Setting the Antenna Encoder Zero Points

    This is done via the SETENC program.  The SETENC program reads the encoder
periodically until the encoder senses the fidicial mark on its disk.  Assuming
that the antenna is tracking at a constant rate, SETENC predicts where the
antenna should be at the next reading.  If the fiducial has been recognized,
the difference between the current reading and the predicted reading is how
much the encoder reading was changed by the fiducial.  (To make this accurate,
the antenna should be driven at only about 10% of full speed.)  After the power
has been off, the encoder reading is a random number, so the first time the
fiducial is encountered, the differerence is meaningless.  It is a "feature" of
the AXPUG encoder cards that they occasionally get the position wrong by one
cycle: 36 arcsec.  Therefore, the procedure below specifies that the
zero-points be set several times until the readings before and after agree to
within a few arcsec. 

    Login to the dumb terminal on the table across from the CAMAC rack.  (It
can be your personal account if you include @SYS$SMT:[SMT]SYMBOLS in your
LOGIN.COM file or execute it manually.  Or you can login as user SMTSYS.)  Type
SETENC.  This procecure will ask whether you want to set the Azimuth or
Elevation encoders.  Answer AZ or EL.  If you choose EL, it will ask if you
want to set #1, #2, or both.  Answer BOTH.  (Only the first two letters are
needed.)  It will then give instructions to move the antenna manually to a
position about three degrees below the fiducial point.  (They are near 23
degrees in elevation and 179 degrees in azimuth.) 

    The antenna may be moved manually by the controls on the door of the Krupp
SCU.  The row of white buttons on the left door controls azimuth and on the
right door controls elevation.  With the key in the center of the row of
buttons in the vertical position (Local Control), select the direction with the
toggle switch above the row and the rate with the potentiometer (10 in the
pot's window = 100%).  Before rotating in elevation, first check that there are
no obstacles in the chamber which could hit the antenna surface.  (Especially
check the focus platforms and the door crane.  They ought to be interlocked to
prevent this, but interlocks can fail.)  When rotating in azimuth for the first
time, move at a slow rate of about 10% for about 20 seconds.  If the building
fails to follow the antenna, it will hit the limit switches at a slow speed and
won't coast further.  To rotate the antenna, hold the elevation or azimuth Rate
button down.  One should see motor currents on the meter and the rate selected
by the potentiometer on the rate meter.  Releasing the Rate button will stop
the antenna. 

    (If the azimuth limit switch is encountered, the Difference Angle lamp will
light and the drive will stop.  Either Bypass CW-Ang or Bypass CCW-Ang will
also be on.  Put the toggle switch in the direction illuminated.  Check that
the rate has been turned down to about 10%.  Hold down both the Rate button and
the Bypass button until the Difference Angle light goes out and then hold them
for another 10 seconds beyond this to center the antenna.  Finally, enable the
building drive as described in Steps 4 and 9.) 

    When the antenna has been moved to about three degrees below the fiducial,
move the direction toggle switch to either CW or Up as appropriate, and press
<Return> on the terminal.  (It is not so important how far below the fiducial
the antenna is before <Return> is pressed, so long as there are enough readings
before the fiducial is found to make an accurate prediction of the encoder
position at the time the fiducial is found.  It IS important not to cross the
fiducial in the negative direction, as this can cause the AXPUG to set the
encoder to the wrong angle.)  The SL LED's should light on the corresponding
encoder module in the antenna crate.  (Slots 1: #1 elevation encoder, 2: #2
elevation encoder, 3: azimuth encoder.)  Move the antenna toward the fiducial
at the 10% rate.  Either watch for the SL LED's to go out (on both elevation
modules) or for a line to be printed on the terminal (two if elevation). 

    Repeat the above by first reversing the direction toggle switch and
rotating the antenna at 10% for about 10 seconds.  Put the toggle switch back
to CW or Up.  Type SETENC again, select the same direction as before and strike
<Return>.  Rotate the antenna again at 10% until the line(s) again are printed
on the terminal (after about 10 seconds).  The last number printed on these
lines is the difference between the predicted and actual encoder readings after
the fidicual was sensed.  When setting elevation, repeat the procedure until
both elevation lines end in a number less than 4 arcsec.  The two elevation
readings on the color monitor should agree to about 10 arcsec.  When setting
azimuth, repeat the process until the last number on the line is below 4 arcsec
three times in a row. 

    Turn the two key-switches on the Krupp SCU doors to the horizontal
position: computer control.  The two Computer Enabled lights should come on.
The Computer Accept lights should come on too if FAHREN is running on Kronen
and if the azimuth and elevation buttons on the Antenna Control Rack in the
Control Room are illuminated.