\section{{\bf Soft X-Ray Telescope (SXT)}}

\index{instrument description, SXT}
\index{SXT instrument description}

The following sections are intended as an extension of the ``Red Book'' paper which
describes the fundamentals of the Soft X-ray Telescope (SXT)\@.  We strongly recommend
that first time users read ``The Soft X-Ray Telescope for the Solar-A Mission''
which is in Solar Physics 136, pp 37-67.

Many sections in this chapter refer to the SXT Calibration Notes.  A partial set of these notes
can be found in the \$DIR\_SXT\_DOC directory.  Users should send a mail
message to the account specified in the preface of this document if they
want copies of the notes.

\subsection{Overall Response Function for SXT}

\input fig_sxt_area1.tex				% 4 panel plots
\input fig_sxt_area2.tex \index{effective area, SXT}	% 6 panel plots
\input fig_sxt_resp.tex \index{response function, SXT}
\input fig_sxt_ratios.tex \index{filter ratios, SXT}

The SXT response functions have been characterized for a solar thermal
spectrum as observed in each of the SXT analysis filters.  The SXT is
a broad-band instrument and is sensitive over a range of energies between
approximately 0.28 and 4 keV{}. The analysis filters have been carefully
selected so that their intensity ratios are diagnostics of temperature.

\subsubsection{CCD Camera Signal}
\index{CCD, SXT}

The SXT makes use of a front-illuminated, virtual-phase CCD to detect
both X-ray and visible photon fluxes.  While it is possible to operate
CCDs in a photon counting mode, the SXT CCD is operated in a
camera-integrating mode.  This means that the shutter is opened for a
short interval during which photon events are summed into the CCD's
pixels.  The amount of charge read out of the CCD by the camera
represents the energy in the photon flux which has been integrated in
each pixel, not the actual number of photons.  If the incident beam is
mono-energetic, then the amount of charge is directly proportional to
the photon flux.  However, since the Sun's X-ray flux is far from
mono-energetic, we must model the response of the CCD for a given solar
spectrum. 

When a photon is absorbed in the CCD it produces electron-hole pairs in
the silicon.  The number of photons, $N(\gamma)$ that produced the
charge or number of electrons, $N(\mbox{e}^-)$, which is read out of the
CCD is given by

$$N(\gamma) = {\lambda \over 12398.5 \,\mbox{eV}} \times 3.65 \,
\mbox{eV/e$^-$} \times N(\mbox{e}^-),$$ 
% 
where $\lambda$ is the incident photon wavelength (\AA) and 3.65
eV/e$^-$ is the energy required to produce one electron-hole pair in
silicon.  Note that this relation applies only to X-ray energies.  The
SXT CCD pixels have a full-well capacity of about 250,000 e$^-$ per
pixel.  \index{CCD, full-well}\index{full-well, SXT}

The CCD camera electronics contain analogue-to-digital converters.  In
this Guide the digital camera output is referred to as DN or Data \index{DN (Data Number), SXT}\index{SXT DN}
Number.  These values are read from the camera with 12-bit accuracy, but
in the case of X-ray images, are compressed to 8-bit values using a
pseudo-square root compression algorithm prior to being telemetered. 
Thus, when looking at raw SXT images, one should remember that the
intensity values are typically compressed.  The relation between the DN \index{compression, SXT}
values and the integrated charge or number of electrons is given by 

$$\mbox{DN} = N(\mbox{e}^-) / G + 13,$$
%
\index{CCD, digital offset}
where $G$ (e$^-$/DN) is the so-called gain constant and 13 is a
digital offset in the CCD camera for full resolution images (see Section~\ref{ccd_zero} for more information).  The value of $G$ was adjusted to be
about 100 e$^-$/DN prior to launch.  This value is a compromise to \index{CCD, gain}\index{gain, SXT}
enable adequate performance at both X-ray and visible light energies. 
The routine SXT\_DN2PH will convert SXT DN values into photon counts for \index{SXT\_DN2PH}
a user-supplied wavelength.  The digital offset is removed automatically
during dark frame subtraction (SXT\_LEAKSUB, SXT\_DARKSUB), since the
dark frame images also have the same digital offset. 

\subsubsection{SXT Effective Area}
\index{effective area, SXT}

The effective area of the SXT to X-rays is described more fully in SXT
Calibration Note 31 (J. R. Lemen, 11 Feb 93, Rev A) and also in
SXT Calibration Notes 30 and 25. 

The on-axis effective area of the SXT depends on four different factors: the double
entrance filter transmission, the mirror reflectivity, the analysis filter
transmission, and the CCD quantum efficiency (QE){}.  All these factors are
functions of wavelength and off-axis angle (see section~\ref{vignette}).  
Fig.~\ref{fig_sxt_area1}~(a) shows the double entrance filter
transmission, Fig.~\ref{fig_sxt_area1}~(b) shows the mirror reflectivity (including the
effects of obscuration by the support structures), and Fig.~\ref{fig_sxt_area1}~(c)
shows the CCD QE as a function of wavelength.  Fig.~\ref{fig_sxt_area1}~(d) shows the
product of Figs~\ref{fig_sxt_area1}~(a)--(c) and corresponds to the SXT effective area
when both filter wheel positions are ``open'' and sometimes referred to
in SXT documentation as ``noback'' (for no back filter). Fig.~\ref{fig_sxt_area2}~ contains
six panels which show the SXT effective area through the open position and
through each of the X-ray analysis filters.  Calibration Note 31 gives 
details about how these curves were derived, but in short, measurements
were made a selected wavelengths, to which model calculations were fit and
used to predict the responses at any desired wavelength.
NOTE: Be and Al12 are mistakenly reversed in Tsuneta {\it et al.} (1991) for Fig~\ref{fig_sxt_resp}.

