denababy I have a zwo294 mc pro camera and a zwo ASIAIR pro, I’m thinking of getting an Altair Astro Triband filter.
As long as your main aim is to image emission nebulas.
That filter will not be suitable for reflection nebulas for example, and you may have more difficulty with balancing the colors from galaxies, in addition to not really improving wide band galaxies by much, except to require long exposure times.
As passive filters, narrowband filters do not bring up (amplify) a passband. They work (improve signal to noise ratio) by cutting off "noise" from wavelengths that are not in the passband.
Notice that the wavelengths that these narrowband filters pass are typically set to H-α, H-β, Oxygen-III and Sulphur-II. Unless your target only emits these wavelengths, their other wavelengths will be as much attenuated as your sky background (and thus not improve signal to noise ratio).
As such, they are most effective (and really, only effective) for emission nebulas. Or if you want to bring out the H-α parts of a galaxy.
Let's say your sky background emits evenly between a wavelength of 560nm and 700 nm, and the red sensor of your camera also have a constant sensitivity between those two wavelengths. (They won't, but this is just to illustrate how filters work). That is a passband of 140nm.
Lets say, your filter is spec'ed to pass 5 nm around the H-alpha wavelength (656nm), and pass nothing else (again, real-world filter are not as brickwall as this). What this means is that the filter will knock your background noise by a factor of 28x. Meanwhile, it will pass your target (if your target emits H-alpha) completely (again not true since filters will not pass 100% of the emission wavelength).
A Bortle 8 vs Bortle 2 sky differ by about 3.6 stellar Magnitudes, or about 27 times (2.5123.6 = 27.5 ...why I chose to compare Bortle 8 and Bortle 2 :-).
So, using said 5 nm filter will make shooting a Hydrogen-α target under a Bortle 8 sky as good as someone shooting under a Bortle 2 sky, as far as the red component goes.
Not quite... in practice, the filter will only pass 85% to 95% of the H-α, depending on the quality of the filter. Additionally, the other components of the Bayer array will also be receiving sky background from other filter passbands. So, perhaps Bortle 8 to Bortle 3 type improvement is more likely in practice with 5nm ffilter passbands.
And of course, a filter that is 7 nm wide would not do as well (by a significant factor actually), while a 3 nm wide filter will do better.
Good filters are not cheap, cheap filters are not as effective. The narrower they are, without sacrficing the transmission efficiency, the more expensive they are.
When choosing narrowband filters, be careful too about the halos that they produce when you are including bright stars in your images. Unless you have money to spend to be a guinea pig, don't buy any narrowband filters until you have seen posted images of bright stars next to an emission nebula. Pay attention to Alitak with the Horsehead nebula, for example. Sadly, 90% of images I have seen of the Horsehead have halos around Alnitak -- and even sadder, the people posting the images are unaware of the halos!
Another thing to consider is the response of the camera sensor. Many Bayer sensors produce almost the same response on the Blue and Green components to O-III and H-β. As such, you need to play post processing tricks to get a deep bluish hue from a nebula since the equal blue/green response will produce a cyan hue. And if there is also H-α present, you won't see any bluish hue at all, but neutral hues (i.e., R=G=B). The blue can only be brought up by reducing the red component, and all the stars become blue too! (Monochrome cameras are the only ones that can do this properly.)
Chen
