[Fwd: Mars project]

From: Laura Bashlor <lauralou@gatecoms.gatecom.com>
Subject: [Fwd: Mars project]
Date: Wed, 18 Sep 1996 22:57:23 -0400

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Date: Wed, 18 Sep 1996 16:42:26 -1000
To: lauralou
From: Jim harwood <harwood@hubble.IFA.Hawaii.Edu>
Subject: Mars project

Hi, Laura Lou-
        Glad you liked my early Mars experience.  That's fine, to pass it on
to your discussion list.  Would be delighted to help you with anything I can.
        Only thing is, I'm not a "Real Scientist".  I'm a "Real Software
Engineer".  I got this way during that very same Mars experiment.  When we
got back to Hawaii with our data on punched cards, I was asked to reduce it
on a process control computer that had recently been purchased to control
the 88" telescope. I had never worked with a computer before.   I found my
way through Fortran and the computer's assemply language and the electronic
hookups, and eventually became responsible for the computer control of the
88" telescope.  The rest is history.
        Prior to Hawaii I was an optical physics instrumentation specialist,
working for a Connecticut prime contractor to ARPA on ballistic missile
reentry research, using various special purpose cameras and opto/electronic
instruments on a tracking ship covering ICBM reentries.
        Your contour mapping sounds uncannily like our Mars experiment.  We
were poking blindly in infrared through Earth's and Mars' atmosphere.  Mars'
atmosphere is of course mostly CO2, which is highly absorbent of certain
infrared wavelengths invisible to humans.
        Our instrument (designed by Dr. William Sinton, retired) used a
cylinder about the size and shape of a 1-pound coffee can divided down the
middle lengthwise.  The ends were optically transparent to infrared (and
visible) radiation.  The divider down the middle separated the cylinder into
two sections, one of which was filled with nitrogen and the other with
carbon dioxide, to a pressure of a few atmospheres each.
        The cylinder was made to rotate at a rate of a few revolutions per
second.  Infrared light collected by the telsecope was beamed through the
cylinder so that for half a rotation the beam would go through the nitrogen
(transparent to IR at that wavelength), the other half through the CO2.  If
there was a lot of CO2 external to the cylinder, IR light from the sun
reflected off the object (Mars) would not show much change between the
nitrogen and CO2 halves of the cylinder rotation, because much of the
CO2-sensitive radiation was already gone.  If there was very little CO2 in
the external path, the signal during the nitrogen half would be strong and
the CO2 half weak, because of being absorbed in the CO2 gas in that cylinder
half.  So this was a very sensitive indicator of the amount of CO2 external
to the instrument.
        The infrared radiation that passed through the rotating cylinder was
sensed by a lead sulfide detector.  The electrical signal from that detector
was digitized and punched into cards, which was the computer input medium in
those days.  Thus, the signal became a bunch of punched holes in hundreds of
IBM cards, and it wasn't until weeks later when they could be analyzed back
in Hawaii that we discovered there wasn't enough dynamic range.  Nowadays,
astronomers can see a representation of their signal (usually a picture on a
screen) while they are on the telescope, and can repeat something if it
isn't working right.
        Let me know how the contour mapping goes.  Would be interesting to
see the map on the web.  Am also curious as to what kind of object is under
the foil.  Will have to wait until I see the map, I guess.  Poking through
the foil will also give the students an intuitive feel for the effects of
sample size.  If they could poke every millimeter, they will have a very
detailed contour map.  Poking every inch will miss much of the detail in
whatever the object is (dead frog?).  Our Mars experiment was equivalent I
guess to poking every quarter inch.  Maybe a little better.

        Best wishes-
	Jim Harwood
	Institute for Astronomy, U.H.