\subsubsection{Reading the SXT Effective Area File}

Although it is not normally necessary to access the effective are data file directly, it is
very easy to do so.  In IDL you must type:
%
\idlinputa{file = CONCAT\_DIR('\$DIR\_SXT\_SENSITIVE','sra930211\_01.genx')}
\idlinputb{restgen,file=file, str=area}
\idlinputc{help,area,/str}
\begin{verbatim}
	** Structure MS_125793397004, 5 tags, length=11600:
	   WAVE            FLOAT     Array(290)
	   ENTRANCE        FLOAT     Array(290)
	   CCD             FLOAT     Array(290)
	   MIRROR          FLOAT     Array(290)
	   FILTER          FLOAT     Array(6, 290)
\end{verbatim}
~
\idlinputa{plot\_oi,area.wave,area.entrance,xr=[1,100]}
\idlinputb{plot\_oo,area.wave,area.filter(0,*),xr=[1,100],yr=[.001,1]}
%
The last two plot commands will reproduce Fig.~\ref{fig_sxt_area1}~(a) and the first panel
in Fig.~\ref{fig_sxt_area2}, respectively.  In the data structure, {\it WAVE} is a vector of
wavelengths, {\it ENTRANCE} is the double entrance filter transmission,
{\it CCD} is the CCD QE, {\it MIRROR} is the mirror effective area (including
obscuration), and {\it FILTER} is the effective area through the various
filters.  The order of the filter data is according to filter thickness going
from 0=open to 5=Be119.  Note: this is not the normal SXT filter numbering scheme.

\subsubsection{SXT Response to Thermal Plasma}
\index{response function, SXT}

The response of the SXT to a thermal plasma has been computed using the
thermal spectrum of Mewe {\it et al.} (1985, 1986), but with the abundances
adjusted to coronal values (Meyer, 1985).  The SXT thermal responses are shown
in Fig.~\ref{fig_sxt_resp} for the various analysis filters.  One can see from the curves
that there are essentially two ``thick'' filters and three ``thin'' filters 
plus the open position.  The open position has almost never been used during 
flight.  This is because, prior to the entrance filter failure in November \index{entrance filter failure, SXT}
1992, the aspect sensor door was left open almost continuously in order to
take optical exposures.  After the entrance filter failure in November 1992,
the open position became unusable.

The SXT response functions are available through the IDL routine SXT\_FLUX.
For example, 
%
\idlinputA{3.5}{te = 5.5+findgen(51)*.05}{; Log10(te) vector between 5.5 and 8.0}
\idlinputb{plot\_io,te,SXT\_FLUX(te,1),yr=[.01,1000]}
\idlinputb{for i=2,6 do oplot,te,SXT\_FLUX(te,i)}
%
should reproduce Fig. ~\ref{fig_sxt_ratios}.  Note that the second argument is the normal
SXT filter-B number.

\vspace{0.2in}\begin{tabular}{|l|l|} \hline
1 & { Open/Open or Noback case } \\
2 & { Al 1265 \AA } \\
3 & { Dagwood sandwich } \\
4 & { Be 119 $\mu$m} \\
5 & { AL 12 $\mu$m } \\
6 & { Mg 3 $\mu$m } \\ \hline
\end{tabular}\vspace{0.1in}

\begin{sloppy}
The units of the plot are DN/sec for an emission measure,
EM=$10^{44}$~cm$^{-3}$.  SXT\_FLUX has an optional switch: 
\mbox{\it /photons};  if specified, the routine 
will return the equivalent photons s$^{-1}$ detected by the SXT{}. 
This will allow the user to determine the value of DN/photon as a
function of filter and electron temperature.  For example, try the
following IDL commands to plot DN/photon for each filter as a function of
electron temperature.
\end{sloppy}
%
\idlinputA{4.5}{plot,te,SXT\_FLUX(te,4)/SXT\_FLUX(te,4,/photons)}{; plot DN/photons vs te}
\idlinputb{for i=1,6 do oplot,te,SXT\_FLUX(te,i)/SXT\_FLUX(te,i,/photons)}
%
The partial entrance filter failure in November 1992 caused a small
change to the SXT response function.  SXT\_FLUX has a {\it date= }
option which will return the new response function if the date is given
as {\it date='13-nov-92 17:01'} or later (the date can be given in any
standard Yohkoh format, such as a roadmap or index structure).  In this
case, the fractional open area of the entrance filter (the size of the
hole) is taken to be 0.0833. 

For information about how to obtain temperature estimates from the
intensity ratios through the various filters, see the User's Guide.

