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Going into exotic narrowband even more


riklaunim
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I've just received a set of Edmund Optics narrow bandpass filters - for He-I, He-II, O-I (not O-III) and even Ar-III bands :)

a1pm48.jpg

I've described them on my webpage. I've used broader He filters before and they did provide interesting results. Now it's time to test this even more in autumn/winter on M42, Rosetta and maybe other targets :D

There isn't much professional images in such bands, as scientists usually want spectroscopy and not pictures ;) He is high/highest ionisation. O-I is neutral gas - quite different than Ha/O3/S2. Ar-III is... something.

6s5ant.jpg

nn5zph.jpg

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Right now I'm trying to fit them in a filter wheel. I have TruTek manual filter and they won't fit inside (to high by 1-2 mm). Can anyone check if other wheels like the Brightstar could handle them (screw two Astronomiks together for the height) ?

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Right now I'm trying to fit them in a filter wheel. I have TruTek manual filter and they won't fit inside (to high by 1-2 mm). Can anyone check if other wheels like the Brightstar could handle them (screw two Astronomiks together for the height) ?

Did you try swapping the wheel around - so that what would normally be the camera side is attached to the scope? It does mean that the knob used to turn the wheel is at the back but I have found this works with Baader filters.

EDIT: Just noticed you are trying to use 2 filters, I doubt my suggestion would work.

Edited by ColB
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I made a gasket/spacer to give me more space with one of my wheels... a bit of a fiddle but i didn't fancy turning down the filter cells in the lathe...fortunately i could adjust the spindle bearing to accomaodate the larger gap...

For a quick trial use washers and seal the edge with insultaing tape...

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for He-I, He-II, O-I (not O-III) and even Ar-III bands ...There isn't much professional images in such bands, as scientists usually want spectroscopy and not pictures

As a professional :) yes there is - for example, strong He-II emitters like Wolf-Rayets can be found in photometric surveys by looking at the He-II - continuum excess from two close-spaced filters, one covering a He-II line and the other a nearby continuum region. A normal star will show no excess, as both filters will be recording broadband quasi-black-body emission, whereas the emission line object will show a strong excess in the He-II band. Take a look here for an example (although it uses He I and He II lines in the IR, not visible)

http://arxiv.org/abs/astro-ph/0608356

However, I don't know quite what you expect to achieve with these filters, as few objects will show much true emission in these bands and many of those that do will be point sources (e.g. [Ar III] is seen in B[e] stars which will be point sources, although planetary nebulae commonly show it too). You may get some signal, but these are specialist filters for a reason, and not that well suited for general backyard use. It's a bit like H-beta, which for amateur use looks to effectively be an H-alpha filter with very poor transmission (in almost all nebular conditions H-beta traces the same gas as H-alpha, but with roughly a third of the emissivity). People use it and get something, but it's really just making life harder.

O-I is neutral gas - quite different than Ha/O3/S2. Ar-III is... something.

You're missing square brackets for a few of these, as they're forbidden lines. [O I] is a ground-state transition of neutral oxygen, typically seen from OH disassociation in dusty disk conditions but occurs in a number of other low-energy settings too. [Ar III] is a high excitation line and quite rare. As a general thought, it's really worth looking into the conditions that give rise to these lines, as it will strongly affect your choice of targets.

Edited by Ben Ritchie
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Pardon my ignorance here, Ben, and I know that you know what you're talking about - but my H beta filter seems to pass blue light, not red. You mean, perhaps, that it traces the same gas features, though it might be useful for colour balancing NB images maybe? I only used it once (on the Bubble) and it remains my only hardback book credit to date, in a book on string theory... in Dutch. Now that is making life harder!

Confused of Etoile St Cyrice...

Edited by ollypenrice
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Hi Olly,

You're right that H-beta is in the blue, at 4861 angstroms, whereas H-alpha is in the red ( at 6582 angstroms). But in narrowband mappings to RGB channels are pretty arbitrary, e.g. in the Hubble palette the red H-alpha line isn't used for the red channel, instead it's used for green - [O III], which is green, is instead mapped to blue. Having filters that are really blue, green and red emission lines might help star colours (i.e. H-alpha -> red, [O III] -> green, H-beta -> blue), although depending on how the filters are mapped to RGB they still might not be real. But I don't think that outweighs the disadvantages of H-beta.

