\hypertarget{chapter3}{}
\chapter{X-Ray Telescope Instrument Guide}
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% XRT System Overview
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\section{XRT System Overview}
The XRT includes a single Wolter I X-ray optic and a companion 
visible light optic. The optics are supported at one end of a 
telescope tube. At the opposite end are two filter wheels with 
analysis filters, the main shutter, and a camera. There is an 
additional set of heat rejection filters in front of the optics, 
and a visible light shutter that allows light through the visible 
light optic. A focus mechanism controls the spacing between the 
optical elements and focal plane. See~\citet{Golub07}.

The Mission Data Processor (MDP) coordinates the operation of 
the XRT and the CCD camera, in addition to the other components 
on the satellite. Camera image data is delivered directly to 
the MDP and is processed there. The only electrical connection between the XRT and 
the camera is the CCD\_EXPOSE line, which initiates an exposure.
\section{Telescope Performance}

\begin{table}[H] \label{xper}
\centering
\caption{\textbf{X-Ray Performance}}
\rowcolors{1}{gray!35}{}
\begin{tabular}{p{2in}p{3in}}
%\multicolumn{2}{c}{\textbf{X-ray Performance}} \\
\midrule
\T Focal Length \B &  2707.2 mm $\pm$ 0.3 mm \\
\T Aperture Size \B & 34.1 cm\\
\T Bandwidth \B & 0.2 to 1.2 keV  \\
\T Image Performance \B &  68\% encircled energy within 27 micron at 0.523 keV at the focal plane\\
\T Field of View \B & $35.11^{\prime} \times 35.11^{\prime}$ \\
\T Visible Light Suppression \B & $10^{11}$ \\
\T Effective Area \B & $>$ 2.2 cm$^2$ at 0.523 keV \\
\end{tabular}
\end{table}

\begin{table}[H]\label{oper}
\centering
\caption{\textbf{Optical Performance}} 
\rowcolors{1}{gray!35}{}
\begin{tabular}{p{2in}p{3in}}
%\toprule
%\multicolumn{2}{c}{\textbf{Optical Performance}} \\
\midrule
\T Focal Length \B &  2708 mm $\pm$ 5 mm (adjusted to focus at the X-ray focal plane) \\
\T Central wavelength \B& 4305 \AA $\, \pm 20$ \AA~(G-Band) \\
\T Bandpass \B & $< 170$ \AA~FWHM \\
\T Resolution \B &  $< 2$ arcsec half power diameter\\
\T Field of View \B & $>$ 30 arcmin \\
\T Co-alignment \B & 17 arc seconds\\
\T Confocality with X-ray optic \B & $\pm$ 150 micron \\
\end{tabular}
\end{table}
\section{MDP/XRT Communications}
Communication occurs using a set of discretes (MDP-to-XRT signals) 
and bilevels \\(XRT-to-MDP signals), as well as serial commands 
sent from the MDP and status packets sent from the XRT. Discretes
and bilevels propagate essentially instantaneously on the 
time-scale of interest. Serial commands consist of status requests, 
regularly spaced at 500 ms 
intervals, and other commands, interspersed between status requests, 
that perform specific actions. Status requests act as a heartbeat, 
resulting in receipt of a similarly spaced stream of status packets. 
\section{CCD Camera System}
XRT uses a back-illuminated three-phase CCD with 13.5 $\mu$m 
pixel-size and 2048$\times$2048 array, which was manufactured by 
E2V Technologies~\citep{Kano08}. The CCD has two identical read-out ports; 
R-port and L-port. XRT uses R-port as the default port, and L-port 
as a backup. From either port, an entire CCD image can be read. 
Characteristics of the camera are described in the table below.\\
\begin{table}[H]\label{ccd}
\centering
\caption*{\textbf{CCD Specifications}} 
\rowcolors{1}{gray!35}{}
\begin{tabular}{p{2in}p{3in}}
%\toprule
%\multicolumn{2}{c}{\textbf{CCD Specifications}} \\
\midrule
\T CCD Type \B & Back-illuminated CCD (E2V/CCD 42-40) \\
\T Pixel Format \B & 2048$\times$2048 pixels \\
\T Pixel Size \B & 13.5 $\mu \mathrm{m}$ $\times$ 13.5 $\mu \mathrm{m}$ \\
\T Field of View \B & 35$\times$35 arcmin \\
\T Pixel Binning Mode \B & 1$\times$1,  2$\times$2, 4$\times$4, 8$\times$8\\
\T Dark Current \B & 0.1 e$^{-}$/sec/pixel at -65$^{\circ}$C \\
\T CCD Temperature \B & Passive cooling: $< -43^{\circ}$C \\
\T CTE \B & Parallel $>$ 0.999996, \hspace*{10pt} Serial $>$ 0.999999 ($-93^{\circ} \mathrm{C} < \mathrm{T} < -50^{\circ} \mathrm{C}$)\\
\T QE (X-rays/EUV) \B & 0.93 at 13 \AA, 0.61 at 45 \AA, 0.46 at 116 \AA,  0.56 at 304 \AA \\
\T QE (Visible Light) \B & 0.44 at 4000 \AA, 0.66 at 5000 \AA \\
\T Full-well capacity \B  & 2.0$\times$10$^{5}$  e$^{-}$ \\
\T Camera Gain Constant \B & 57 e$^{-}$/DN \\
\T Camera System Noise \B & $<$ 30 e$^{-}$ \\
\T Output Data Resolution \B & 12 bit \\
\end{tabular}
\end{table}
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% Pointing
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\subsection{Pointing}
Hinode can point anywhere on the solar disk. XRT, SOT and EIS 
planners decide on the day's pointing every morning. Hinode has 
four pointing tracks; thus it is not capable of tracking more 
than four objects over the course of one day. 

