5nm vs 3nm Ha Filters- the NII question

[SimmoW]

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

[gregbradley]

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.

[SamD]

There's a post here that might be useful:
http://www.iceinspace.com.au/forum/showthread.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

[troypiggo]

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/3/4/9/0/34905502/astrodonnarrowbandfaq.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.

[RickS]

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.

[rustigsmed]

just an aside, does Oiii work well under full moon conditions @ 3nm?

[SimmoW]

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!

[RickS]

My experience is that I get gradients using a 3nm Oiii unless the FOV is a long way from the moon.

[rustigsmed]

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 :thumbsup: I'm not familiar with the tak f3.3 which scope is it?

[SimmoW]

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).

[Andy01]

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 :)

[SimmoW]

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!

[SimmoW]

What the heck, full 3nm set will be ordered tomorrow, have not seen any justification to mix 5nm with 3.

'Sponsored by VISA'!

[RickS]

There go the balance of payments figures for July :lol:

[clive milne]

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.

[SimmoW]

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.

[Slawomir]

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... :lol: (but I was told that such filters are not going to materialise...)

[SimmoW]

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!

[clive milne]

Not exactly... you can actually get a 1nm H-a for a couple of grand from Alluxa.

[Slawomir]

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-bandpass/6562-15-od6-ultra-narrow-bandpass-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.

[luka]

Few years ago when I worked in a physics lab we used some very narrow (sub nm) filters for our experiments. They were not astronomical filters and the transmission was usually about 50% but the price was much cheaper than the comparable astronomical filters.

For example, 1nm FWHM 50mm diameter Ha filter is US$424 here (https://www.andovercorp.com/products/bandpass-filters/standard/600-699nm/).

And if that is not narrow enough, here (https://www.andovercorp.com/products/bandpass-filters/semi-custom-bandpass-filters/) is 0.15nm FWHM 50mm diameter Ha filter for US$2078.

The tradeoff is that you may need to double your integration time.

[Slawomir]

In astronomical narrowband filters you not only need high transmission (ideally close to 100%) for a given emission line, but equally importantly you'll need very good out-of-band blocking since monochrome CCDs cover from about 300 - 1100 nm.

Otherwise very narrow FWHM offers no real advantage.

Just my five cents.

[luka]

Out-of-band transmission is not a problem for the filters I mentioned. They (optionally) block from x-ray to far infrared (6um to 30um) so out of band transmission is not a problem. They also have "imaging quality" filters with smoother surfaces.

Transmission, of course, is about 50% only...

[Slawomir]

So it looks like such filters could potentially be useful in astrophotography in heavily light polluted areas. Any volunteers to test these filters? :)

[SimmoW]

Ha, I'd love to help, I can pm you my bank account details so you can send the funds :lol::P:rofl:

[SimmoW]

Oh, forgot to mention, paid the customs and GST charges for my filter set today, so delivery imminent! Pity I don't have the filter wheel yet:(

[luka]

Extremely narrow filters like I mentioned (0.15nm) may result in "interesting" effects because of redshift/blueshift.

For example, z = 0.0001 (http://simbad.u-strasbg.fr/simbad/sim-id?Ident=trapezium) for Trapezium in Orion. This gives a shift (redshift) of Ha line by 0.06nm. This is a almost 1/2 of the FWHM of the 0.15nm filter, hence for that region the star intensity will be almost 50% dimmer than in a "wider" filter.

Faster moving objects may not even be visible through the filter...

[Slawomir]

Great news, these narrowband filters are awesome and second to none that are available to amateur astro imagers. FW is a necessity IMO though, in particular when having such expensive filters.

[Merlin66]

We regularly use a combination of a 10A (0.1nm) ITF (induced transmission filter) and a 0.3A (0.03nm) etalon to give a very narrow band Ha filter.
OK for solar but NBG for deep sky stuff.

[SimmoW]

Hmm, anyone have any example images from such narrow filters? Could make images rather distinctive I think, as a 4th channel

[Merlin66]

These very narrow band filters will not change the Ha detail, just suppress the nearby continuum wavelengths.....seen by some as light pollution.

[Merlin66]

Another option is to double stack a couple of 5nm filters.
This will result in a bandwidth of 2.5nm and about a 50% reduction in transmission.

[SimmoW]

Ken, the vendor you link to, they don't seem to sell 36mm filters, do you know if they could (he asks sheepishly, hoping they DONT! )

[Merlin66]

Simon,
It was Slawomir who posted the vendor....
I used Baader filters and some from Omega Bob (he only does 25mm diameters)

[troypiggo]

?? Won't both filters just allow the same bandwidth through? So you'll get 5nm bandwidth, but decreased signal because filters not 100% transmission (ie not perfect).

[Merlin66]

No.
That's how the Solar Ha filters work in double stack mode.
I have a spreadsheet (somewhere! I'll find it..) which shows the results of multiplying two Gaussian curves...
Internal reflections between the filters may be an issue....
Edit - can't upload the spreadsheet - but the attached screen shot shows the results. 3.5nm FWHM

[clive milne]

Yes, true enough, but it should also be stated that there are a few assumptions here that may not be true...
for example, if the transmission is mesa shaped as opposed to Gaussian, then the above statement no longer holds.



Also... if we are talking about ultra narrow band interference filters, the discussion becomes pointless without
including a caveat with respect to bandwidth shift and angle of incidence... vis-a-vis - focal ratio

~2c

[Merlin66]

Clive,
Agreed...
Any narrow band ITF filter will be susceptible to off axis wavelength drop.
I assume all/ most of the extreme narrow band filters we're talking about - <5nm FWHM bandwidth will "approximate" to the Gaussian wings...
http://astrodon.com/uploads/3/4/9/0/34905502/oiii3latestt.jpg
That being the case, there will be a reduction in the FWHM when stacked...