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(meteorobs) Re: Holographic acetate grating spectroscopy.
For anyone considering the use of Holographic acetate (thin film type)
transmission gratings for meteor spectroscopy, I have a few suggestions.
When I did my initial tests with these gratings, I mounted the grating
between two sheets of flat glass. If you inspect the meteor spectrum
secured with such a grating on the spectroscopy section of the AMS home page
at http://www.amsmeteors.org/index.html you will note that the spectral
lines are not as sharp as they should be. The camera I was using has a film
registration problem (keeping the film flat in the focal plane). This may
only be part of the problem! There may be possible internal reflections
between the two pieces of glass I used to keep the holographic grating flat.
With small diameter lenses such as a standard 50 mm fl 35 mm camera lens or
a slow f-4.5 lens on a large format press camera, the glass sandwich could
perhaps be dispensed with. Properly mounted, the grating should remain
reasonably flat on its own. For larger format lenses such as F-2.5 - 7 inch
or 12 inch focal length aero lenses a possible solution would be to use a
Beseler Negaflat film holder. This type of film holder is used to keep 4X5
negatives flat in the focal plane of an enlarger by keeping the negative
under tension. This Negaflat device could probably be used to keep acetate
film type gratings flat also without the glass sandwich. There should be
less light absorption also perhaps increasing efficiency a small amount.
Such gratings need all the help they can get as they are less efficient than
a precision blazed replica grating. Anyone know where I can get a 4X5
Negaflat cheap? Our Canadian dollar has hit the skids again! :-(
Ed Majden - AMS Meteor Spectroscopy Project Coordinator
Refr: JRASC Vol 92, No. 2, April 1998, p.91-92
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