Marty, are you telling me I can use my Optolong 2" green filter that I've been using for night as a good enough stand-in for a continuum filter? I literally just added a continuum filter to a shopping cart...guess I can take it out? Don't know why I haven't thought of this.
There's nothing special to it. Just look at the centered wavelength and the bandpass. None of these are really that narrow band and include different wavelengths, they're not targeting a specific wavelength based on solar emission/absorption lines. You'll find the closest thing to 540nm is 546nm which is a defined absorption line based on Mercury (Hg), after that is 587nm based on Helium (He) and before all of those is 527nm which is based on Iron (Fe). 540nm itself doesn't correspond to anything specific other than its what most optics are highest corrected for as our eyes are most sensitive to green. Using it as a narrowband filter, well, it's not narrowband enough here (10nm) to represent anything specific really. Yes, it's more narrow than a typical green imaging filter. But again, you're not viewing something unique in 540nm since it has a bandpass of 10nm deviating from that centered wavelength and none of those wavelengths correspond to a specific emission/absorption line.
Here's the defined classic lines:
Here's more info on different lines:
And here's some interesting tests comparing different filters.
I have both the Solar Continuum filter, Baader green filter, 395nm filter, 430nm filter, etc. And I rarely use the Solar Continuum filter when imaging. 430nm & 395nm give more contrast on cells than 500+nm does. And when it comes to sunspots, these shorter wavelengths actually darken the inside of a sunspot, so it's actually better if you like sunspots to image them with long red filters like 610nm~656nm.
Edited by MalVeauX, 18 September 2019 - 11:58 AM.