\section{{\bf Introduction to the Yohkoh Instruments}}

\subsection{Yohkoh Observing Modes}

There are generally two Data Processor (DP) modes when Yohkoh is taking\index{DP (S/C Data Processor)}
scientific data, FLARE mode and QUIET mode.  Yohkoh uses the WBS HXS and
SXS instruments to monitor solar activity and when a threshold is
passed the spacecraft (S/C) enters FLARE mode.  There are times when Yohkoh is in\index{S/C (spacecraft)}
FLARE mode but there is no flare.

There are three different telemetry rates, but only two are used when
observing the sun.  HIGH rate is 32 Kbits/s (a major frame every 2 s) 
and MEDIUM is 4 Kbits/s (a major frame every 16 s).
When first entering FLARE mode, the DP goes into HIGH rate.  After
10 minutes, it will go into MEDIUM rate if the intensity of the flare
has subsided, but is not below a minimum threshold.

\subsection{Bragg Crystal Spectrometer (BCS)}
\index{BCS instrument description}
\index{instrument description, BCS}
\index{cadence, BCS}

\begin{verbatim}
Instrument:                  Bent Crystal Spectrometers
Spectral lines:              Fe XXVI (1.76 - 1.81 A)    (Chan 1)
                             Fe XXV  (1.83 - 1.90 A)    (Chan 2)
                             Ca XIX  (3.16 - 3.19 A)    (Chan 3)
                             S XV    (5.01 - 5.11 A)    (Chan 4)
Spectral resolution (l/dl):  3000 - 8000
Angular resolution:          Full disk
Best time resolution:        0.125 sec
Typical time resolution:     3.0 sec in FLARE/HI
\end{verbatim}

\subsubsection{Instrument Modes}
The BCS has the capability to bin the different channels in a variety 
of ways.  Each binning group is defined by a separate ModeID.
For a given ModeID is it possible to determine how the data original
bins were combined before being telemetered to the ground.

\subsection{Hard X-Ray Telescope (HXT)}
\index{HXT instrument description}
\index{instrument description, HXT}
\index{cadence, HXT}
\index{field of view (FOV), HXT}

\begin{verbatim}
Instrument:                Fourier Synthesis Telescope
Energy bands:              (15 - 100 keV, 4 channels)
                           Low       14 - 23 keV
                           Medium-1  23 - 33 keV
                           Medium-2  33 - 53 keV
                           High      53 - 93 keV
Angular resolution:        ~5 arcsec
Effective area:            1.5 cm2 avg. x 64 elements
Field of View:             35 x 35 arcmin
Best time resolution:      0.5 sec
Typical time resolution:   0.5 sec in FLARE/HI
\end{verbatim}

\subsection{Soft X-Ray Telescope (SXT)}
\index{SXT instrument description}
\index{instrument description, SXT}
\index{cadence, SXT}
\index{field of view (FOV), SXT}

\begin{verbatim}
Instrument:          Glancing incidence mirror/CCD sensor
                     Co-aligned optical telescope using same CCD
Wavelength ranges:   2.5-46 A     no analysis filter (or Noback)
                     2.5-36 A     1265 A Al
                     2.4-32 A     2930 A Al, 2070 A Mg, 562 A Mn, 190 A C
                     2.4-23 A     2.52 micron Mg
                     2.4-13 A     11.6 micron Al
                     2.3-10 A     119 micron Be
                     4600-4800 A  Wide band optical filter
                     4290-4320 A  Narrow band optical filter
Spectral discrimination:   Filters
Angular resolution:        3 arcsec
Field of View:             42 x 42 arcmin
Best time resolution:      0.5 sec
Typical time resolution:   2.0 sec in FLARE, 8.0 sec in QUIET
\end{verbatim}

After the failure of an entrance filter in Nov-92, the
narrow band, wide band optical filters, and Noback X-ray filter became unusable.

