View Full Version : Splitting Hairs: Separating Ha from [NII] with filters

Richard Crisp
December 28th 03, 03:52 AM
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:


As you can see there is a separation of the two emission lines and contrast
can be seen in the Crab structure.