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H alpha.

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thing

thing

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I get a little confused by low wavelength filters being called H alpha filters. I always thought that H alpha emitted at around 650 nm, and the other hydrogen emissions were H beta, H gamma etc. How can you have an H alpha filter at 6nm? Someone put me out of my misery.

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beamish

beamish

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I might be wrong but if you think of it terms of a "narrow slit of a window" allowing the Ha thru that ocurrs at " around 650nm :(

So its a 6nm window or a 13/12nm window, just tightens the whole thing. It's not referring to the wavelength per se

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SteveL

SteveL

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I think you are confused by terminolgy. Narrowband filters have two important values attached to them. The wavelength of the filter (i.e the center of the zone that the filter is aimed at), and how wide that zone is. 6nm or 12nm tells you how wide that zone around the desired wavelength it will allow through.

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thing

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I might be wrong but if you think of it terms of a "narrow slit of a window" allowing the Ha thru that ocurrs at " around 650nm :(

So its a 6nm window or a 13/12nm window, just tightens the whole thing. It's not referring to the wavelength per se

But 6nm isn't around 650 odd nm, it's a considerable difference, it's the difference between visual red and x-rays.

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thing

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I think you are confused by terminolgy. Narrowband filters have two important values attached to them. The wavelength of the filter (i.e the center of the zone that the filter is aimed at), and how wide that zone is. 6nm or 12nm tells you how wide that zone around the desired wavelength it will allow through.

Ha! Thanks Steve, that one's been bothering me for a while.

Actually rereading Beamish's post, I think that's what he was saying too.

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Ben Ritchie

Ben Ritchie

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The main difference is the amount of 'background' light they let through.

The rest wavelength of H-alpha is 6563 Angstroms (or 656.3nm), red- or blue-shifting that by 1nm requires motion of approx. 500km/s which is relatively large for any galactic object, so it's fair to say that whatever width you pick (6nm, 12nm or 35nm) will capture all the H-alpha emission from your target - it's not like the much narrower solar H-alpha filters, where motion along the line of sight can move the emission off-band. However, the H-alpha emission is superimposed on a background level of light pollution, scattered moonlight and broadband emission, so what you're getting by choosing a narrower bandwidth filter is not extra H-alpha but lower background. The practical effect is the lower the bandwidth of the filter you pick, the greater the contrast between H-alpha emitting gas and the background - stars, for example, are broadband emitters and appear much more point-like in a 6nm filter than 35nm.

After all that, i'd suggest that it doesn't matter too much, although i'd probably avoid the 35nm filters, especially if you want to image on moonlit nights. IMHO 12nm is a good compromise between performance and cost. Transmission tends to drop a bit on the narrower bandwidths unless you spend even more money, but that's not a huge issue either, maybe +10% exposure time.

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Ben Ritchie

Ben Ritchie

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Here's a real-world example, which is an extract of the H-alpha line from a VLT spectrum i'm working on. The H-alpha emission is all concentrated in the big 'spike' at 656.3nm, while the whole spectrum covers roughly 12nm.

Notice that there's a background level of approx. 250 counts across the whole spectrum, while the H-alpha line peaks at around ten times the background. However, the integrated flux from the background is actually greater than the line emission, so with a 12nm filter you're actually getting more non-H-alpha light in this case than H-alpha. Halving the bandwidth obviously reduces the background without affecting H-alpha, and you can drop to 3nm or even 1nm without cutting into the H-alpha line. With a 35nm filter you're obviously getting lots more background.

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narrowbandpaul

narrowbandpaul

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There is another subtle issue here...

when you use a 6nm (I do) or greater bandpass filter, the emission you pick up is not entirely pure Ha.

There is an emission line (doublet in fact) from N[iI] (nitrogen) which flanks the Ha emission, separated by about 2nm.

The case in point is M27, which has some red mottling in the core area.

This is usually attributed to Ha, but an image taken by Don Goldman (astrodon) shows that this mottling is actually N[iI].

To split these wavelengths requires a filter with bandpass of about 3nm...which are expensive to make and have a slightly lower transmission efficiency.

I dont know if N[iI] is present in diffuse emission neb or if its just planetaries....

heres the link to the Ha/ N[iI] comparison....worth a look

Paul

http://dg-imaging.astrodon.com/gallery/display.cfm?imgID=148

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Ben Ritchie

Ben Ritchie

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There is an emission line (doublet in fact) from N[iI] (nitrogen) which flanks the Ha emission, separated by about 2nm.

It's [N II] 6583, you can see it in the spectrum above although it's pretty weak. Very useful line though, the ratio with [N II] 5754 is a useful diagnostic of temperature and electron density in the nebula. However from a backyard point of view it's not really something to worry about unless you're going after Don Goldman-levels of narrowband imaging, it's one of a group of several emission lines that are accessible to amateurs but generally rather low intensity and/or present in a limited range of targets.

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Ben Ritchie

Ben Ritchie

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If you're intersted in the detailed physics of the emission lines, i'd suggest a copy of Osterbrock's book if you don't already have one, very good.

The standard set of VLT/FORS narrowband filters are listed here

http://www.eso.org/sci/facilities/paranal/instruments/fors/inst/Filters/#both

to give you an idea of some of the other commonly used ones, although some (like He I and He II) are used for looking for emission-line stars, e.g. Wolf-Rayets, rather than nebular emission.

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