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512 Channel Magnetograph Retired After 20 Years...(1Jun93)
of Service
(from NSO, NOAO Newsletter No. 34, 1 June 1993)

Just short of its twentieth birthday, the 512 channel magnetograph is being
retired. This has been the major focal plane instrument at the Kitt Peak
Vacuum Telescope. The telescope and instrument were originally designed by
Bill Livingston, Dale Schrage, Don Trumbo and others in 1972 to support
solar observations made on board the Skylab space station during 1973 and 1974.
The goal was to provide seeing-limited maps of the longitudinal magnetic field
on a daily basis during that mission and subsequently. Following the discovery
that coronal holes (sources of high-speed solar wind) were detectable in maps
of the Helium 10830 AŹ line, maps with this line were added to the daily
observing program in 1974. The name of the instrument refers to the number of
spatial positions mapped simultaneously along the slit of a spectrograph.

The program and instrument have been highly productive. The bibliography of
refereed papers using data from the facility numbers nearly 500. Among the
discoveries with the instrument are the existence of an intermittent, mixed
polarity magnetic field over the entire solar surface, a magnetic field canopy
extending above photospheric magnetic concentrations, and gigantic two-ribbon
events in the 10830 AŹ chromosphere associated with filament eruptions and
coronal mass ejections. Several theses were based on data from the instrument.

Until recently, the instrument probably produced more data than any other one
on Kitt Peak. It was used every clear day for more than nineteen years and
generated 16 Mbytes each day for a total of nearly 100 Gbytes. Progress in
recording technology is such that at first only one full disk observation
would fit on a 2400 foot, seven-track tape. When 1600 bpi nine-track tapes
became available, five observations would fit on a tape. Now the archive of
data is being converted to Exabyte tapes, and the entire set of observations
will fit in a small cardboard box.

Support for the facility has been provided by NOAA since 1974 and NASA since
1976. The observations are made available in nearly real time to the Space
Environment Laboratory of NOAA in Boulder, Colorado. The data are also now
placed on an anonymous ftp disk for general community access.  All of the data
are in the public domain, and some products are published monthly by NOAA.

Technological improvements, science imperatives, and servicing difficulties
have made the instrument obsolete. It has been replaced by a
spectromagnetograph which provides better measurements of more quantities. When
it was designed, the 512 channel magnetograph was state of the art. A new
company, Reticon, was beginning to offer linear arrays of diode light sensors.
Their product line did not include what we needed, so they designed a 512
element array with 25 x 600 um pixels to meet our needs. Two of these arrays
were placed in the focal plane of the spectrograph on the red and blue wings of
a spectrum line so that circular polarization strength could be measured.
Reduction of the 2 x 60 x 512 samples per second was done using an assembly
language program and a Varian 620f computer-a hot machine in the early 1970s.
The program very nearly consumed all the capability of the computer. A few
years ago, the Varian (the last one on Kitt Peak) was replaced with a DEC
11-73 and the program rewritten in FORTH.  However, it was necessary to add a
custom 68020 processor to match the capabilities of the venerable Varian.

The 512 channel magnetograph is an example of a successful program. We expect
that the new spectromagnetograph, which has replaced it, will continue the
tradition.

Jack Harvey

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