Narrowband filter recommendations

[kosh]

Hi everyone,

I'm researching filters for my newly acquired atik 314l+ mono. Having established that 1.25" filters should suit, I am now looking into which narrowband filters I should look for.
The reason I'm a bit confused is with the varying bandwidth of say 12nm down to 3nm for the Ha filters. Is the 12nm Ha filter less suitable for imaging during times when the moon is out and in areas of moderate light pollution or do I have to go for something with a narrower bandpass?
Also, if I could ask whether the 3nm astrodons are available locally because I only seem to be able to find the 12nm astronimik and the 7nm Baader.
Thanks.

[Amaranthus]

Short answer is, the wider the band, the more non-target (wavelength) photons get through. This means the image is brighter, and the subs can be shorter, but the effects of background skyglow (moon, urban LP etc.) are also more prominent. The narrower you go, the longer subs you'll need, but the less light pollution and other extraneous contamination you'll suffer. Trade-offs...

[RickS]

As Barry said, the narrower filters will reject more of the sky glow. This will give you better contrast and won't really need much longer exposure time - the light that you're rejecting isn't signal that you want anyway.

With Ha, 5nm is a good compromise because it will give you NII as well as Ha and that will help with a lot of objects (e.g. the Helix Nebula is significantly fainter with a 3nm Ha filter.)

You didn't mention whether you were considering other narrowband filters as well as Ha? Assuming you are: Moonlight will affect OIII more than Ha, so if you want to image with the moon up a 3nm OIII filter is a good investment. If you're getting SII as well then a 5nm is adequate.

The other filter parameter that is important is transmittance. This tells you how much of the light you're trying to capture gets through the filter. A filter with 90% transmittance will give you quicker results than one with 70%.

Oh, and narrow filters won't work so well with very fast optical systems but unless you're near f/3 or faster it's not a problem.

Sorry, don't know of anyone that carries Astrodon filters locally. I've always ordered mine direct from the US.

Cheers,
Rick.

[glend]

Does this information apply to non-modded DSLRs as well? I have heard that i'd be wasting my time with trying narrow band imaging with my Canon 450D. Is a LP filter the logical alternative?

[SamD]

Putting some numbers to what Rick and Barry say, I get about 7e-/min skyglow at zenith with my 7nm Baader Ha (15km from Brisbane, no moon, with 314L+, and at f/7.5). This skyglow number is similar to what Craig Stark reports in his 314L+ pdf review at a similar f ratio.

If I doubled up from a 7nm to a 14nm Ha filter and follow the SNR calculations through, I would expect to need double the integration time to get the same SNR, or half the integration time if I went down to 3nm (assuming you don't chop off any signal like NII).

Even with NB I find that I'm better off to image when moonless (ie lower integration times). Less of a problem of course for the brighter NB stuff, but for the deeper stuff and a reasonably dark suburban sky I find no moon is the way to go.

Another factor of course is that the narrower filters are pretty expensive !

[Amaranthus]

Glen, the problem with trying narrowband with a OSC DSLR is that you are already filtering through the RGGB Bayer matrix. So you'll end up unavoidably extracting a true colour signal that has been filtered through a 'false' colour (NB) band. Obviously, this is hardly ideal! NB only really makes sense for monochrome sensors.

[SamD]

Same kind of logic applies ...

I get some great results with a modded 450D and a 7nm Ha/OIII filter at f/5, but I reckon non-modded would be tough in Ha, unless you can image at something like f/2 or lower, or stick to the bright Ha emission nebulae, or only image when pretty cold.

Basically, with non-modded (and presumably uncooled DSLR), the weak Ha signal detected on the sensor will be swamped by thermal noise. Other NB lines like SII are hard work even when everything is in your favour !

[SamD]

Quite ! For DSLR Ha imaging you discard all but the R pixels, although you can get the resolution back with dithering logic.

I agree, mono is the way to go for NB if you can.

[MrB]

Not ideal, but hardly a waste of time:
http://www.astrobin.com/98888/

[kosh]

Thanks Barry. That's how I was thinking too. Yes, the expensive trade-offs...



Hi Rick, yep I am intending to get the OIII and SII also, but I still haven't purchased my LRGB set so I'm going for that, plus the Ha filter first. I'm not satisfied with the Horsehead nebula I did last summer, so I want to go again with the Ha added when Orion is in season.
Also, I'm using a newt at f4 and a refractor at f7.5 so the 3nm shouldn't be an issue I don't think.



Hi Sam,
Thanks for that info. I think a set of narrowband filters would help with getting a bit more use out of my kit, for the money spent, a few moonless nights a month makes it a hard sell at times.
It appears that maybe a 7nm or similar maybe a good compromise for the Ha filter?

[SamD]

Hi Goran
Yes, I think the moderate bandpass width filters (ie around 7nm Ha) are a good compromise. If you want to get started with NB, you can do a lot of mono imaging with Ha alone to start with. On the other hand, you can sometimes get discounts if you buy a NB filter set together.