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(meteorobs) selecting a grating for meteor spectroscopy



SELECTING A GRATING FOR METEOR SPECTROSCOPY

    I am often asked by would-be amateur spectroscopists. how to select a
grating for this type of work.  The best and most efficient grating of
course is a precision blazed replica transmission type large enough to
accept the light cone entering your camera lens.  For normal cameras this is
probably one with  a ruled area of 50X50mm or better.  Unfortunately
precision blazed replica gratings are expensive.  Less efficient gratings
have been used but these are only capable of recording the spectra of very
bright meteors.  A precision blazed replica grating mounted on a fast lens
using fast b&w film will record spectra of meteors brighter than -2.0
magnitude in most cases.  Other gratings such as thin film acetate
holographic gratings and the Cokin type camera filter diffractors will work
but with much less efficiency.  They are however an in-expensive way of
getting into this field.

        It is highly desirable to obtain the largest dispersion that is
possible but there are practical limits that one must adhere to.  Generally
speaking, a coarser grating, one with less g/mm, is more efficient than a
high density grating, one with more g/mm.  One must select a grating that
will fit your film format size or image intensifier screen size.
Dispersions of 100 A/mm or better are highly desirable so lines that are
close together can be resolved.  This is not always practical as such high
dispersions are too wide to fit the screen of an image intensifier so one
must compromise.  Normal pan films are sensitive from 0A to around 7000A and
image intensifiers are sensitive from 0A to around 9000A.  Using film
cameras such as the K-19 Aero camera with an F-2.5 - 12 inch focal length
lens on 8X10 inch film, if you use a 600 g/mm grating you will achieve a
dispersion of around 54A/mm in the 1st order.  Very nice if you can afford a
grating large enough for such a lens and also pay for the high cost of 8X10
film.  This is probably not practical for most amateurs.  On the other hand
you could use a 600 g/mm grating on a 50 mm focal length 35mm camera and
record low dispersion spectra at around 326A/mm.  It would be much better to
use a large format 4X5 inch camera with a longer focal length lens to
achieve greater dispersion.  The film using photographic systems above will
record spectra of most meteor brighter than -2.0 magnitude.  Somewhat a rare
event so don't expect to capturing many spectra by these methods.
    Today some amateurs are using 2nd and 3rd generation image intensifiers
to record faint meteors.  When you add a dispersive  element such as a
grating this reduces the faint magnitude detectabity by 2 or 3 magnitudes
because the light of the meteor is spread into a number of monochromatic
lines of the spectrum.  There is also another problem to contend with.  The
image intensifiers used by amateurs come in two sizes, 18mm and 25mm
screens.  One must select a grating with a lens that will fit the spectrum
on such a small format screen.  Image intensifiers are sensitive from 0A to
around 9000A.  If you want to record the zero order image plus the first
order spectrum you must select a grating/lens combination that will give you
a dispersion of 500A/mm with an 18mm intensifier or 360A/mm for a 25mm
intensifier.  This assumes that the meteor producing the spectrum is located
in the center of the intensifier with the zero order recorded at one edge.
This almost never happens as you cannot predict where the meteor will occur.
Even using the above parameters these spectra are considered low dispersion
an many lines will be blends because they are to close together to be
resolved.  This is the only practical method to obtain spectra of fainter
meteor members however.  During last years Leonid shower I recorded 110
meteors with my system which used a surplus experimental grade 25mm 2nd
generation image intensifier imaged with an F-1.4 - 50mm focal length lens
fitted with a 600 g/mm precision blazed replica grating.  60 were  zero
order or direct meteor images and 50 recorded some portion of the 1st order
spectrum depending on where the meteor was positioned.
    For an 18mm intensifier I would select a grating with a ruled density of
around 300 g/mm.
This will give you a dispersion around 652A/mm with a width of 13.8 mm if
you record the zero order and Ist order  spectrum.  If you use a 25 mm
screen intensifier you can use a 600 g/mm grating providing a dispersion of
around 326A with a width of 27.6 mm if you use a 50 mm focal length lens.
These are low dispersion spectra and many lines will be overlapping blends
but the spectra are still interesting.
    Now, will they be useful to professionals.  That is a difficult question
to answer.  For the Leonids, professional programs have been intensive and
they have their hands full with the spectra they have recorded.  Unless you
have a very unussual or different spectrum it will probably not be measured
at this time but may be kept in an archive for future study.  Spectra of
minor shower members are of interest and you may be also lucky enough to
capture the spectrum of a very bright fireball that may drop a meteoite.
Such a spectrum would be of consideable interest but its just like winning
the lottery getting one of these.  If you don't try however none will never
be obtained.

Ed Majden - AMS Meteor Spectroscopy Project Coordinator
Courtenay, B.C. CANADA

PS:  You can do the spectra dispersion calculations by refering to the
formula in a good College level physics text book.  I hope I haven't made to
many errors in the above. ;-)

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