\subsubsection{Reading the SXT Response Function File}

The routine SXT\_FLUX reads the thermal response function data from
a file.  Although it is not normally necessary, the response function file can be read directly
using the following commands:
%
\idlinputa{file=concat\_dir('\$DIR\_SXT\_SENSITIVE','sre930211\_02.genx')}
\idlinputb{restgen, file=file, str=response}
\idlinputc{help,response,/str}
\begin{verbatim}
	** Structure MS_125794338005, 5 tags, length=5280:
	   TEMP            FLOAT     Array(51)
	   ELECTS          FLOAT     Array(12, 51)
	   PHOTONS         FLOAT     Array(12, 51)
	   ELEM            STRING    Array(15)
	   ABUN            FLOAT     Array(15)
\end{verbatim}
%
Note, that the response file may be updated from time to time.  The name contains
the date of the last update.
In this data structure, the variable {\it TEMP} is Log10($T_e$), {\it
ELECTS} are the number of CCD {\em electrons} (not DN) per sec for
EM$=10^{44}$ cm$^{-3}$, and {\it PHOTONS} is the corresponding number of
photons.  The first dimension of these two dimensional arrays is
arranged so that columns 0 to 5 correspond to the filters in increasing
thickness from 0=open to 5=Be119 and assuming the pre-launch entrance
filter transmission.  NOTE: This is not the normal SXT filter numbering scheme.
Columns 6 to 11 correspond to the same filters but with no
entrance filter. By combining the corresponding columns in the appropriate
ratio, it is possible to quickly compute the SXT response for any size hole
in the entrance filter.

\subsubsection{Relative Alignment of X-Ray and Optical Images}

\insertplot{fig_help_offsets.eps}{Relative alignment of the X-ray and optical images
(as of 1992 analysis). According to the final calibration in 2008,
the NB and WB boresights are both in the NE of the X-ray boresight.}{help_offsets}{6.25}{3.5}

The relative alignment of the X-ray and optical images is discussed in
SXT Calibration Note 36 (R. Fuller, Jan-94).  The actual values were
initially determined by T. Metcalf in 1992. 
The final calibration was done by L. Acton in 2008, using all available
optical images. The final offset values were determined as:

\index{pixel size, SXT}
\index{entrance filter, SXT}
\index{response files, SXT, reading}
\index{optical and X-ray alignment, SXT}

\vspace{0.2in}\begin{tabular}{|l|c|c|l|} \hline
{~} & { x (East/West)} &  {y (South/North)} & {~} \\ \hline
{NaBan boresight -- X-ray boresight} & {$-0.90$ ($\pm 0.23$)} & {+1.36 ($\pm 0.27$)}
                                     & {\small final values (2008)} \\
{~} & {$-0.36$ ($\pm 0.18$)} & {+1.47 ($\pm 0.28$)} & {\small old values (1992)} \\ \hline
{WdBan boresight -- X-ray boresight} & {$-0.25$ ($\pm 0.23$)} & {$+1.08$ ($\pm 0.25$)} 
                                     & {\small final values (2008)} \\
{~} & {+0.24 ($\pm 0.21$) } & {$+1.26$ ($\pm 0.29$)} & {\small old values (1992)} \\ 
\hline
\end{tabular}\vspace{0.2in}

\noindent where values are given in terms of SXT full-resolution (2.453 arcsec) 
pixels, the negative values indicate East and South, respectively.  
Fig.~\ref{help_offsets} shows the relative alignment of the X-ray, 
narrow band and wide band boresights (as of 1992 analysis). The most current 
values can be obtained with the routine GT\_SXT\_AXIS{} using the command:
%
\idlinputA{3}{print, gt\_sxt\_axis(1, /rel)	}{; NaBan - X-ray}
\idlinputC{3}{print, gt\_sxt\_axis(2, /rel)	}{; WdBan - X-ray}
%
\subsection{The CCD Camera}

\subsubsection{Saturation Effects  }
\index{saturation, SXT}

A CCD pixel can contain about $3 \times 10^5$ electrons before charge begins to spill
into adjoining pixels.  This effect is called ``pixel saturation''.  The
charge is not lost but appears in pixels surrounding (preferentially
above) the saturated pixel.  Exact saturation level varies from pixel to
pixel but, in general, a signal level of DN=235 (compressed) in a full resolution
image is considered
to be saturated, i.e., the response of the CCD becomes non-linear.  For half and
quarter resolution, saturation occurs at DN=255 (compressed). These are
the default levels used in SXT\_SATPIX which is called by SXT\_PREP.  It is \index{SXT\_SATPIX}\index{SXT\_PREP}
probable that DN=235 (compressed) is about 5\% conservative as histograms of
optical images indicate saturation setting in at DN=240 (compressed).

The 12 bit analog-to-digital-converter (ADC) of the CCD camera saturates at
about $4.1 \times 10^5$ electrons (4095 compressed to DN=255).  Charge above this level
is lost.  Thus, the signal in saturated half or quarter resolution features
is a lower limit.