\subsubsection{Sequence Tables}
\index{sequence tables, SXT}

SXT uses sequence tables to define the order that the images will be taken.
There are thirteen `slots' which are available to set the parameters.  For each
slot a separate filter, resolution and observing region can be
specified.  The observing region selection is from a table of nine entries.
Each entry specifies a location and the size of the output image in
pixels (this means that the field of view is different depending on the
pixel resolution used).

The sequence is started from the beginning whenever the ``SXT CNT AUTO''
command sequence is issued from the ground or by programmed control, or
if the telemetry rate or mode changes.
Therefore, the sequence does not automatically begin at the top at the
beginning of a new orbit, for example.

\newpage
\begin{verbatim}
LOOP 1 (n1=infinity)
                              Img 1-1   --------------------+
   LOOP 2 (n2= )                                            |
                                 Img 2-1   ------------+    |
                                 Img 2-2               |    |
       LOOP 3 (n3= )                                   |    |
                                    Img 3-1   ----+    |    |
                                    Img 3-2       |    |    |
                                    Img 3-3       |    |    |
                                    Img 3-4   ----+ ---+    |
   LOOP 4 (n4= )                                            |
                                 Img 4-1   ------------+    |
                                 Img 4-2               |    |
        LOOP 5 (n5= )                                  |    |
                                    Img 5-1   ----+    |    |
                                    Img 5-2       |    |    |
                                    Img 5-3       |    |    |
                                    Img 5-4   ----+ ---+ ---+
\end{verbatim}

\subsubsection{Automatic Exposure Control (AEC)}

SXT has automatic exposure control (AEC) available for partial frame images.  The
algorithm works on a per observing region basis and runs separately for each 
``slot'' shown in the sequence above (image 3-3 exposure control is independent
of image 5-3, even if they are for the identical filter and location).  An upper and lower intensity, or Data Number (DN), threshold
is set and the number of pixels above and below that threshold is set.  An allowable number of pixels above and
below is also set, and if the image exceeds this set level, then the exposure duration is
shortened or lengthed, depending on whether it is over exposed or under exposed.
See the ``Red Book'' article for more details on the exposure control.\index{SXT exposure control}\index{AEC, SXT}

\subsubsection{Automatic Region Selection (ARS)}

The automatic region selection (ARS) allows two different selection methods.  ARS-1
takes an exposure of the sun and locates the brightest points and selects the brightest
four locations.  Normally the sequence table simply uses the single brightest active
region.  ARS-2 allows the operator to specify a sub-region of the sun, and the tracking
algorithm will point to the brightest region within that sub-region.  ARS-2 is normally
used to force the pointing to stay on a fixed region, and to correct for solar rotation.
See the ``Red Book'' article for more details on the region selection.\index{SXT region selection}\index{ARS, SXT}

\newpage
\subsection{Wide Band Spectrometer (WBS)}
\index{cadence, WBS}

\begin{verbatim}
Instruments:
           Soft X-ray Spectrometer (SXS)
           Hard X-ray Spectrometer (HXS)
           Gamma-ray Spectrometer (GRS)
           Radiation Belt Monitor (RBM)

           SXS: Gas Proportional Counter (3-30 keV (nominal), 128 channels)
           HXS: NaI scintillation counter (20-657 keV, 32 channels)
           GRS: BGO scintillation counter (0.3-100 MeV, 144 channels)
           RBM-SC: RBM NaI Scintillation Detector (5-300 keV)
           RBM-SD: RBM Si Detector (>20 keV)
Angular resolution:     Full disk
Best time resolution:   0.125 sec
\end{verbatim}

Most of the sub-instruments have pulse height (PH) data and pulse
count (PC) data.  PH is essentially a spectrum with counts as a function
of energy, and PC is the sum of all counts for a given energy range.

The structure for the WBS data has numerous tags.  Some of the data tags
and types of data available are listed below.