As you know, H-alpha signal is strong and [O III] is often pretty good too, but [s II] can be a struggle so people look for alternative third channels. H-beta seems a good option - H-alpha works well, right? - but if you look at the emissivity under nebular conditions then the 4-2 transition (which gives the H-beta line) is a factor of roughly three weaker than the 3-2 transition that gives H-alpha, it's tabulated in Osterbrock and Ferland's Astrophysics of gaseous nebulae and active galactic nuclei which is a pretty good reference for this kind of stuff. So the emission is coming from the same hydrogen gas that gives you the H-alpha channel because the conditions don't vary enough in the nebula to really change the H-alpha/H-beta emissivity ratio, but because H-beta is weaker the S/N is worse ... expose long enough and you get signal, and you can build an H-alpha, H-beta, [O III] image, but you're not really doing much different to assigning H-alpha to two channels and taking longer to do it. Or, alternatively, expose equal times and you have a second effective H-alpha channel with worse S/N. See the problem? [s II] at least traces different gas conditions, so gives you a third channel with real information in it. Or, if you want to short-cut, you can go for the third blended H-alpha/[O III] channel that some people do.

edit: if reddening is significant (which it generally is for professionals, less so for the nearby bright objects in the backyard lists) then the H-beta/H-alpha ratio becomes even worse. A number of the objects I look at have extremely strong H-alpha emission but nevertheless barely detectable H-beta due to the loss of the blue end of the spectrum.

Edited by Ben Ritchie
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[s II] is often hard to catch, so this test will also show if [s II] could be replaced by something else. There are few people using He-II already and I've decided to test the [O I] and helium bands.

Various spectrograms show that those emission lines can be found and used for imaging (at least in theory):

http://astrosurf.com/buil/lhires3_faint/poster.png

http://www.nauka.rk.edu.pl/site_media/resources/nauka.rk.edu.pl/images/emisje_mglawic.png

I have the book "Astrophysics of Gaseous Nebulae and Active Galactic Nuclei" :) and I have to read it before season for Orion, Rosetta and friends will start. :)

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http://astrosurf.com/buil/lhires3_faint/poster.png nicely illustrates the H-alpha/H-beta emissivity problem too, look at the top-left spectrum

Worth looking at how weak some of those lines are in comparison with the commonly used lines though, He II is very weakly present in a couple of planetary nebulae (which are objects that do show high-excitation lines) and while He I will be present (e.g. in the Wolf-Rayet nebulae or planetaries) it will also typically be (much) weaker than the canonical H-alpha/[O III] lines. The strongest non-UV He I lines are in the near-IR. As for [O I], remember it's a very low-excitation line (the ground state of the transition is at 0eV) so it generally won't be present in objects that show higher-excitation lines.

I'm not saying it can't be done, but it's tricky and the filters you have chosen are very object-specific. (AGN)^2 is a good book, so enjoy :)

Edited by Ben Ritchie
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From what I saw in spectras He-II is in some PN (He I weak or none), and He-I may be in some H II regions. I did test broarder He-II on few PN:

- NGC7009, and Ha-O3-He2 color, and deconvolved UHC-S for comparison :)

- M27 and Ha,O3,He2 color... and not much of Ar III (but with crappy filter :))

- M57 (15 sec exp. only)

They are weak, they are hard to catch, they won't make suuuper pretty pictures, but it's something unknown, challanging... and fun. My new Atik 314L+ with low noise will help too :(

Edited by riklaunim
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Hmmm, but if you have an unmodded DSLR, isn't a H beta filter quite useful? H beta might have 33% of the intensity of Ha, but the DSLR's filter cuts the Ha signal to an even greater extent...

H beta signal must be fairly strong for most bright nebulae, as it is the basis, along with OIII, of the UHC filter.

Edited by Ags
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Very interesting thread!

Really interested to see how you get on with these. I think it's fab when people start trying new/unusual things...

H beta signal must be fairly strong for most bright nebulae, as it is the basis, along with OIII, of the UHC filter.

The UHC is a visual filter, and unfortunately H beta is our only chance of seeing Hydrogen emission in nebulae visually, as our eyes' response to H alpha is negligible.

I think you might be right about the unmodded DSLR one though. If I remember right, the unmodded camera response to Ha is about 1/10th that of Hb, so in theory Hb should be about 3x brighter.

Andrew

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Managed to shoot [O I] barely, only 5 x 120 before roof totally shaded the M27 (had to crop to kill the initial shade)... and the Moon was glowing so much I could barely see the nebula in the eyepiece:

6138009296_b148695df3_z.jpg

It's underexposed, low on frames and a lot of stretching and curves, but it does show the same shape of the nebula as the book example, so the filter does work.

Edited by riklaunim
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