Re: (meteorobs) Re: NRL Instrument Makes First UV Observation of Meteor in Space

Subject: (meteorobs) Re: NRL Instrument Makes First UV Observation of Meteor
in Space

    I have finally had a chance to read the text of the message on UV-meteor
spectroscopy that you have referred to and now I think I can comment further
on it.

Lew Gramer quoted and wrote:

However, this line:
> > "Ground-based observations of meteors cannot detect many of the
> > important elements and compounds expected to be present in meteoroids."
> really gave me pause. Ed and others, is it in fact true that ground-based
> visible and IR meteor spectra are useless for studying certain constituent
> elements in the meteoroid itself??

    In the above statement, "Ground based observations of meteors cannot
detect MANY ---------etc.  This statement is true.  It refers to the
spectral lines present in the far UV region that cannot be observed from the
ground because of atmospheric absorption.  This takes nothing away from the
importance of ground based spectroscopy but adds more information to the
picture.  Early meteor spectra recorded meteor spectra well, at the blue end
of the spectrum.  Why, prisms were used as the dispersive element and they
only provide good dispersion in this region.  With a prism, the red end is
very crowded so lines overlap and are difficult to measure.  Also, blue
sensitive ortho plates were used.
    Later, panchromatic plates were used extending the region to slightly
below 4000A to around 6500A.  In the 1950s, transmission gratings blazed for
the visual region of the spectrum were introduced.  This solved the problem
of the poor dispersion of a prism at the red end of the spectrum.  Later,
gratings blazed for the red end, provided photographic spectra out to about
9000A or the near IR.  Again new optics were introduced which allowed
spectroscopy in the near UV region down to about 3300A.  Quartz optics had
to be used as standard glasses used in normal lenses are not very
transparent much below 3900A.  The atmospheric window from a ground based
observatory will not allow spectroscopy below  the ~3000A region.  Thus the
importance of extending the spectral range below this, as this NRL project
reports.  This does not take anything away from the importance of
spectroscopy in the other regions of the spectrum such as the ones obtained
from ground based systems but adds IMPORTANT information not observed
before.  Just as the use of video spectroscopy is important, as it extends
spectroscopy to fainter meteors.  Photographic systems using film will only
record spectra brighter than magnitude "zero" (prism systems) or ~-2.0> for
transmission gratings.  Video intensified systems have extended the faint
detection capability to about ~+3.0 magnitude.  It is too bad that the
resolution, as yet, is not as good as film based spectroscopy.  Video still
provides important information on the spectra of fainter meteors and in some
cases train spectra.
        I hope my explanation clears things up somewhat.

Ed Majden - AMS Meteor Spectroscopy Project Coordinator

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