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Questions about NB filters

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#1 AKHalea

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Posted 18 September 2019 - 05:40 PM

I am a bit perplexed by high prices of really narrow band filters (like AstroDons). I presume they are worth it based on how many people go for them. I understand that the narrower the bandpass, the cleaner the emission line picked up. However, we are not doing spectroscopy here, so I wonder what these filters buy for the NB Astro-imager.   

 

I have a dual band NB filter that does a good job of cutting out most of LP and moonglow and catches the HA & OIII bandpasses well. It is inexpensive and therefore, has relatively wide bandpasses of 12nm for HA and ~30nm for H-Beta to OIII. The cut-offs of bandpasses seem to be sharp enough that there is not stray emissions picked up beyond the bandpass widths.

 

So, can one achieve similar final quality images as from even narrower band filters if I extract the Red (HA channel) and the OIII channel (G&B channels) separately and later recombine them? I presume the answer is "No", but I do not understand why that should be the case. That is, what is the magic about very narrow band filters that gives us the wonderful images (of course at a large expense for filters, longer exposures required etc). Your help will be most appreciated ..... Anil


Edited by AKHalea, 18 September 2019 - 05:41 PM.


#2 Jon Rista

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Posted 18 September 2019 - 06:57 PM

If you are using a DSLR, then get the one of the multi-band NB LP filters. I wouldn't spend the kind of money on ADon 3nm filters if you are imaging with a DSLR. I would only spend that kind of money if you are imaging with a mono camera, where you can really get the most out of it all.

 

There are some pretty good multi-band NB filters out there these days. And I think the higher end offerings will only get better as time goes on.

 

The main benefit with 3nm filters is when you have high LP, and specifically with Ha if you also want to image NII separately. With OIII and SII, the very narrow pass can help a good deal with very high LP. Even with 3nm filters, most of the time my median background level is higher with OIII and SII than with Ha, as odd as that may sound. That is how bad the LP is and how much it affects those passes. With a 3nm Ha filter, I am also excluding the NII, which I was eventually hoping to get another 3nm filter so I could do multi-band planetary nebula imaging (I've even thought about ordering filters for some of the more exotic emission bands that you can pick up from planetary nebula, sometimes gasses like Argon or Helium emit stronger signals in planetaries.)

 

With a 5nm or wider Ha filter, you will pick up both Ha and the two neighboring (straddling) NII emission bands. This could be thought of as a good thing, though, since you'll get more signal in the same amount of time. Depending on the nature of the two emissions, this could help or hurt overall contrast...depends on whether NII is the same structure or different structure than Ha. In some nebula, the NII can be markedly different, and the distinction can be quite interesting. In other nebula, NII is largely the same as Ha.



#3 AKHalea

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Posted 18 September 2019 - 08:06 PM

Hi Jon : Thanks for your appropriate and prompt response. I only have the DSLR at this point, so certainly the multi-band filters make sense for me - that is in fact why I have one. Cheers ..... Anil 



#4 AKHalea

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Posted 19 September 2019 - 10:17 AM

A related question : I have been trying to get a nice blue coloration in my Bicolor images of emission nebulae thru various combinations of HA/OIII. I imaged the Clamshell nebula (Sh2-119) recently and got this image with a DSLR & a multi-band filter. When I look around, there are a few images of this object like this one which have some stunning colors. 

 

What is the magic behind many of the super nice looking (perhaps over the top for some people) images made in SHO palette?

  • Is this not possible with Bicolor (HA+OIII) images?
  • Is it the very narrow band filters (with mono cameras) that give clean separation between the emission lines,
  • Or is it all in processing - like Extreme saturation

Anyway, appreciate your thoughts on this question ..... Anil


Edited by AKHalea, 19 September 2019 - 10:18 AM.



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