The integrated X-ray intensity in a saturated full resolution image is not
lost---only the spatial information is lost.  The sum of all of the
saturated pixels plus one pixel above (north) and one below (south) yields
the integrated intensity of the area thus sampled.  Only in very extreme cases does
charge spill east or west into adjacent columns.  NOTE that the signal in
the pixel immediately above and below saturated pixels is questionable as
it will usually contain spilled charge.   We believe that the difference in
saturation level between SXT X-ray and optical images is caused by the
charge-spill effect which is not important for optical images --- which have
much lower contrast.

The routine SXT\_SATPIX can be used to flag the location of
saturated pixels.\index{SXT\_SATPIX}

\subsubsection{Effect of CCD Contamination}\index{contamination, SXT CCD}

Over time the SXT CCD collects some unknown contaminant.
This is still happening, apparently at about the same rate,
after more than 2 years in orbit!  The problem is first apparent in optical
images as noise in the form of short horizontal ``dashes'' randomly sprinkled
over the image.  Somewhat later, dark absorption ``freckles'' appear in the
thin filter images at the same location as the ``dashes''.

The contamination is removed by heating the CCD to +20 C about every 3
months.  Sometimes we have waited too long and these contamination
artifacts are evident in the images.  Although the effect of this
contamination on observability of solar features is minimal, we have not
yet quantitatively evaluated effects on things like measurement of
temperature, emission measure, errors in intensity measurements, etc.

The periods of CCD bakeout are recorded in the appendix of the
Reference Guide and can be obtained with the program PR\_DATES\_WARM.
A sample call is:
%
\idlinputd{pr\_dates\_warm}\index{PR\_DATES\_WARM}
\index{CCD, warm}

\subsubsection{SXT Images Taken with the CCD Warm  }
\index{spikes, SXT}\index{image spikes, SXT}

The CCD dark current increases about a factor of 2 for each 8 degree C
increase in CCD temperature.  The dark current at our normal operating
temperature of $-20$ C is about 1 DN/sec for each full resolution pixel.  Dark
spikes or ``hot pixels'' have a corresponding rate of increase but, with
a much higher dark current to begin with, can become quite dominant at CCD
bakeout temperatures of +20 C.  Although the analysis software tries to
match dark frames of the appropriate temperature to the data being analyzed
users should be alert for incorrect background subtraction during periods
surrounding CCD bakeout.  Dates of CCD bakeout can be obtained with the
program PR\_DATES\_WARM and are listed in the appendix of the
Reference Guide.

\subsubsection{Artifacts in Optical Images from Radiation Damage  }

Areas of the CCD with high integrated X-ray dose, such as the east and west
limbs, are observed to lose sensitivity for visible photons.  The reason for this
is still unknown.  
Thus, the aspect sensor images recorded before 14-Nov-92 when
the entrance filter failed contain ``ghost images'' of the X-ray sun.  The
darkness of the ghosts fades with time to some degree.

For most uses of aspect images the ghosts have no effect.  For quantitative
studies with aspect images they may become important.  Primitive flat-fielding
routines are available.  Hugh Hudson is the expert to consult. 
\index{flat-fielding, SXT optical}

\subsubsection{SAA Effects on Images  }
\index{SAA, SXT}
\index{South Atlantic Anomaly (SAA), SXT}
\index{spikes, SXT}\index{image spikes, SXT}

The SXT operates through the South Atlantic Anomaly, for better continuity
of coverage, even though the Yohkoh high energy sensors are switched off. 
SAA images are subject to lots of ``hot pixels'' and particle tracks from
proton hits.  These artifacts can be removed, to a fair degree, through use
of the program DE\_SPIKER. 

\subsubsection{Subtleties of Background Subtraction  }

In general, the background subtraction provided by SXT\_PREP, DARK\_SUB, and
LEAK\_SUB work amazingly well for such a complex problem.  If background
subtraction is a little off, substantial errors can accumulate in computing
the intensity of large areas of faint emission.  To better match exact
exposure times the keyword {\it /dc\_interpolate} should be used.  For cases where
even this is not satisfactory, the keyword {\it /shift\_floor} will use the average
of a $5\times5$ area in the lower left corner of the image as the background. 
This is usually only needed for the very shortest exposures.

Note that these routines leave the negative and zero values remaining from
dark frame subtraction.  These may need to be replaced with a positive
value for, e.g., logarithmic display or ratio forming.  By default SXT\_TEEM
will ignore negative intensity pixels when computing temperatures.

Dark subtraction will not remove cosmic or SAA artifacts which can be
treated manually by replacing the offending pixel with the average of the
surrounding pixels or through the use of DE\_SPIKER.
\index{spikes, SXT}\index{image spikes, SXT}

After Nov-92, some background due to a visible light leak also needed to be
removed.  See section~\ref{leak_sub} for more details.\index{entrance filter failure, SXT}

\subsubsection{Charge Bleed-back}
\index{CCD, bleed back, SXT}

During fast transfer of a CCD image to reach the Region of Interest (ROI)
of a PFI, charge builds up in the serial shift register of the CCD.  This
charge must be shifted out lest it bleed back onto the CCD and corrupt the
image.  A commandable guard band is provided to prevent this problem. 
However, on a few occasions charge bleed back is apparent in PFIs taken
with the aspect sensor.  The result is saturated columns appearing in the
image, usually in the middle where 2 ROIs have been pasted together to
create a 2 exposure PFI.  A good (bad) example is

\begin{verbatim}
 18-AUG-92  04:46:30  QT/H  NaBan/Open  Full Norm C  20   948.0  128x128
\end{verbatim}
%
There is no way to recover data corrupted by bleed back.