XRT can offset its pointing from the spacecraft pointing using Region
of Interest (ROI) tables. These tables allow the XRT Chief Observer to
construct rectangular field of views, though the smallest
scientifically valuable field of view is 256$\times$256. The amount
XRT can offset depends on the size of the image being read out from
the CCD. If reading out a 256$\times$256 image, XRT can offset up to
896 arc seconds from the Hinode pointing. Reading out images larger
than 256$\times$256 causes the offset amount to decrease. Reading out
the full 2048$\times$2048 CCD requires the XRT pointing to be equal to
the spacecraft pointing. 

Due to the position of the instrument on the spacecraft, XRT X-ray 
image pointings are inherently offset from the spacecraft pointing.
The XRT X-ray images are offset from the SOT pointing by 40.0 
arc seconds in the {\it x}-direction and 22.3 arc seconds in the 
{\it y}-direction. The XRT VLI images are offset from the XRT 
X-ray images by 32.4 arc seconds in the {\it x}-direction and 42.3 
arc seconds in the {\it y}-direction. These offsets vary in time 
due to the spacecraft orbit and jitter.
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% Exposures
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\subsection{Exposures}
XRT enables several algorithms to maximize the quality of
observations. 
They are succinctly described below.
\paragraph{Automatic Exposure Control} The Automatic Exposure Control 
(AEC) adjusts the exposure duration by analyzing the most recent
X-ray image on board in a pipe-line manner. AEC is available only 
for the image with size smaller than or 
equal to 256 k pixels (i.e. a 512$\times$512 pixel image). If an
X-ray image does not achieve the proper exposure with the shortest 
exposure, AEC automatically changes the X-ray analysis filter to a
pre-specified thicker filter.
\paragraph{Automatic Region Selector} The Automatic Region Selector 
(ARS) is the function to automatically update ROI targets. This 
function does not work during the flare observation or during the 
passage of radiation belts. There are two functions in ARS. One is
a global search to select the brightest region in the full frame of 
CCD and to update ROI1 table. The other is a local search to track a
bright region by searching only around the current ROI location, and 
it updates ROI2, ROI3, and ROI4 independently. 
\paragraph{Flare Detection Algorithm} Because Hinode is not equipped 
with any dedicated X-ray detectors to detect solar flares, XRT has 
to do so by itself. The Flare Detection (FLD) is the function to 
detect the occurrence of a flare, to identify the 
flare location on CCD, and to raise a flare flag not only for XRT 
but also SOT and EIS. XRT takes full frame CCD images with 8 arcsec 
resolution (called FLD patrol images) at regular intervals. The 
intervals during the normal observation and during the flare 
observation can be set independently in the FLD Control Table. The 
baseline interval to take FLD patrol images is about 30 sec. Each 
FLD patrol image is first divided into 16$\times$16 blocks, called 
macro-pixels. The macro-pixel image is created by summing the 
intensity in each macro-pixel. From a macro-pixel image, the MDP 
calculates a parameter  which indicates the increase of the X-ray 
intensity normalized by the photon noise; if this parameter 
exceeds the threshold for the flare start in more than one 
macro-pixel, the MDP sets the flare flag and proceeds to calculate 
the flare position. When this parameter is smaller than the 
threshold for flare end, the MDP drops the flare flag. 
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% XRT Mechanisms
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\section{XRT Mechanisms}
The X-ray Telescope is composed of entrance filters, mirror, focal 
plane filters, visible light shutter, visible light imager, focus 
mechanism, filter wheel and shutter assembly system. Detailed descriptions are available in~\citet{Golub07}. Short descriptions of the mechanisms relevant to those proposing joint observing programs are included below.
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% Visible Light Shutter
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\subsection{Visible Light Shutter}
The VLS is opened to admit visible light to assist in aiming the 
telescope and aligning X-ray images with optical images from the 
