PART 1: About the chopper and polycyclic aromatic hydrocarbons


[Editor's note: 
This is the fifth in a series of updates which will continue through
November.  An archive of the previous journals is available on the Gopher 
at quest.arc.nasa.gov (look in Live From the Stratosphere under "Updates 
and Journals").  Or you can use the following URL to access past updates:

The passage below continues a series of journals from last
year as a preview of what is to come.  It is another version of the
broken mirror flight which is also described in LFS Updates #3]

KAO flight 5/7/94 from Honolulu to Honolulu 
From Jesse Bregman in Hawaii  

Last night's flight didn't last very long since a crucial piece of the
telescope, called the  chopping secondary, did not work. Observing in
the infrared (light with wavelengths  between about 1 and 1000
micrometers, and far redder than the reddest light we see with our
eyes) is difficult since everything at room temperature glows
brightly at these wavelengths. It is similar to trying to observe
stars with you eyes during the day. To accomplish this difficult
task, we use a mirror which rocks back and forth, alternately
putting the object we want to observe and blank sky on our
detectors. The mirror moves  back and forth 7 times per second, and
we have programmed our computers to read the light intensity in our
spectrometer* at the same rate. This technique enables us to
measure  signals that are 10,000 to 100,000 times weaker than the
general glow of our  surroundings. The moving mirror is called a
chopper, and it was this part of the telescope  that was not working.

The KAO staff will most likely be able to fix the chopper so we can
have a good flight on Monday to do the observations planned for last
night. If the chopper had worked, we would have observed a few
more bright infrared stars in  icy regions, similar to those on the
first flight on May 5.

We also had a group of objects on the list  which are completely
different. Instead of showing the chemical nature of the material
surrounding the star by absorbing light from the star, we can tell
what is around the stars by the light that the gas emits, having been
excited by ultraviolet (bluer than our eyes can see) light from the
stars. We have evidence that this material is similar to the soot
that comes out of car exhaust and leaves the black marks you see on
the road. They are called polycyclic aromatic hydrocarbons (PAHs)
and are very large molecules, especially by astronomical standards,
composed entirely of carbon and hydrogen. The observations we had
planned would test this idea by searching for emission features

which others at NASA Ames have found in laboratory spectra of
PAHs. Finding these new features in the spectra of the objects we
will look at with the KAO telescope will confirm that these
molecules are  indeed PAHs. We will then be able to use spectra of
PAH molecules to determine the kinds of environments in which the
molecules can exist. It may be that when the earth was formed,
PAHs were included and became the building blocks for more
complex organic molecules which eventually led to life on our

(Note by Edna DeVore)
*The spectroscope is the instrument attached to the KAO telescope
which allows astronomers to spread out the infrared radiation like a
prism or diffraction grating spreads out visible light into a band of
colors called the spectrum. When the infrared radiation is spread
out, astronomers can look at a small piece of the spectrum, and see
bright (emission) or dark (absorption) lines. These strength of the
lines (the "signal strength") is stored as numbers in a computer for
later analysis and comparison with the spectra of dust and ice
viewed by spectroscopes in the laboratories at NASA Ames. By
comparing the spectra of astronomical objects with the spectra of
dust and gas that is of known composition, the astronomers are able
to identify the composition of the dust and gas in space.

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