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% Name:
% memo_mean_bias_1.0.tex
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% History:
% 29 May 07, created (v1.0), Glenn E. Allen
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\documentclass{article}
\usepackage{cxo-memo-logo}
\usepackage[dvips]{graphics}
\usepackage{gea}
\begin{document}
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% 1. Header
\memobasic{
Jonathan McDowell, SDS Group Leader }{
Glenn E.\ Allen, SDS }{
Mean biases }{
1.0 }{
http://space.mit.edu/CXC/docs/docs.html\#mean\_bias }{
/nfs/cxc/h2/gea/sds/docs/memos/memo\_mean\_bias\_1.0.tex }
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% 2. Introduction
\vspace*{0.3in}
%
Anomalous features have been identified in many ACIS bias images. Events
that occur on pixels adversely affected by an anomaly can have inaccurate
pulse-height amplitudes and grades. To address this issue, the ACIS
instrument team created mean bias images for each CCD at a focal-plane
temperature of $-$120~$^{\circ}$C.
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% 3. Processing
\section{Processing}
\label{proc}
To construct a bias $B'$ from a mean bias $\bar{B}$ and an
observation-specific bias $B$, perform the following steps for each node $n$
of each CCD $c$:
\begin{enumerate}
\item
Compute the median value $\bar{O}_{nc}$ of the 256 columns and 1024 rows
of the mean bias $\bar{B}_{nc}$. Exclude pixels that have a bias value
of 4094 and 4095 from this calculation.
\item
Compute the median value $O_{nc}$ of the observation-specific bias
$B_{nc}$. Again, exclude pixels that have a bias value of 4094 and 4095
from this calculation.
\item
Set the estimated bias value
%
\begin{equation}
B'_{nc} = \bar{B}_{nc} - \bar{O}_{nc} + O_{nc}.
\end{equation}
%
\item
Round $B'_{nc}$ to the nearest integer to avoid introducing a systematic
offset of $-$0.5 adu when the real-valued bias $B'$ is converted to an
integer image by truncating the decimal portions of the numbers.
\item
Write the bias $B'$ to a file as an array of two-byte integers. Also
write the four values of $O_{nc}$ to the keywords INITOCLA ($n=0$),
INITOCLB (1), INITOCLC (2) and INITOCLD ($n=3$).
\end{enumerate}
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% 4. Finish
\end{document}