Hydrogen Alpha and Beta filters

[John in Penzance]

Any bit of Hydrogen will be spitting out both alpha and beta frequency photons so I'd have thought either narrowband filter will show the same image.

Obviously I'm mistaken since both filters are available, but Google as I might I've failed to find out what difference there is between the image each produces or why the images differ.

There will be a simple answer?

[Dinu Adrian]

The idea behind narrowband filters is that they are centered one a specific wavelength and basicly allow only the photons with that wavelength to pass through. Of course, this would be for an ideal filter. Even the best filters have a bandpass of a few nanometers. You may find their bandpass to vary from 2 to 32 nanometers depending on the filter. This is not an issue, is just an engeneering problem, I guess. Anwway, the images given by the Hydrogen alpha filter will be different than the Hydrogen beta filter because they correspond to different emission lines. The H alpha is around 656.28 nanometers and the H beta is around 486.1 nanometers. As to why these to emission lines are different, well this is due to the way bodies emit light. If you aproximate a body to a Black Body than you can say that the flux of light it emits is described by Planck's law for Black Body Radiation. Since stars are a good aproximation to a Black Body we may very well take them as one. Depending on their temperature they emit more light at some wavelenghts that at onthers. Many of them emit far more light in the Hydrogen alpha line than in the Hydrogen beta. For the same time expusore, the Hydrogen alpha image would be brighter the the Hydrogen beta one. That's why they are different 

 

Adrian 

[Owmuchonomy]

We need a NB imager to come along but following the nice description above I believe some Deep Sky Objects benefit from attention with a Hydrogen beta as well as or instead of alpha. Supposedly, this is the case for the California nebula and the Horsehead. I imagine the distribution of those emission lines could vary through the gas cloud thus rendering depth to any image? Let's see what the expert imagers say.

[ultranova]

Adrian's description is spot on really, all depending on the Target you are capturing

most Nebula will be rich and bright in the HA wave length and very weak in say OIII

 for the same given exposure length, the main idea of having / combining various

NB filters is to create a false colour image by assigning the various wave lengths  

and assigning them to a particular colour, I.E OIII you could assign to the blue channel

There are so many colour variations to be had by this method,

or you can even mix the NB data with RGB.

It would almost be like On RGB imaging assigning the red to blue the green to red etc.

if that makes sense

Paul

 

 

[StuartJPP]

Hydrogen Alpha (~656nm) is the wavelength of the photon generated when the electron of a hydrogen atom falls from the 3rd energy level to the 2nd...Hydrogen Beta (~486nm) is when the electron falls from the 4th to the 2nd energy level.

A Hydrogen Alpha narrowband filter will only allow a narrow portion around Hα through (a couple of nm to a few tens of nm either side) and a Hydrogen Beta will only allow a narrow portion around Hβ wavelengths to be passed through. Both will give a representation of how much hydrogen there is in a specific part of the sky and as far as I can remember there is roughly 50% more Hα as there is Hβ for that same area...I am not sure what effect causes either to be produced (natural decay?).

I always thought that a Hydrogen Beta filter was mostly used for visual since your eyes are more sensitive to green light which is closer to the Hβ line. So even though there is less Hβ, the sensitivity of your eyes to that wavelength compensates...but honestly I do not know the real difference...hopefully someone can elaborate? They may also be more useful for non-modified DSLRs that would traditionally chop off the Hα line severely (although NB is not really recommended for DSLRs).

 

Quite interesting is that for radio astronomers the "Hydrogen Line" (21cm) is the wavelength generated when the "spin" of the electron changes from being parallel to being anti-parallel.

Please note that the above is only my own "noddy" interpretation and shouldn't be classed as being "scientific"

 

[John in Penzance]

Thank you all.

Stuart may have cleared up why both filters exist for visual observation: increased retinal sensitivity to green light compared to red sounds like a reason for using a Hβ filter.

Adrian notes that the intensity ratio of the two frequencies shows where is hotter or cooler. For imaging, if each filter feeds a false-colour into a merged image, perhaps any resulting colour variation would show temperature gradients within a gas cloud?

[ollypenrice]

For an imager the H beta line traces pretty much the same gasses and structures as the H alpha but with a fraction of the signal. Is it worth it? Why not add a touch of Ha to the blue channel as surrogate H beta? Many do. But for visual hunters of the Horse then our eyes can see H beta and so this filter can pass just enough of the emission to render the dusty HH visible by contrast. Yes well - I reckon I've 'seen' it three times in our 20 inch Dob but I'm using the word 'seen' in a rather optimistic way!!

Olly

[Dinu Adrian]

On 26.06.2016 at 11:47, John in Penzance said:

Thank you all.

Stuart may have cleared up why both filters exist for visual observation: increased retinal sensitivity to green light compared to red sounds like a reason for using a Hβ filter.

Adrian notes that the intensity ratio of the two frequencies shows where is hotter or cooler. For imaging, if each filter feeds a false-colour into a merged image, perhaps any resulting colour variation would show temperature gradients within a gas cloud?

There is something like this too, only that it is used to find absolute temperature instead of its gradient. It is called B-V index and relies on the UBV filters on the Johnson scale. It's just another set of filters which happens to be the standard for scientific measurements :))). You measure the magnitude of a star in the B filter and in the V one, you subtract the V magnitude form the B one and some math gives you the absolute temperature on the surface of the star.( I don't know how to use this for other objects :)) ). So.. yeah... B-V index. 

 

Adrian

[michael.h.f.wilkinson]

I use my H-beta filter only for visual, and indeed for the Horse-Head nebula (got it in my C8). H-alpha is much brighter in nebulae, but the dark adapted eye can hardly see that. By contrast, the dark adapted eye does see H-beta. I cannot recall seeing an H-beta filter for imaging, most are sold for visual, and as such may pass quite a bit of IR (to which we are blind, anyway, but a CCD isn't!).

[Physopto]

Astronomik did at one point sell H beta CCD imaging filters, I have one, but now only sell it for visual astronomy. Astrodons in H beta are still available.

Derek