quadratic degap
coefficients (see the corresponding projection image
here). The projection for U-axis
coarse positions 50 - 60 is more puzzling. The saw-tooth pattern along
the distribution is almost certainly not due to variations in the
illumination pattern and it is also difficult to understand why the
detector efficiency would show such a pattern. Additional examination
of the data reveal that the saw-tooth pattern is mainly produced by
events with the highest pulse-heights. There is most likely some
feature of the instrument electronics that is causing this systematic
effect. A linear degap coefficient will not remove this systematic
effect. A quadratic correction may be more successful. However, the
corresponding quadratic projection
still exhibits odd systematic behavior.

## Expanded views of the projections

Details of the quadratic degap coefficients can be found
here.

Back to "Deriving HRC-I Degap
for XRCF Settings (Flat Field Data)"

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*

*E-mail:
mjuda@cfa.harvard.edu*