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Splitting Hairs: Separating Ha from [NII] with filters
My new [NII] filter from Cust Sci is rated at a 3nm FWHM with a center
wavelength of 658.3nm. On the other hand my Ha filter from them is a 3nm/656.3nm filter. The result is that there is 2nm separating the two emission lines. The Schuler Ha filter I have is rated at 9nm/656.3nm so it will pass the upper and lower [NII] lines (there's another [NII] line at 654.8nm) as well as the desired Ha line. The Astronomik is something like 12nm from what I recall, but could be mistaken on that. It also will pass both [NII] lines and the Ha line. The Schuler and Astronomik filters could be more accurately referred as an Ha + [NII] filter in my opinion. Cust Sci has a 4.5nm wide Ha filter too and it looks like it may pass the [NII] lines as well. To separate the two lines it seems the 3nm filters are the better choice. To see how well the Cust Sci filter separated the two lines I made an image of the Crab using the Ha and [NII] filters only. I assigned Ha to Red, and [NII] to both Green and Blue. Here is the image: http://www.rdcrisp.darkhorizons.org/...ha_n2_page.htm As you can see there is a separation of the two emission lines and contrast can be seen in the Crab structure. |
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