Combing ha and hb filters?

[ebdons]

Hi, hope this makes some sense, but if you combine (screw together) a hb and ha with the same nm spec ie 12nm, does the higher wavelength of ha 656nm filter thru the hb filter which is 486nm, not sure as to how the light wavelengths behave, obviously you can use these filters independently of course, just a query on my understanding of either the same nano-meter spec or using a different spec filter with another nano-meter to bring out a different result. does this make sense on here? Tony

[vlaiv]

No, it does not work like that - in fact you'll get nothing - or rather no light will pass thru such combination of filters.

To understand why - just think of what is going with each filter - let's start with Hb filter - it passes only wavelengths around 486nm (+/-6nm or there about for 12nm filter). All other light is blocked and does not make it even thru that filter. Now any remaining light that goes thru that filter hits Ha filter which passes 656nm +/-6nm. But only 486nm light had passed Hb filter so it will be blocked by Ha filter because it blocks everything except it's pass band. And since there is no light around 656nm because Hb blocked it all - no light will pass both filters.

[ebdons]

Yep ,makes sense,  does a 12nm ha filter have the same wavelength as a 6nm ha? is it the same bandwidth? but what makes the 6nm different to the 12nm, just the filter material? just trying to get my mind around filter types. Tony

[vlaiv]

If both are Ha filters, then it is very likely that they have same central wavelength (abbr CWL) - designers of filter aim for this to be wanted wavelength - like 656.28nm, but it does not matter if its slightly off - like 656nm or 656.3. Width of filter denotes width of the band of wavelengths that are passed by filter, or more specifically - it should denote full width at half maximum, as not all filters have sharp cut off edges.

So something like 6nm Ha filter will have CWL of 656 and FWHM of 6nm, while 12nm Ha filter will have CWL of 656 and FWHM of 12nm.

Best way to understand filter performance is to look at its frequency response curve - a graph that represents percent of light passed at any given wavelength. Here is for example Baader Ha 7nm filter:

You can see that max transmission for example peaks somewhere at 88%, and that filter response is not rectangle (ideal frequency cut off filter), but rather "bell shaped" sort of curve. If you measure width at around 45% you should find that it is about 7nm wide - that is width of filter. If you divide that width in two equal pieces and draw vertical line - it should land somewhere at 656 nm.

There is some light leak that can be seen on the right - but it is far into IR and most cameras are not sensitive in that area. But if you have camera that is sensitive in those wavelengths and you use refractive optics for example - you can get slight halos around bright stars because of that in your Ha subs.

Contrast that to 35nm Baader ha filter:

Here curve is obviously wider because of 35nm pass band, but it shows interesting feature - Ha wavelength is at 95% but actual CWL is not 656.28nm but rather somewhere around 648nm or so. If filter is wide enough to include wanted line, that line does not need to fall exactly on center of pass band.

In relation to your original question - easy way to understand combination of multiple filters is to look at their graphs, align wavelengths and multiply percents, or values on the graph. If you do this with Ha and Hb filter curves, you can easily see that there will be no light going thru because graph is at 0% most of the time and anything times 0 is 0.

[ebdons]

Ok, many thanks for your always comprehensive explanation of the answer, clear skies, Thankyou sir!

[ollypenrice]

A couple of related things: H beta usually traces the same gasses as H alpha but with far less signal so it is not regarded as terribly useful by most imagers. Also, the key thing about narrowband filters is not so much what they pass as what they block. A red filter will pass Ha comprehensively. What it will not do is block those non-Ha red wavelengths, so it will not allow you to isolate, visually, the structures which will be isolated by an Ha filter.

Olly

[Knight of Clear Skies]

2 hours ago, ollypenrice said:

H beta usually traces the same gasses as H alpha but with far less signal so it is not regarded as terribly useful by most imagers.

Just to add, I believe H beta filters are more useful for visual astronomy as the human eye is more sensitive to blue light than red.

[ollypenrice]

8 minutes ago, Knight of Clear Skies said:

Just to add, I believe H beta filters are more useful for visual astronomy as the human eye is more sensitive to blue light than red.

Famously good for the Horsehead, visually.

Olly