\subsubsection{CCD Pixelization}
\index{CCD, pixel definition, SXT}
\index{pixel size, SXT}

SXT has three on-chip summation modes available: full resolution (FR),
half resolution (HR), and quarter resolution (QR).  The pixel sizes 
are 2.45, 4.90, and 9.80 arcsec respectively.  A factor to consider when working
with images is the proper adjustments necessary when changing resolutions.
The corner address saved in the index are the IDL CCD coordinates of the
center of the binned pixel in the lower left corner.

Fig.~\ref{ccd_map} on page~\pageref{ccd_map} shows some of the
details of the CCD pixels, and how the pixels are summed on the CCD.
Due to the camera design, the FR, HR and QR images are not perfectly
aligned which each other.  There is a shift of up to 3 full resolution
pixels which needs to be accounted for when inter-comparing different resolution images.

There is also the age old question of whether to refer to the center of
a pixel or to the corner of a pixel.  We have decided to refer to the center
of the pixel since it translates much better when working with fits between
different resolutions.

See SXT Calibration Note 35 for a detailed discussion of the SXT CCD pixels
and all of the various coordinate systems associated with the CCD: ground-based
coordinate system using the camera system test set (CSTS), CCD readout pixel
coordinate system, and others.

\subsubsection{\label{ccd_zero}What is Really Zero Signal Level in Data Numbers (DN)?}
\index{CCD, digital offset}
\index{background, SXT}

The DN value passed out of the camera when there is no light incident on
the CCD and the dark current generation is zero will not be 0 DN.  The
minimum level is set electronically to be slightly higher, plus the
spurious charge during readout causes another offset.  
The following are the approximate DN levels for no light incident on the cooled CCD.

\begin{itemize}
\item Full Resolution Pixels = 13 DN
\item Half Resolution Pixels = 30 DN
\item Quarter Resolution Pixels = 69 DN
\end{itemize}

This means that if you have one raw image (which has not had
the dark current image removed) with an average signal of 15 DN and
another with 20 DN, that is not a 25\% difference, it is over 300\%
different.  This offset must be watched carefully when working with
low signal levels, even after the dark image has been removed.

The above values are appropriate for data acquired prior to 13-Nov-92.  After
the entrance filter failure, all values increased 1--2 DN due to a
change in the spurious charge.  NOTE: A separate effect is that the dark current has been increasing
gradually during the mission, and will continue to do so, which is caused by
X-ray damage.

DARK\_SUB (which is what SXT\_PREP calls), will remove a single dark
frame which is closest in exposure level to the input image.  When
removing that single dark image, it takes off the digital offset, plus the
dark current (exposure dependent) portion.  An even better method
is to use the /DC\_INTERPOLATE option, which will interpolate two
dark images to correct for the exposure dependent dark current, and will also remove the
digital offset. \index{DARK\_SUB}\index{dark current, SXT}\index{background, SXT}

%TODO - Jim - Review last paragraph (new)

\subsection{The Mirror, Lens or Filters}

\subsubsection{Scattering by ND X-ray Filter  }
\index{ND filter, scattering, SXT}
\index{scattering, SXT}

In order to extend the dynamic range of SXT for flare use, the forward
filter wheel is equipped with a stainless steel mesh (``ND'' filter) with a
transmission of $0.0805 \pm 0.0008$.  While the mesh causes no noticeable
degradation of the image it has been found to scatter X-rays (SXT
Calibration Note 34) at about the 0.7\% level.  This is a minor effect for
the brightest features in a flare but can provide the dominant signal in
nearby faint features.  No adequate means for correcting for this effect
has yet been derived.  Therefore, for greatest accuracy it is presently
advisable not to mix images with and without the ND filter in determining
the $T_e$ and EM of bright flare features and never to use ND filter images in
the faint features.  SXT\_TEEM warns the user if the ND filter is included.  E.g.,
the light curves, intensity,  $T_e$ , and EM of faint areas of ND flare images
may be totally in error.  Because the scattering is diffuse and smooth, the
images themselves are quite adequate as pictures of the flares for morphological studies. 

\subsubsection{Point Spread Function  }
\index{point spread function, SXT}
\index{PSF, SXT}
\index{spatial resolution, SXT}
\index{SXT spatial resolution}

The point spread function (PSF) of the SXT has a narrow core and wide
scattering wings.  Laboratory measurements of the core of the PSF have been
analyzed thoroughly by Martens (SXT Calibration Note 34).  The shape of the
PSF is well characterized by an elliptical Moffat function with a FWHM of
about 1.5 CCD pixels (3.7 arcsec) over the central area (diameter = 470
pixels or 2300 arcsec) of the CCD.  Between a radius of 1 and 10 CCD pixels the slope
of the PSF falls off roughly as a power law of index $-4$. 

There is as yet no released software for deconvolution of SXT images,
although McTiernan has done some work on this using the maximum entropy
method and Roumeliotis has used a modified Lucy method.

