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Re: (meteorobs) Fw: continuum emission in meteor spectra



Lew wrote:
>
> BTW, from Ed's excellent site and other info on the Web, it seems a video
> spectrum is nothing more than a spectrum captured on video rather than
film,
> so allowing for the use of an image intensifier between the grating and (I
> ASSUME) the video camera. Is that right, Ed?

Lew:
    Video spectrographs are quite simple and work pretty much the same way
as a film spectrograph.  A transmission grating is placed over the front of
a camera lens, (no slit is used and in this configuration one refers to this
as a slitless spectrograph).  The lens then focuses the image on the imput
screen of the image intensifier.  The intensifier amplifies this image and
produces an intensified image on its output screen.  A video camera then
records the image produced on the intensifier output screen.  Film
spectrographs operate in a similar manner.  A transmission grating is placed
in front of the camera lens, the lens then focus the image on the film.
Grating spectrographs using film will record meteor spectra of around -3.0
magnitude and brighter.  Video spectrographs are more sensitive because of
the "intensifier" and will record spectra as faint as +2 or +3 magnitude.
It should be noted that if either system is used without the transmission
grating you will record meteors approximately three magnitudes fainter.
Even fainter with intensifier systems.  Why arn't intensified spectrographs
used all the time???  Resolution is not as good as film.  Intensifier
screens are small, 18mm, 25mm, 40mm in diameter.  In order to fit an entire
spectrum on this screen dispersion is rather low so many lines are not
resolved but overlap.  Film cameras can use large format films so high
dispersion is possible.  Dispersions of 10A/mm have been achieved on film.
At 10A/mm dispersion, a spectrograph covering a range of 3500A to 6500A you
would need a film 300mm wide to capture all of the first order spectrum.
Image intensifiers just arn't that large.  For a detailed study of a
spectrum dispersions should be 100A/mm or better.  Even then, only around
50% of the spectral features are resolved into single lines.  Video
intensified spectrographs at present can't do this. Their advantage lies in
the fact that they will record fainter spectra than film will.

Ed Majden



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