Chandra calibration review meeting - 10/31/01
HRC Event Filtering
-
Event processing in the HRC is limited to digitizing selected signals from the readout device and transmitting
these to the ground. As a result, it is possible to examine and screen data as part of processing. This
screening allows non-X-ray events to be identified, reducing detector background, as well as detecting and
filtering out "bad" events that might otherwise degrade image quality.
[see S. Murray memo (3/9/2000) and Murray et al., Proc. SPIE vol 4140, pp 144-154 (12/2000)]
- In automated processing, events filtering takes place between level 1 and level 2 data products,
screening on any of 32 status bits.
[Table of status bits] [Status Bit Definitions]
-
The status bit filters serve a valuable function in that they remove
events that are likely not the result of x-rays or are misplaced.
["jet rejection" comparison
in OBSID 1385]
HRC-I filtering
- Filtering of events of HRC-I laboratory flat field data
shows pronounced position- and energy-dependent loss of events,
mostly from the filtering of events flagged by the H-test and
Saturation test.
[Images of flat field ratios]
-
The change in HRC-I HV settings in October 1999 results in
far fewer events being rejected by the standard filters
than at the laboratory settings. Before 4 October 1999, 10-20% of source
events were removed by filtering, and a more modest 2-10% afterward.
[Graph comparing source filtering
at different HV settings]
- Background events prior to the HV change were reduced 60-80% by standard filtering. Presently,
about half of background events are eliminated by the standard filter.
[Graph comparing background
filtering at different HV settings]
HRC-S filtering
- HRC-S voltages are set so as to make saturated events far less likely than
with the HRC-I. There is no substantial loss of events due to filtering, as
there is in the HRC-I.
- AR Lac data from 12/2000 show that there is little spatial variation
due to filtering, and that while few source events are lost, background is
reduced approximately 10-25%.
[Graph comparing source
and background filtering]
- The selection of filters used in level-2 products represents
a good selection for most users and applications.
Some users may want to be able to select custom filters to be applied
to level-1 data depending on what their application (such as high
precision photometry or timing). There is always a trade-off: filtering
removes many background events, which is generally good, but
will also remove some x-ray events with spatial and energy-
dependent bias.
- Analysis of both HRC-I and HRC-S filtering is continuing.
Charles Wilton
Chandra X-ray Observatory Center
Harvard-Smithsonian Center for Astrophysics
60 Garden Street MS 31
Cambridge MA 02138
office: S 463 (Porter Exchange)
phone: 617-496-7959 fax: 617-496-7700
cwilton@cfa.harvard.edu