The Chandra X-ray Observatory

Destreaking ACIS Data

Introduction

There is a flaw in the serial readout of the ACIS chips, causing a significant amount of charge to be randomly deposited along pixel rows as they are read out. Although not much coincidence is expected for low-rate/low-exposure sources, ACIS-S4 (ccd_id=8) is significantly affected by this problem. The destreak tool detects coincidence of events in adjacent pixels along a row (i.e. the serial read), flags probable streak events, and (optionally) removes them.

The destreak tool became part of standard data processing (SDP) at software version DS 7.3.0. It is applied by default to the S4 chip for all ACIS observations. In SDP, the tool is run such that events are only tagged in L1 and filtered in L2; the L1 products in the archive, therefore, still contain the streak events, so users can retrieve that information if desired.

Background Information

On account of the serial readout problem, the event file shows a variable pattern of linear streaks. For the purpose of this tool, a "streak" is defined as any CHIPY row which contains more than M events in a single frame time in a single CCD node. By default, M=1, which is to say that any row containing 2 or more events on a single node in a single frame time is identified as a streak. This default should be fine for low-rate data; more details are given in the destreak memo.

The tool should always be used with caution because it removes counts from any source sufficiently bright to generate multiple events in a single row of one CCD node within a single frame time. Bright point sources are certain to be affected.

Applying destreak to the ACIS-S4 chip is justified when the source count rate per row is small compared to the streak event rate. It is generally most applicable to grating observations of faint point sources and imaging observations of extended, low-surface brightness objects.

Examine the Streak Events

To determine whether significant source counts are being removed, examine the spatial distribution of streak events flagged by destreak.

First, run destreak with filter=no. This flags the streak events in the output file, but does not remove them:

unix% destreak acis_evt2.fits acis_destreak.fits filter=no

Next, filter the data to select all events with status bit 15 marked as "bad" (1) and all other bits set as "good" (0); note that the bits are counted from the right, starting at zero. This filter retains only those events that have been flagged by destreak:

unix% dmcopy \
      "acis_destreak.fits[EVENTS][status=00000000000000001000000000000000]"\
      streak_events.fits

unix% ds9 streak_events.fits

If the events are distributed across the field, as shown here, it is fine to filter out the events. However, if the source is visible, raise the streak event threshold (max) and run the tool again. The default threshold value is appropriate for low count-rate sources, but higher threshold values may be required for brighter sources.

VFAINT mode

If you have run acis_process_events with check_vf_pha=yes, destreak will not be as efficient at detecting (and removing) streak events because of the large number of nonzero status bits set by the background-cleaning algorithm. Follow the recommendations in the Clean ACIS Background in VFAINT Mode thread to destreak this data.

Continuous Clocking Mode

In continuous clocking (CC) mode, all CHIPY information is lost, so the streaks off the spectral trace will contaminate the spectrum. Therefore, there is contribution from a much larger area. In real CC-mode data, streaks have been known to cause heavy corruption; running destreak with the default parameters cleaned up the data quite a bit.

Although the destreak parameters have not been optimised, the tool is able to produce diagnostic information for a specific source. One should be able to estimate the expected coincidence given an approximate source spectrum and the value of the EXPTIME keyword (not EXPOSURE); EXPTIME is the frame time in TE mode or row transfer time in CC mode.

The destreak help file contains information about the statistics files which can be written out (see "ahelp destreak"). These files can be used to see if you are on the Poisson source counts region or on the noise tail. The destreak memo has detailed information on how to determine the allowable contamination limit for a particular observation; also see the max parameter in the help file.