SOT, but must be closed during X-ray exposures. The fail-safe mode 
for the VLS is closed, since X-ray exposures can only be made with 
the visible light shutter closed. It passes no more than 10-10 of 
visible light when closed. A stepper motor opens (positive direction) 
or closes the shutter. Moving the shutter from open to closed requires 
about 165 steps. 
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% Visible Light Imager
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\subsection{Visible Light Imager}
The visible light imager (VLI) is mounted in the center of the Sun 
shield. X-rays pass through and are focused by the ring near the 
edge of the Sun shield. Visible light passes through the visible 
light shutter, if it is open, then through the visible light 
imager, and to the camera CCD array. 
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% Focus Mechanism
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\subsection{Focus Mechanism}
After calibration, a position of 0 places the mechanism in the
middle of the range. In this position, there is no stress on the 
CCD array. The stepping rate is approximately 300 actual steps/s. 
It takes about 67 s to move from one extreme to the other. 
Each 200 steps of the stepping motor corresponds to a single 
revolution of the motor shaft, and 40,000 steps (200 revolutions) 
are necessary to complete a cycle of the geared down cam. One half 
cycle, 20,000 steps, moves the focus rod from one focus extreme to 
the other, a range of $\pm$2.433 mm. This is converted to $\pm$1 mm 
camera deflection. There is a magnetic detent every fourth step. For this reason, position 
tracking remains reliable over time only if positions are specified 
in multiples of four focus motor steps. A focus step is defined as 
four motor steps. The focus odometer counts motor steps.
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% Filter Wheel and Shutter Assembly
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\subsection{Filter Wheel and Shutter Assembly}
The FSA subsystem consists of two filter wheels and the focal 
plane shutter, and the associated controllers. The focal plane 
shutter (FPS) is a disk containing narrow, medium and wide slots, 
shown here with the motor behind. The narrow (1.8$^{\circ}$) and 
medium (14.4$^{\circ}$) slots are swept over the camera aperture at 
a constant rate to make exposures. That is, the motor accelerates 
to a maximum speed before the slot is reached, then begins braking 
after the slot has passed. For wide (80$^{\circ}$) exposures, the 
shutter may dwell in the open position as needed to complete the 
exposure. Thus the shutter brakes to a halt in the center of the 
wide slot, then reaccelerates to complete the pass. The shutter 
rotates either clockwise or counterclockwise to perform an exposure, 
so it is necessary to skip an intermediate slot only one third of 
the time. Exposure timing is somewhat asymmetric depending upon 
direction of motion. The shutter is not completely opaque to X-rays. 
The CCD is swept whenever it is not exposing to flush accumulated 
charge. A brushless DC motor operates the shutter. Timing marks 
placed at 20$^{\circ}$ intervals determine the resolution with 
which shutter position is known to the controller. 

The FSA subsystem operates two independently-positioned filter 
wheels. Each filter wheel has six slots, one of which has no 
filter. The filters provide a temperature resolution of 
0.2 log T, 6.1 $<$ log T $<$ 7.5. They are numbered as shown
in \hyperlink{Fig3_1}{Figure~\ref{Figure3_1}}. 
 
Filter wheels require nearly 1.5 s to switch between adjacent 
positions, and thus can make up the longest part of an expose 
cycle. To minimize positioning time, select exposure order to 
reduce motion and/or use the expose command for filter wheel 
pre-positioning. Filter wheels are positioned sequentially, 
not in parallel, so time is additive (though typically both 
filter wheels are not used in the same exposure).
\begin{figure}[ht] \label{fws}
\hypertarget{Fig3_1}{}
\centering
 \begin{subfigure}[b]{0.45\textwidth}
\includegraphics[width=\textwidth]{FW1.pdf}
\caption*{Filter Wheel 1}
\end{subfigure} \quad
\begin{subfigure}[b]{0.45\textwidth}
\includegraphics[width=\textwidth]{FW2.pdf}
\caption*{Filter Wheel 2}
\end{subfigure}
\caption{XRT filter wheels as viewed from the sun.}
\label{Figure3_1}
\end{figure}