\subsubsection{Mirror Scatter  }\index{mirror scatter}
\index{scattering, SXT}

The scattering wings of the SXT PSF begin to dominate the narrow core at about 20 arcsec
from the center---at a level about 5 or 6 orders of magnitude down from the peak (see Fig.~4 in
the Tsuneta, {\it et al.} (1991 `Red Book').  The
scattering wings may be characterized from over-exposed flares observed in
FFI mode such as 6-Sep-92 18:48.  Hara (to be published) has derived the
following expression which describes the shape of the PSF wing for a flare
of $6.9\times 10^6$ K, through the thin filters

\[      Wing(r,6.9 \times 10^6) = 10^{-1.839} \times r^{-2.0}   \hskip 1cm  [r > 20^{\prime\prime}] \]
%
%TODO - Jim/Loren - "per FR pixel?"
%TODO - Jim - put point spread funtion plot on-line.
per full resolution SXT pixel and per unit intensity in the flare, with {\it r} 
specified in arcsec.  The scattering
properties of the mirror are wavelength dependent.  This introduces a
dependence upon input spectrum in correcting for the PSF scattering wings
in data analysis.  Hara represents this dependence in terms of a ratio of
the mean wavelength of the flare and the mean wavelength of the reference
($6.9 \times 10^6$ K) spectrum and gives the following expression for the PSF wing for
other source temperatures. 

\[      Wing(r,T_e) = Wing(r,6.9 \times 10^6) \times (L_{avg}(6.9 \times 10^6)/L_{avg}(T_e)) \]
%
where $L_{avg}$ is the average wavelength recorded by SXT through either the
Al.1 or AlMg filters at temperature $T_e$.  

Scattering wing corrections have not yet been derived for the thicker
filters and programs for use of the preliminary algorithms derived by Hara have not yet
been released for general use because of concerns of their applicability away from sun center.

\subsubsection{\label{vignette}Vignetting Correction}\index{vignette}
\index{effective area, SXT}
\index{off-axis effective area, SXT}

\insertplot{fig_vignette.eps}{
The effective area (or vignette function) as a function of off-axis
angle.  The solid curve was derived from \mbox{C K} and \mbox{Al K} data
and the dashed curve from \mbox{Ag L} data.
}{fig_vignette}{6.25}{4.0}

\index{field of view (FOV), SXT}
The SXT X-ray effective area varies across the $42\times42$ arcmin\index{effective area}
field-of-view.  The effective area and response functions that are
reported in other sections of this manual always refer to values for the
optical axis of the telescope, where the effective area is maximum.  The
off-axis response has been measured during pre-launch calibration and
has been refined with in-flight data.  To a first approximation, the
off-axis variation of effective area, or the vignette function, can be
approximated by two non-concentric cones (see SXT Calibration Note 37). 
The inner- and outer-cones intersect one another at a radius of about 21
arcmin.  Analysis of flight data indicates that the vignette function
is actually more complicated and should at least be approximated by a
cone with some flat distortions.  These distortions were not detected in
pre-launch calibration, and several questions about them remain.  The
vignette function is known to vary as a function of incident photon
energy.  Pre-launch measurements were made at three energies: 0.277 keV
(C K), 1.49 keV (Al K), and 2.98 keV (Ag L).  Measurements at the two
lower energies gave results that were consistent with one another. 
Thus, we have characterized the SXT vignette functions as either ``low"
or ``high" energy.  The shape of the vignette function can be seen using
the IDL routine HELP\_VIGNETTE for which a sample plot is shown in
Fig.~\ref{fig_vignette}. 

There are no fully approved routines to remove or correct for the SXT
vignette function, however, there are a two preliminary routines. 
SXT\_VIGNETTE will compute the vignette function as it is currently
understood.  The form and parameters of the shape may be changed as
additional data and analysis become available.  The routine SXT\_OFF\_AXIS
will correct for the vignette function by calling SXT\_VIGNETTE{}. 
Users are warned that these routines are preliminary and may not be the
most appropriate technique to use, depending on the scientific
application. \index{SXT\_VIGNETTE}\index{SXT\_OFF\_AXIS}
However, they are certainly more accurate than no correction when intensities from different
parts of the CCD must be compared.

\subsubsection{\label{leak_sub}Stray Visible Light}
\index{straylight, SXT}\index{entrance filter failure, SXT}

The SXT entrance filter consists of a small amount of titanium and
aluminum deposited on Lexan.  In order to guard against the
possibilities of pinholes, two such filters were flown back-to-back with
the Lexan surfaces facing each other.  These filters cover the X-ray
entrance annulus and are arranged as six back-to-back pairs, or twelve
individual filters in all.  On 29-Oct-92 between 04:30 and 06:30 UT it
was noticed that the optical images contained an halo of diffuse light. 
On 13-Nov-92 at 16:50 UT the visible light images became completely
saturated.  It is now thought that in October 1992 one of the outer
entrance filters broke either completely or partially.  The resulting
halo was produced by the light that leaked through the inner filter that
was behind the broken outer filter.  Once the outer filter failed, the
inner filter had its Lexan surface exposed to solar UV radiation and
after three weeks of continuous exposure, the Lexan material
deteriorated, causing the filter to at least partially fail.  The
resulting hole in the entrance filter allows visible light to be
reflected by the X-ray mirror into the focal plane. 

