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Old 26-07-2016, 01:48 PM
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5nm vs 3nm Ha Filters- the NII question

Hi All, yay I just took delivery of a spanking new ASI1600 mono cooled cam!

Will be ordering a 7 by 36mm FW from Starlight Xpress.

Along with 3nm Astrodon OIII and SII filters.

I have done plenty of web research including this forum. Am happy with the 3nm Ha option with my Tak F3.3 setup and relatively darkish skies in the mornington peninsula.

But I am curious at the extent of NII signal that will be missing from such an Ha filter. I can't find any convincing discussions or images that prevent me from picking the 3nm filter.

I'm more into the 'big'n pretty' nebs, doubt I'll be taking many pics of planetary nebs. Maybe helix at the most.

What ya reckon? Thanks in anticipation
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Old 26-07-2016, 08:52 PM
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I have 5nm Astrodon filters. I was considering the 3nm Ha filter for my F3.8 AP Riccardi Honders. I asked Don about it. He recommended I stay with 5nm for the Honders but use 3nm for O111.

I was getting wonderful Ha images from the 5nm so I took that advice.

At faster F ratios the filters lose some of their narrowness so your F3.3 Tak setup would not be running at 3nm but a bit higher. I don't know how much higher but you could ask Don.

Greg.
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Old 27-07-2016, 02:25 AM
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There's a post here that might be useful:
http://www.iceinspace.com.au/forum/s....php?p=1166135

Based on the measured line strengths near the centre of M42, I reckon there's very little SNR to be gained in going from 5nm to 3nm. The main benefit would be that your image would truly be Ha, not a mixture of Ha and NII.

On the other hand, if a source had very little NII signal compared to Ha, then there would be a decent SNR improvement with the 3nm
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Old 27-07-2016, 05:42 AM
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My recollection when considering 5nm vs 3nm for Ha filters was that the NII was more significant in planetary nebulae and Wolf-Rayet bubbles. So for emission nebulae like M42, you're not going to see much benefit or difference. You need to make the comparison on planetaries like M27.

Sounds like you've done some research, so am sure you've read the section on pages 4 and 5 here: http://www.astrodon.com/uploads/3/4/...rowbandfaq.pdf which is what I based the above comments on.

For me it was a no-brainer, 5nm for Ha and 3nm for the OIII and SII and be done with it. Could not see any benefit of the 3nm Ha for practically all cases.
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Old 27-07-2016, 09:55 AM
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I went with 3nm Ha and the only time I regretted it was on the Helix. The 3nm filter is bulletproof when it comes to imaging under moonlight!

Cheers,
Rick.
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Old 27-07-2016, 11:12 AM
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just an aside, does Oiii work well under full moon conditions @ 3nm?
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Old 27-07-2016, 11:49 AM
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Thank you Sirs, and Don just kindly replied very promptly, mainly reaffirming that F3.3 is no problem for the 3nm. Decision almost made, will let you know soon. Hmm, this will not be cheap!
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Old 27-07-2016, 12:46 PM
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Quote:
Originally Posted by rustigsmed View Post
just an aside, does Oiii work well under full moon conditions @ 3nm?
My experience is that I get gradients using a 3nm Oiii unless the FOV is a long way from the moon.
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Old 27-07-2016, 01:36 PM
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My experience is that I get gradients using a 3nm Oiii unless the FOV is a long way from the moon.
thanks Rick, I've noticed actually that astrodon don't do 2" filters anyway so my wallet is safe...ish.


good luck Simmo, looking forward to the results I'm not familiar with the tak f3.3 which scope is it?
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Old 27-07-2016, 04:57 PM
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Hi, it's the Takahashi Epsilon E-130D, 430mm FL. I'm building another 10" F4 scope with Al Sam, but parts are still trickling in (Protostar spider!!!??? WTF, the guy does not communicate at all).
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Old 27-07-2016, 06:28 PM
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Here's a thought... I'm imagining tricolour narrowband imaging with just 3nm Ha, N2 & S2 filters.
Maybe suited to Ha rich objects like the Lobster, Cats Paw etc. where there's stuff all O3 present.
As soon as funds permit I'm going to sell my 5nm's and get these 4x3nm filters to try this out.
Reckon there's a whole new fresh approach to be considered here.
Was going to keep this under wraps until I'd actually tried it, but the $2k US investment has held me back to date, so maybe someone else can try it out.
Also check out Bert's images (Avandonk).
He uses a 3nm N2 with great results

Last edited by Andy01; 27-07-2016 at 07:09 PM.
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Old 27-07-2016, 07:30 PM
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Yeah Andy, it sort of dawned on me that if the Ha is too narrow, you could always just add NII, though I have no idea of the targets that are big enough and rich enough in NII.

Big learning curve up ahead!
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Old 28-07-2016, 09:35 AM
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What the heck, full 3nm set will be ordered tomorrow, have not seen any justification to mix 5nm with 3.

'Sponsored by VISA'!
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Old 28-07-2016, 10:33 AM
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What the heck, full 3nm set will be ordered tomorrow, have not seen any justification to mix 5nm with 3.

'Sponsored by VISA'!
There go the balance of payments figures for July
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Old 28-07-2016, 10:58 AM
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Quote:
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What the heck, full 3nm set will be ordered tomorrow, have not seen any justification to mix 5nm with 3.

'Sponsored by VISA'!
There is one possible justification in having both (3 & 5nm) filters.

If you subtract a 3nm frame from a 5nm frame... in theory, what's left is NII signal.
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Old 28-07-2016, 11:06 AM
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Ha, don't encourage me to start collecting Ha filters!

Hmmm, but wouldn't the net image also comprise a small amount of LP etc not filtered by the 5nm filter?

I notice the Astrodon shop doesn't show any 36mm versions of the NII filter, may be telling.
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Old 29-07-2016, 04:34 PM
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Quote:
Originally Posted by SimmoW View Post
What the heck, full 3nm set will be ordered tomorrow, have not seen any justification to mix 5nm with 3.

'Sponsored by VISA'!
That's the way to do things!

Life's too short and clear nights are counted too...

BTW, I would not swap my 3nm Astrodons for anything else, maybe except for 2nm Astrodons... (but I was told that such filters are not going to materialise...)
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Old 29-07-2016, 05:30 PM
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Ha, thanks for the encouragement Suavi, makes me feel better.

Don't even mention 2nm, that'll give them ideas. A set of 50mm 2nm filters, cheaper than a new German car!
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Old 03-08-2016, 11:04 AM
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Quote:
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Ha, thanks for the encouragement Suavi, makes me feel better.

Don't even mention 2nm, that'll give them ideas. A set of 50mm 2nm filters, cheaper than a new German car!
Not exactly... you can actually get a 1nm H-a for a couple of grand from Alluxa.
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Old 03-08-2016, 03:42 PM
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Not exactly... you can actually get a 1nm H-a for a couple of grand from Alluxa.
That's true, but I was told that Astrodon will not make such filters.

For those who might be interested in 1.7 nm Ha filter: http://buy.alluxa.com/ultra-narrow-b...dpass-210.html

EDIT: In the past (a year or so ago) I enquired Alluxa if they would make a 2nm Ha 1.25" filter for me, so they asked about my budget. Alluxa never got back to me once I told them I could spend up US$1500 for such filter.

Last edited by Slawomir; 03-08-2016 at 04:12 PM.
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