Checking the Consistency of the Two Elevation Encoders

    There are two read heads on the elevation encoder disk.  When the antenna
is driven in elevation, the average of the two readings is used.  The zero 
points for each read head are adjusted so that they give about the same 
reading.  Ideally, the difference between the two readings should be constant
and no greater than a few arcsec.  However, the encoder disk was not perfectly
centered on the elevation axis when it was installed.  This eccentricity makes
the difference into a sinusoid with elevation angle.  Since this deviation has
the same functionality as one of the pointing model flexure terms, it simply
gets absorbed into this pointing model constant. 

    So long as the eccentricity remains the same, it will have no further 
effect on the telescope pointing.  However, we don't currently know how stable
this eccentricity is.  Therefore, it should be measured occasionally to 
check its value.  This is done by comparing the two readings as the antenna is
moved over nearly its full range in elevation.  The measurement procedure is
as follows:

    1) Turn on the elevation drive and enable elevation computer control with
the Control Rack button.  (Azimuth can be left off, however, if the sun is up,
the azimuth should not be such that the antenna will be pointed closer than
about 30 degrees to the sun during the elevation scan.  That is, the sun must
be below 60 degrees elevation and the azimuth of the telescope must not be
closer than 60 degrees to that of the sun.)

    2) Turn off the Control Rack's Backend, Frontend, and Data Acquis buttons
so that the "observation" will not attempt to acquire backend data, tune the
receiver, or write backend data to disk.  Depress the Start and Wait buttons.

    3) Set up the antenna to scan in elevation from about 3 degrees to 87
degrees by typing the following commands to the OBST program:

    	SOURCE 45NORTH				(Azimuth = 0, Elevation 45)
	CMAP /MAPSIZE 0 10 300000 10 ELEV /TIME 300 /SUB 2
	START

The antenna should move to about 3 degrees elevation and the Wait Lamp should 
come on.

    4) Meanwhile, find a window logged in to Kronen as SMTSYS (or log in one).
At the SMTSYS> prompt, type:

	SET DEFAULT SYS$SMT:[SMT.TEST]
	RUN ELV_ENC

and answer the two questions, but not the one to "start":

	.2			(for Polling Time)
	ELV_ENC.<something>	(for File To Write; make <something> distinctive
				such as today's date, e.g. 12MAR97)

    5) Release the Wait Button and strike <RETURN> to start the measurements by
the ELV_ENC program.  After the Start Lamp comes on, depress Wait again so the 
antenna will pause at the beginning of the second subscan in elevation: 87 deg
to 3 deg.

    6) The antenna should now be scanning in elevation.  When it completes the
scan, strike <RETURN> again to stop the ELV_ENC measurements.  The program will
prepare a plot file, will print a summary, and ask whether to continue.

    7) Respond with Y (and <RETURN>) and repeat steps 5) and 6).  (The WAIT
button need not be depressed again, as this will be the last measurement.)

    8) Respond with N (and <RETURN>).  ELV_ENC will exit.  To see the plots,
type PLOT and they will be printed on the Laserwriter.