From an analysis of the data acquired after 14-Nov-92 it has been
calculated that the fraction of the open area is 0.0833.  The SXT\_FLUX
and SXT\_TEEM routines take this into account when computing the
response functions.  The effect is negligible for the thick filters and
makes only a small effect for the thin filters, increasing the response
slightly at longer wavelengths.  No adequate explanation for the partial
entrance filter failure has been determined. This value and the date
are available from the routine GET\_YO\_DATES(/entrance,/verbose).\index{GET\_YO\_DATES}
Analysis and description of these artifacts
are given in SXT Calibration Notes 38 and 39.

When the 8\% transmission X-ray ND mask is used, the stray light intensity
increases a hundred fold.  This is not as severe a problem as it sounds
because exposures using the ND filter are generally quite short.

It has proven possible to acquire images with the SXT between 10 and 30
seconds before optical sunset---for which the atmosphere completely
attenuates the X-ray sun with little effect on the visible illumination. 
These ``terminator'' images are routinely taken through all combinations of
SXT filters for use in subtracting this stray light signal.  This
correction, along with dark frame subtraction, is provided by the program
LEAK\_SUB which will be updated from time to time as the terminator data
base and correction algorithms improve.  At present, LEAK\_SUB is not perfect\index{LEAK\_SUB}
and over/under correction can be a problem you should be alert for. 
Because of complexity and partial randomness (especially for the thin Al filter), perfect straylight correction
will never be possible.

\subsection{Other Instrument and Operations Features}

\subsubsection{Data Compression}\index{compression, SXT}
\index{compression, SXT}
\index{decompression, SXT}
\index{data compression, SXT}
\index{data decompression, SXT}
\index{compression error, SXT}
\index{decompression error, SXT}

The CCD camera employs a 12-bit ADC but data are transferred to the DP (telemetry) 
as 8-bit numbers.  For 
purposes of gain calibration and helioseismology it is important to maintain the
full 12-bit accuracy.  This is accomplished by commanding compressed data and 
the low-order 8 bits for the same region, and then restoring the full 12 bits (see
the routine RESTORE\_LOW8 described in the Reference Guide).

The compression of ADC numbers uses the following algorithm.  Taking DN,
or Data Number, to be the CCD digital camera output, and $X$ the 8-bit
compressed value, and $M$ the 12-bit decompressed value,

%\IItem		%???
\begin{itemize}	%???
\item for DN $\leq$ 64 \\
	\[ X(\mbox{DN}) = \mbox{DN}, M = X = \mbox{DN} \]
\item for DN $>$ 64 \\
	\[ X(\mbox{DN}) = \mbox{round}\bigl[59.249 + \sqrt{3510.39 - 9.50 (431.14-\mbox{DN})}\,\bigr]\,, \]
	\[ M = \mbox{round}\bigl[0.10526 X^2 - 12.473 X + 431.14\bigr] \,.\]
\end{itemize}
%
These expressions are used by the routines SXT\_DECOMP and SXT\_COMP for
decompression and compression, respectively. 

The SXT compression algorithm is not lossless, and so the decompressed
values, $M$, contain a small error due to decompression: $\delta M =
|\mbox{DN}-M|${}.  The algorithm has been selected so as to make the
decompression error, $\delta M$, comparable to the statistical
uncertainty.  The decompression error is given by:

$$\delta M = 2 \times 0.10526 X - 12.473\,,$$
%
which are optionally returned by the SXT\_DECOMP routine.

\subsubsection{Use of Low-8 Images in Optical Data}
\index{low-8 bit SXT data}

For X-ray images the error introduced by the SXT data compression is
less than counting statistics so the compression from 12-bit (out of the
CCD camera) to 8-bit numbers loses little information.  For aspect
images the errors in counting statistics are far less than the data
compression errors.  In order to obtain the best statistical precision, the
aspect images were often taken in both low-8-bit and compressed data
modes separately.  These are combined to provide full 12-bit precision
through the use of the program RESTORE\_LOW8.  This procedure works
best if the two exposures are close together in time to minizize the
effects of spacecraft pointing drift and jitter. 

\subsubsection{Filter Alternation During AEC}\index{filter alternation, SXT}\index{AEC, SXT}

The Automatic Exposure Control of the SXT is described in the ``Red Book''
and briefly in the Reference Guide.  One aspect of the AEC 
algorithm is that if the image is overexposed, and the AEC software has
reduced the exposure to the lowest exposure level possible, then the
logic could insert a ``thicker'' filter to avoid saturation.  This option
is enabled or disabled by the sequence table.  There has been confusion on
occasion when the filter has changed for a given sequence ``slot,'' even
when there has been no table upload, or change in DP mode or DP rate.  In
most cases, the change in filter was due to AEC Filter Alternation.

The bottom line is that if exposure and ND filter adjustment do not suffice to maintain the desired
exposure a thicker or thinner filter could be selected.  Beware of this when
analyzing flare data and an unexpected filter suddenly appears in the
sequence!

