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% SAMPLE1.TEX -- ADASS XII (2002) ASP Conference Proceedings sample
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\documentclass[11pt,twoside]{article}  % Leave intact
\usepackage{adassconf}

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\begin{document}   % Leave intact

%-----------------------------------------------------------------------
%			    Paper ID Code
%-----------------------------------------------------------------------
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\paperID{P10-8}
%%%% ID=P10-8

%-----------------------------------------------------------------------
%		            Paper Title 
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% Enter the title of the paper.
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\title{Maintaining Software for Active Missions: a Case Study of Chandra's CTI Problem}
\titlemark{Maintaining Software : a Case Study of Chandra's CTI}
%\titlemark{ }

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%		          Authors of Paper
%-----------------------------------------------------------------------
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\author{J.\ S.\ Masters, H.\ He, W.\ McLaughlin, K.\ Glotfelty}
\affil{Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138}
\author{G.\ Allen}
\affil{MIT Center for Space Research, Cambridge, MA 02139}

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\contact{Joseph Masters}
\email{jmasters@cfa.harvard.edu }

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\paindex{Masters, J.}
\aindex{He, H.}
\aindex{McLaughlin, W.}
\aindex{Glotfelty, K.}
\aindex{Allen, G.}

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%                     Author list for page header
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% Please supply a list of author last names for the page header. in
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\authormark{Masters, He, McLaughlin, Glotfelty \& Allen}

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%			Subject Index keywords
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% Enter up to 6 keywords describing your paper.  These will NOT be
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\keywords{Chandra, CTI, ACIS}

%-----------------------------------------------------------------------
%			       Abstract
%-----------------------------------------------------------------------
% Type abstract in the space below.  Consult the User Guide and Latex
% Information file for a list of supported macros (e.g. for typesetting 
% special symbols). Do not leave a blank line between \begin{abstract} 
% and the start of your text.

\begin{abstract}          % Leave intact
During   early   flight   operation,   the   Chandra   X-ray
Observatory's  Advanced   CCD  Imaging  Spectrometer  (ACIS)
suffered degradation  of charge transfer  inefficiency (CTI)
due  to   radiation  induced  charge   trapping.   The  ACIS
front-illuminated  detector CTI  has  significantly degraded
over time with respect to the intrinsic energy resolution of
the chips.   In this presentation we use  the degradation of
CTI in  the ACIS  camera as an  example of how  software was
adapted  to  fix a  significant  and unforeseen  post-launch
problem in on-board hardware.  We will discuss the impact of
CTI on  events, RMF and  gain files.  Then, we  will discuss
the software updates  necessary to alleviate the degradation
effects  and to  improve spectral  resolution.   Finally, we
will  highlight  other  ACIS  hardware  problems  and  their
corresponding software  solutions.
\end{abstract}

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%			      Main Body
%-----------------------------------------------------------------------
% Place the text for the main body of the paper here.  You should use
% the \section command to label the various sections; use of
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% automatically. 

\section{Introduction}


As hard as engineers may try to foresee and hardware
problems  leading   up  to  the  launch   of  a  space-based
telescope,  these problems are  nearly impossible  to prevent.  For an
unserviceable mission such as the Chandra X-Ray Observatory,
the issue of hardware  problems takes on greater significance
for the success  and ultimate fate of the  mission. Even for
serviceable  missions, to  fix  a problem  with software  is
usually  more  cost effective  than  replacing or  repairing
on-board hardware.

\subsubsection{CTI and Chandra}

When either X-rays or cosmic  rays deposit charge in an ACIS
CCD, the deposited charge is  read out from one of four sets
of read-out  nodes (256 x  1024 pixels).  Because  charge is
read out at  only one location on a node,  the charge at all
other locations  must be moved  to the read-out.   Charge is
moved  both  vertically  (i.e.  in  the  negative  CHIPY  or
``parallel''  direction)   and  horizontally  (i.e.    in  the
positive  or  negative  CHIPX  or ``serial''  direction).   As
charge is moved,  some may be lost to  charge traps that are
distributed across the  detector. The mean fractional amount
of charge  lost per pixel  transferred is called  the charge
transfer inefficiency (CTI).

Shortly  after the  launch of  the Chandra  Observatory, the
ACIS  detector  (one  of  two primary  science  instruments)
suffered   severe  degradation   to   its  charge   transfer
inefficiency due to soft proton damage, resulting in loss of
energy resolution  and the  elimination of some  events that
would otherwise be considered good.

\begin{figure}
\epsscale{1.0}
\plotone{P10-8_graphic.eps}
\caption{Energy plots across a CCD before (left) and after (right) the CTI correction.}
\end{figure}

\section{Data Impact of CTI}

The increase in CTI had a significant impact on the quality
of Chandra science data.  Listed below are some of the
effects and how they were caused by CTI.

\subsubsection{Grade Migration}

When  charge is  captured by  a  CCD the  energy is  divided
within an ``event island'', usually  3 pixels by 3 pixels.  The
distribution  of  charge within  the  island determines  the
GRADE of the  event.  When the event is  read out across the
detector,  its  charge  is  ``smeared out''  in  the  read-out
direction  due  to  the   effects  of  CTI.   This  smearing
redistributes some of the charge and may change the GRADE of
some events.