SXS has two detectors with a 2-channel PC and 128-channel PH for each detector.  The
energies listed below are nominal values.
\index{energy range, WBS-SXS}
\index{instrument description, WBS-SXS}
\index{WBS-SXS instrument description}

\begin{verbatim}
          sxs_pc11 - SXS-1 Detector, chan 1 (3-15 keV) 
          sxs_pc12 - SXS-1 Detector, chan 2 (15-40 keV)
          sxs_pc21 - SXS-2 Detector, chan 1 (3-15 keV) 
          sxs_pc22 - SXS-2 Detector, chan 2 (15-40 keV)

          sxs_ph1  - SXS-1 Detector, 128-channel PH (3-30 keV)
          sxs_ph2  - SXS-2 Detector, 128-channel PH (3-30 keV*)
\end{verbatim}

HXS has a 2-channel PC and a 32-channel PH.
\index{energy range, WBS-HXS}
\index{instrument description, WBS-HXS}
\index{WBS-HXS instrument description}

\begin{verbatim}
          hxs_pc1  - HXS Detector, chan 1 
                         (20-60 keV for 1-Oct-91 to 9-Jun-92)
                         (25-75 keV for 9-Jun-92 to present)
          hxs_pc2  - HXS Detector, chan 2
                         (65 - 657 keV for 1-Oct-91 to 9-Jun-92)
                         (75 - 830 keV for 9-Jun-92 to present)

          hxs_ph   - HXS Detector, 32-channel PH 
                         (20 - 657 keV for 1-Oct-91 to 9-Jun-92)
                         (25 - 830 keV for 9-Jun-92 to present)
\end{verbatim}

GRS has two detectors with a 6-channel PC and a 128-chan PH (16-channel for the
high words) for each detector.
\index{energy range, WBS-GRS}
\index{instrument description, WBS-GRS}
\index{WBS-GRS instrument description}

\begin{verbatim}
          grs_pc11  -  GRS-1 Detector, chan 1 (0.27 - 1.04 MeV)
          grs_pc12  -  GRS-1 Detector, chan 2 (1.04 - 5.47 MeV)
          grs_pc13  -  GRS-1 Detector, chan 3 (5.47 - 9.3   MeV)
          grs_pc14  -  GRS-1 Detector, chan 4 (9.3  - 13.1  MeV)
          grs_pc15  -  GRS-1 Detector, chan 5 (8 - 30 MeV)
          grs_pc16  -  GRS-1 Detector, chan 6 (30 - 100 MeV)

          grs_phl1  -  GRS-1 Detector, 128-chan (0.3 - 13.6 MeV)
          grs_phh1  -  GRS-1 Detector, 16-chan (8 - 100 MeV)

          grs_pc21  -  GRS-2 Detector, chan 1 (0.3 - 1.24 MeV)
          grs_pc22  -  GRS-2 Detector, chan 2 (1.24 - 5.66 MeV)
          grs_pc23  -  GRS-2 Detector, chan 3 (5.66 - 9.37   MeV)
          grs_pc24  -  GRS-2 Detector, chan 4 (9.37  - 13.6  MeV)
          grs_pc25  -  GRS-2 Detector, chan 5 (8 - 30 MeV)
          grs_pc26  -  GRS-2 Detector, chan 6 (30 - 100 MeV)

          grs_phl2  -  GRS-2 Detector, 128-chan (0.3 - 13.6 MeV)
          grs_phh2  -  GRS-2 Detector, 16-chan (8 - 100 MeV)
\end{verbatim}

RBM has two detectors. RBM-SC has 2-channel PC and 32-channel PH,
and RBM-SD has 1-channel PC.

\begin{verbatim}
          rbm_sc_pc1 - NaI scintillation detector (5-60 keV)
          rbm_sc_pc2 - NaI scintillation detector (60-300 keV)
          rbm_sd_pc  - Si detector (>20 keV)
          rbm_sc_ph  - RBM-SC Detector, 32-chan (5-300 keV)
\end{verbatim}
        
\subsection{Spacecraft Attitude Control System (ACS)}

The attitude control system uses momentum wheels, magnetic torquers, and control-moment 
gyros as the actuators.  As the attitude sensors, two sun sensors and a star
tracker, as well as geomagnetic sensors, are available for determining the spacecraft
pointing relative to the direction of the sun and to the ecliptic plane, respectfully.
An inertial reference unit comprising four gyros detects changes of attitude with time.