\subsubsection{The Roll Angle in the SXT Images}\index{roll angle, SXT CCD}\index{CCD roll angle, SXT}

The azimuthal orientation of the SXT CCD is mis-aligned slightly with
respect to the Yohkoh spacecraft.  Thus, if the spacecraft is oriented
so that the solar north direction is parallel to the ``pitch'' axis, in
the SXT images solar north will not be exactly aligned with the y-axis
of the CCD, but rotated a small amount counter clockwise.  A useful
routine for displaying a diagram of the roll angle relationships is \index{HELP\_ROLL}
HELP\_ROLL{}.  The precise roll angle was not measured prior to launch. 
The value has been determined from in-flight measurements and continues
to be refined.  It is approximately 1 degree.  The value which is used
currently in the on-line software can be checked by typing:
%
\idlinputA{3}{print,get\_roll(/sxt)		}{; Print SXT roll-angle (degrees)} \index{GET\_ROLL}
%
When analyzing SXT images, one should be aware that the Yohkoh
spacecraft roll orientation varies slightly.  GET\_ROLL by default
returns the roll angle of SXT with respect to solar north, incorporating
the CCD roll-angle and spacecraft roll angle.  GET\_ROLL tries to read
the ATT database files if they are available (see the reference guide
for more information).  The ATT database already has the correction for
seasonal variation that exists between the spacecraft's on-board definition
and the true solar north value.  If the ATT database is not available, the
routine will return values based on a sinusoidal fit to the seasonal variations,
but which does not take into account the true S/C roll.\index{alignment, SXT images}\index{ATT database}

\subsubsection{Incorrect Location of Image Pieces in Some PFIs}

Observing regions (OR) are composed of 64$\times$64 PFI ``blocks.''  The simplest
observing region is 64$\times$64, but quite often larger observing regions are used.
For example, when the observing region is 256$\times$192, there are 4 ``blocks''
in the E/W direction and 3 in the N/S.  Because of the way that the SXT telemetry
buffers work, it is necessary to take three separate exposures for a 256$\times$192
observing region.  The assembly of these ``blocks'' into an observing region
is performed properly for over 99\% of the observing regions, however, there 
is a special case when a problem can occur.

When the telemetry mode is FFI dominant (the full frame images get the
larger portion of the telemetry), it requires four major frames to telemeter
a single 64$\times$64 PFI ``block''.  If any of the first portions of the ``block'' (which is
64$\times$16) is missing, it will cause the image to be assembled incorrectly 
because of a lack of telemetry information.  Be aware of occasional PFI portions
which are not aligned properly.

\subsubsection{Preparation of SFD images}

SFD images are prepared from long and short exposure FFIs to extend the
dynamic range of a given image to about $1 \times 10^6$.  The program MK\_SFD is used \index{MK\_SFD}
for this purpose.  These images are used to make
the SXT ``movie'' and are especially valuable for surveying the data archive. 
Users should recognize that the SFD images are automatically prepared on a
production line basis as part of the data reformatting process and should
be alert for artifacts and features which are not of solar origin.  Also,
some areas of the images will have smoothed data inserted to replace
saturation-bleed regions.  

While, in general the SFD data are quantitatively correct, quantitative
analysis should generally resort to the original SFR files which, for a
variety of reasons, will often have images which are not made into SFD
images.  Users wishing to make their own composite images from 2 or 3
images of different exposure are referred to the program SXT\_COMPOSITE.\index{SXT\_COMPOSITE}\index{SFD images}

\subsection{Contact Persons}
\index{contact persons, SXT}

If you have any question about SXT software and/or instrument, or if you
need any advice in SXT data analyses, contact:

\begin{verbatim}
                yla@solar.physics.montana.edu
\end{verbatim}

\subsection{References}

\IItem
\item Tsuneta, S.,  Acton, L., Bruner, M., Lemen, J., Brown, W., Caravalho, R.,  Catura, R.,
Freeland, S., Jurcevich, B., Morrison, M., Ogawara, Y., Hirayama, T., and Owens, J., Solar Phys.
 136, 37-67 (1991), ``Red Book.''

\item Lemen, J. R., ``SXT X-Ray Effective Area," SXT Calibration Note 31, (Rev.\ A, 1993).

\item Martens, P. C., ``White Sands Measurements of the Point Spread Function,''
	SXT Calibration Note 34, (Rev.\ A, 1993).

\item Morrison, M., ``SXT CCD Pixel Description," SXT Calibration Note 35, (Rev.\ A, 1993).

\item Fuller, R. A., and Lemen, J. R., ``SXT X-Ray and Optical Axes," 
	SXT Calibration Note 36, (Rev.\ A, 1994).

\item Fuller, R. A., Lemen, J. R., and Acton, L. W., ``SXT X-Ray Vignetting Function," 
	SXT Calibration Note 37, (Rev.\ A, 1994).

\item Acton, L.  W., ``Origin of Thin Aluminum Straylight Features," SXT
Calibration Note 38, (Rev.\ C, 1993). 

\item Acton, L.  W., ``Analysis of Thin Aluminum Sunset Straylight
	Images," SXT Calibration Note 39, (Rev.\ B, 1993). 

\end{itemize}



