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barry,
At 100 or 200 lines per inch (4 to 8 l/mm) that's a very low line count.
More like a Ronchi grating than a diffraction grating. At 8 l/mm the resolution will be very poor and only macro detail will be observed in the spectrum.
A Cokin B40 "Cosmos" filter would be better, it is a 240 l/mm grating.
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