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.
  
  

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  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

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