Astrodon Ha 3nm & NII 3nm filters

[Gina]

I'm considering getting the Astrodon 3nm Ha filter - I already have the 5nm version but images I've seen show a definite improvement from the narrower passband.  This cuts out NII which passes the 5nm Ha and I'm wondering how much data there is in NII that wouldn't otherwise be captured by 3nm Ha + 3nm OIII + 5nm SII.  In other words is it worth getting the 3nm NII filter (doubling the cost)?

[swag72]

I've never looked for, nor found an issue with my 3nm Ha filter... do I capture any NII? I have no idea  Do I need any NII? Goodness knows!!  Do I miss the NII component of the Ha? I've not missed what I've never had!!!!

I personally wouldn't pay double to get a NII filter as well...... but if someone comes along that tell me I need one, I'm all ears!!!  

It would be interesting to see a comparison with the 3nm Ha and the 3nm NII - But I don't know if many have both?

[Physopto]

The 3nm bandwidth is measured at FWHM. therefore you will still see some NII wavelength photons through the H alpha 3nm filter. Ha 656.28nm +3 = 689.28nm

NII 658.4nm is within that FWHM. But the filter does still let through  a fair bit light beyond below the FWHM point.

This is an overlay graph from Astrodons web  site.

You could always tilt the filter if you want to change the central wavelength anyway.

Derek

[riklaunim]

In many planetary nebulae/supernovae nebulae [N II] signal can be stronger than H-alpha so using 3nm H-alpha filter will damper the photographed signal. If you don't intend photographing H-alpha and [N II] for separate channels then there is no reason for using very expensive and very narrow filter. It won't improve anything.

[Gina]

Thanks for your replies everyone   I was thinking of separate channels but what I would do with 4 colours/wavelengths I don't really know.  I guess that for DSOs with a lot of NII the 5nm Ha filter would be better than the 3nm but it sounds like having all of Ha 5nm, Ha 3nm and NII nm would be a waste of money.  Have to say I've never seen any images which purport to contain 4 separate wavelenghts.

[DaveS]

I found the Rosette very strong in [NII] my avatar is a NHO image. I'm on my 'phone ATM, when I geton the main comp I'll put up something better.

[Gina]

Thanks Dave

[DaveS]

OK, I'm now on the main computer. Here are two images made from the same data, 2 hours each of hydrogen and nitrogen with 4 hours oxygen.

The first is a bicolour made from H+N in the red channel and O in the blue and green, giving something like a HOO with wider filters.

The second is by mapping H to red, N to green and O to blue

Hmmm...I think the backgrounds could do with tidying up a bit, the first has too much blue-green, the second too much purple.

Having four wavelengths then gives you much more freedom as to how you map them, even within a HST palette, keeping Hydrogen separate or combining it with Nitrogen for instance.

[Gina]

Very interesting - thank you

[riklaunim]

As a side note: there are other bands at which some nebulae glow quite nicely - like neutral oxygen (not [O III]) as well as two helium bands.

[Gina]

Interesting   But I've never seen filters for those.

[DaveS]

I've not seen them either, but I suspect Astrodon would make one for you, at a price  !

The He line in the blue sounds like a good idea, as the strongest line in the yellow is too near the 589nm sodium line from streetlights. It would also give you a nice RGB with Hydrogen and Oxygen.

[Gina]

A custom made He filter from Astrodon...  Now that would be something - really pushing the boundaries of astro imaging   I wonder if anyone here has bought a custom filter from Astrodon.

[Gina]

I've sent a query to Astrodon in the USA asking if they make custom filters and if so how much a Helium filter would cost in either 5nm or 3nm 1.25" size.  I also mentioned Ian King Imaging as my supplier of Astrodon filters in the UK in case that makes any difference.  I suspect the cost will be prohibitive but we'll see.  I'm always very keen to try something different

[DaveS]

I'll be very interested in what Astrodon say!

I actually think it would be a useful addition to their line-up if they went into production with it.

[riklaunim]

Optolong or Chroma can provide but some people, including me got such filters from machine vision companies. Thorlabs or Edmund Optics have a lot of them, but most of them are expensive - but they do make promo sales and you can hunt a matching narrowband filter. With luck very very cheap filters may be available on ebay from bjomejag/OmegaOptical shops - leftovers from various batches.

Neutral Oxygen at 630 nm is quite interesting as it will quite unique looking in planetary/supernovae. Yellow helium will worth if you don't have bad light pollution (within sodium lamp emission). Blue helium may be tricky as it will be even more affected by moon-glow than [O III].

[Gina]

Thanks both

I report back when/if I get a reply from Astrodon and I'll check out those other sources.  Light pollution is very low here maybe even the lowest of the CPRE levels according to their map.

[Gina]

Had a reply from Astrodon and unfortunately it confirms what I thought

Quote

Hi, Gina,

 

It would be very expenisve  since we have to complete two coating runs (one for each side of the filter) using a large plate for one filter.   Custom Scientific in Phoenix, Arizona, U.S.A. specializes in custom single filters.  I'd suggest that you inquire there. 

 

 

 

Best Regards,

 

Don

 

Don Goldman, Ph.D.

President

Astrodon Imaging

Roseville, CA 95661 (USA)

 

[DaveS]

Hi Gina

I'm not really surprised, but at least you got a reply and a possible alternative source.

I had a look on the Edmund Optics UK site, but the nearest I found was this;

http://www.edmundoptics.co.uk/optics/optical-filters/bandpass-filters/hard-coated-od4-10nm-bandpass-filters/65223/

For 589nm, passing Ne and Na. A bit pricey and I'm not sure of the optical quality as I think it may be more spectroscopic orientated rather than imaging.

[riklaunim]

You can take a look at my articles:

http://www.rkblog.rk.edu.pl/w/p/helium-argon-and-neon-narrowband-imaging/

http://www.rkblog.rk.edu.pl/w/p/choosing-edmund-optics-filters-helium-oxygen-and-argon-narrowband-imaging/

And even lunar can go narrowband:

http://www.rkblog.rk.edu.pl/w/p/my-filters-and-first-results-lunar-petrographic-imaging/

[riklaunim]

14 minutes ago, DaveS said:

http://www.edmundoptics.co.uk/optics/optical-filters/bandpass-filters/hard-coated-od4-10nm-bandpass-filters/65223/

This is 50mm dia. 25mm will suffice unless you really want to use a very big sensor

[DaveS]

Yes I know it's a big filter but I was thinking it might be tricky to mount the 25mm ones.