\subsubsection{Detection Efficiency}

When event GRADEs with ``good'' values change to ``bad'',
the  result is  that the  events are  excluded from  Level 2
event  files  and there  is  an  apparent  reduction in  the
detection efficiency of a CCD.

\subsubsection{Gain Shift}

The amount  of charge  read out from  an event will  be less
than the amount of  charge deposited on the detector.  The
difference in charge will depend on the number of pixels 
crossed before reaching the read-out node.
Thus, the loss of charge at the top of the chip will be
greater than when charge deposited near the read-out node. The 
pulse-height  distribution for a source  will be shifted
to lower energies and result in an apparent gain shift.

\subsubsection{Spectral Resolution}

CTI also  causes degradation in  the energy resolution  of a
CCD.  The  pulse-height  distribution of line  features is 
broadened.  This spectrum shift is also spatially dependent,
being greatest at the furthest point from the read-out node.

\begin{figure}
\epsscale{0.7}
\plotone{P10-8_psplot.eps}
\caption{An energy spectrum before and after the CTI adjustment.}
\end{figure}

\section{Software Impact of CTI}

In order to resolve the impact of CTI on data, software
changes were made in several areas of the Chandra data
processing system.

\subsubsection{Tools}

The core  event processing  tool that computes  energies and
performs coordinate transformations was changed in order to
incorporate a correction  algorithm developed by teams  at Penn State
and  MIT.   The algorithm  estimates  the  amount of  charge
deposited  on a  CCD for  an event  based on  the  amount of
charge  read  out and  the  location  of  the event  on  the
detector.  New parameters were added to the tool to make use
of new calibration files and for controlling when to
implement the algorithm.  The CTI adjustment reverses nearly
all of the apparent gain shift and can significantly improve
the energy resolution of a detector. The grating event 
processing tool also had minor updates to handle new 
calibration files.

\subsubsection{Calibration}

The  calibration changes  were a  major portion  of  the CTI
correction  effort.   In  particular, the Redistribution
Matrix File (RMF) and gain tables needed to be re-calibrated to use
the CTI-corrected  data.  The  updates for  RMFs alone
involved  dramatically increasing  the number  of  input
Fits Embedded Function (FEF) 
files,  changing the  format of  the FEF  to  handle regions
within a node, creating  weighted RMFs as input for extended
sources, and developing a new method to produce
PI-RMFs to save time.

\subsubsection{Pipelines}

Ultimately all of the software changes were destined for the
standard processing pipelines.  Updating the pipelines meant
using  the  new  calibration  database and  performing  many
regression tests  to verify that the  correction was working
well in the context  of an automated processing environment.
The  software correction  for  CTI became  available to  the
Chandra user  community through standard  data processing on
March 6, 2003.

\section{Examples of Other ACIS Hardware Issues}

CTI is one issue affecting the quality of science data
produced the the Chandra X-Ray Observatory. Listed below are
several other hardware problems that have been or will be
fixed by software.

\subsubsection{Horizontal streaks}

Some ACIS  CCDs, the S4  chip of the S-array  in particular,
were found to have spurious horizontal streaks.  The streaks
were  not removed by  standard grade,  status, or  bad pixel
filtering.   A tool   was created  to remove
these streaks without  affecting the scientific significance
of the data.

\subsubsection{Vertical streak with bright source}

While  ACIS reads  out,  it is  still  taking data.  Photons
detected during the read-out are clocked out in the wrong row
and so  have incorrect CHIPY  values.  For a  bright source,
a streak appears all along  the column (on both sides of the
source, since some events are from the previous exposure).\altaffilmark{1}

A  software tool  was developed to  remove the  streak  from the
image for  cosmetic reasons and  so that it  doesn't affect
source detection.  The tool also retains the  streak
photons, which have more  accurate timing information and  are not affected
by pile-up.

\subsubsection{Quantum Efficiency (QE) at low energies}

The optical  filter above the ACIS camera  has experienced a
gradual  increase  of   a  contaminant  resulting  from  the
out-gassing  of solid adhesives  within the  telescope since
launch. As  a result, the sensitivity of  ACIS has gradually
decreased  at low  energies.  Most  likely in  the near future, the
instrument  will be  heated to  ``bake-off''  the contaminant,
then brought  back to the lower  operating temperature.  There
have  been  and will  continue  to  be  software changes  to
ameliorate this problem.

\altaffiltext{1}{http://cxc.harvard.edu/ciao/threads/acisreadcorr/}


% You can also add an acknowledgments section as indicated below.

\acknowledgments

This work  was supported by NASA contract NAS8-39073.

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%			      References
%-----------------------------------------------------------------------
% List your references below within the reference environment
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\begin{references}
\reference Townsley, L.\ K.\ , Broos, P.\ S.\ , Nousek, J.\ A.\ , Garmire, G.\ P.\  2000, \apjlett, 534, L139
\end{references}

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\end{document}  % Leave intact
