andrea tasselli:
Only if you don't follow the usual trodden path. I can see how I would use such a filter in my workflo
Intriguing comment and a bit of a challenge there Andrea -- the usual paths are sometimes well trodden for a good reason --so tried here to further think through your proposition that the H beta filter could be useful ?
Firstly yet to see an example where imaging for example a type II region with an H beta filter has been used except where the point of interest was itself to measure the exact ratio of H alpha to H beta in different parts of the region ? Or is there H beta imaging stuff out there that I'm just missing?
On the face of it such apparent under use of the HB filter by the wider astro-imaging community over so many years would seem to indicate a problem - or likely a fundamental limitation in what the filter can offer?
So back to the basic physics - for normal HII regions at temperatures of 10-20000 K the H alpha signal will be ~ 2.8 x the intensity of the H beta signal. That is fixed by the fact that n =3 --> 2 Balmer transitions are are a fixed amount more probable than are n =4 --> 2 transitions under the given conditions.
However - this ratio of H alpha to H beta in such an HII region can increase (but not decrease) - up to a value of about 4 (or so I have read). This increase is according to density differences in scattering dust where the H beta is naturally scattered more than the H alpha. So between different HII nebulae and also within different regions of the same nebula one might expect to see differences in images of - for example - purple shading according to red HA/ blue HB ratio differences of between ~ 3 to 1 and 4 to 1 ?
That kind of level of colour differentiation might well be visible if your image was simply a dual narrow filter image with just H alpha versus H beta of some type II region - and also if the effect was not obscured by tonal variation (i.e you would expect the darker tone denser regions to always be the redder)? HA v HB might be an interesting way of imaging type II region density -- but I'd argue that we get plenty of cues to the presence of dust/ scattering/ density anyway just in the course of normal imaging (i.e denser dust regions look darker and redder just in the normal course of RGB , RGB HA etc imaging of type II regions anyway)?
So I can see H alpha/ H beta as a specialized interest perhaps but struggle to see how H beta could be usefully combined into or add to the other types of imaging. Or you could argue that it is included in images anyway just in the course of broadband and broad UHC type filter imaging?
So e.g. if you were to combine the HB image into a triple - HA, HB and OIII - how would that work in practice? The OIII in HA regions normally needs stretching up anyway so OIII plus HA overlaps will look the same as variations in Ha/ HB ?
Part of the real problem with HB I think is that - unlike OII, NII and SII it does not have the potential to add any completely new feature to any image over HA - it will always be spatially exactly coincident with the HA and always dominated by it. It only has the potential to make red areas slightly more purply for example -- in a world where NB colours are all rather virtual anyway and nothing that you could really see because they are not dense enough. To me the potential to differentiate 3-1 versus 4-1 differences colour differences would never be worth the imaging time.
H beta is a useful filter for visual use to spot HII regions because of the limited ability of the human eye to see deep red at night. But for me -- depending on the object and its temperature etc -- I am going to continue simply imaging in H alpha and - if it is a pure NB OOH or SHO type image - using the Halpha to calculate the approximate implicit H beta signal - of about a third for an HII region - not exactly correct but good enough - to then add into the blue channel .
Tim
