Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Why are "visual" narrowband filters typically wider than their imaging equivalents?

Filters Imaging Observing Accessories Astrophotography Equipment
  • Please log in to reply
10 replies to this topic

#1 Kevin Thurman

Kevin Thurman

    Ranger 4

  • *****
  • topic starter
  • Posts: 388
  • Joined: 12 Jul 2020

Posted 31 July 2021 - 05:54 PM

I've been into imaging for a while and seems most serious imagers go for 3nm NB filters, however most of the highly recommended visual filters like the lumicons for example are much wider. 8, 9, 11nm and so on. Obviously the wider you go the cheaper you get, but I have seen from imaging discussions on the topic that there is a big contrast difference between 8, 5, and 3nm filters at least for imaging. Is it just that the difference in contrast isn't as significant when used visually? Has anybody done a visual comparison of this sort of thing, and if so what did you find?

I am getting a new 3nm Oiii filter for my imaging setup and I think I'll check it out on some visual targets through my 10" dob when I go to darker skies in about a month and a half, but I don't really have a wider filter to compare it to. The only "visual" filter in my collection is a UHC which isn't a fair comparison.


  • ram812 likes this

#2 bobzeq25

bobzeq25

    ISS

  • *****
  • Posts: 25,669
  • Joined: 27 Oct 2014

Posted 31 July 2021 - 06:33 PM

Long exposure camera versus short exposure eyes.

 

For long exposure imaging, pretty much the narrower the better.  Which gets expensive.  You're trying to dim the background as much as possible.

 

For short exposure eyes, wider is fine, and _much_ cheaper.  The eye doesn't register the background nearly as much.

 

It's somewhat similar to the situation re mounts.  Visual, they're no big deal.  Imaging, they're the most important part of the setup.  Imaging mounts are more expensive than visual observers need.

 

It seems intuitive that the camera is just an improved version of your eye.  As with many things in DSO AP, that intuition is wrong.  The camera is a completely different thing, and that has many implications.


Edited by bobzeq25, 31 July 2021 - 06:37 PM.

  • Jeff Morgan, Jon Isaacs, ram812 and 1 other like this

#3 Aaron Small

Aaron Small

    Vostok 1

  • *****
  • Posts: 131
  • Joined: 27 Sep 2012

Posted 31 July 2021 - 07:13 PM

What bobzeq25 said.

 

To add to that it simply is a matter of the camera can capture enough photons through the narrower bandwidth by waiting longer.  To gain the brightness needed for the human eye to perceive, you need a wider bandwidth to allow for more photons to reach the retina.


  • Kevin Thurman likes this

#4 ram812

ram812

    Apollo

  • -----
  • Posts: 1,267
  • Joined: 10 Dec 2014

Posted 31 July 2021 - 08:12 PM

And + to what Bob and Aaron said as well! I've done a visual comparison with a mounted 8nm OIII and a 3nm OIII and to tell the truth? I couldn't find a real big difference- not visually. My target that night was Western Veil (using the GEM mounted XT10 in my avatar). My test filters were an old Celestron 8nm "Visual" filter, while the 3nm OIII was a Chroma version on lone from a fellow astro club member. The 8nm was actually a little brighter than the 3nm, and no, I didn't try them with an astro camera- I only had a OSC at the time. Both allowed dim views with averted vision, the Veil a tough one to see anyway!
FWIW
Note: Filters are like voodoo to me but reading and seeing differences done by imagers, which I'm not, would lead one to speculate the 3's,5's, and 7's are best for pics. Anything above has lesser dark contrast and work for the eyeballs😉!
CS, Ralph
  • Kevin Thurman likes this

#5 Kevin Thurman

Kevin Thurman

    Ranger 4

  • *****
  • topic starter
  • Posts: 388
  • Joined: 12 Jul 2020

Posted 31 July 2021 - 08:17 PM

Thanks for the responses! Makes sense to me. I'm still going to try out my OIII visually when I make that dark sky trip. I want to do a better sketch of the veil.


  • bobzeq25 and ram812 like this

#6 freestar8n

freestar8n

    Vendor - MetaGuide

  • *****
  • Vendors
  • Posts: 10,799
  • Joined: 12 Oct 2007
  • Loc: Melbourne, Australia

Posted 31 July 2021 - 08:53 PM

I don't do much visual work these days but I'd be interested if anyone does an actual comparison.

 

If you have light pollution and are trying to see the Horsehead with H-Beta, it will definitely show better with a narrowband filter rather than no filter, and I expect it would show more clearly the narrower the bandwidth - but with some diminishing returns.  The stars may fade out but the nebula would stay just as strong, so there is certainly  no downside to narrower,, except cost, and losses with fast scopes below f/4 or so.

 

If you have clear skies but strong light pollution or moonlight, I can imagine an h-beta at 5nm would work better than 12nm or even 30nm.  So, I'd be interested to hear if someone does a comparison.

 

Frank


  • ram812 likes this

#7 ram812

ram812

    Apollo

  • -----
  • Posts: 1,267
  • Joined: 10 Dec 2014

Posted 31 July 2021 - 11:26 PM

Thanks for the responses! Makes sense to me. I'm still going to try out my OIII visually when I make that dark sky trip. I want to do a better sketch of the veil.


Cool! They work, I mean the visual versions. You obviously have experience out under the stars so about the only advice I can give is the low power eye piece of 34mm (Or so) work best, and bring your "Averted Vision" eyeballs with you! You'll be pleasantly suprised, I was. I looked for Veil w/o filters and it was no-go. So much so that I had to check and double check my go-to's twice just to be sure I was on target.
Frank has an interesting test there in his answer to your post and if I had those filters I'd be all too happy to give it a whirl. At any rate, let us "See" your results on your return! (Pun intended)😁

CS, Ralph
  • Kevin Thurman likes this

#8 BKSo

BKSo

    Messenger

  • -----
  • Posts: 459
  • Joined: 08 Dec 2015

Posted 31 July 2021 - 11:32 PM

Physics. OIII is actually 2 lines: 500.7 and 495.9 nm. Visual filters pass both lines for maximum brightness, photographic filters often only pass one line.
  • Jon Isaacs likes this

#9 Kevin Thurman

Kevin Thurman

    Ranger 4

  • *****
  • topic starter
  • Posts: 388
  • Joined: 12 Jul 2020

Posted 31 July 2021 - 11:40 PM

Physics. OIII is actually 2 lines: 500.7 and 495.9 nm. Visual filters pass both lines for maximum brightness, photographic filters often only pass one line.

This is true, and explains the OIII difference but still the h-beta filters are usually wider too. 



#10 freestar8n

freestar8n

    Vendor - MetaGuide

  • *****
  • Vendors
  • Posts: 10,799
  • Joined: 12 Oct 2007
  • Loc: Melbourne, Australia

Posted 01 August 2021 - 01:23 AM

I think the main thing special about visual filters is they don't need to block IR, and that makes them cheaper.  And for visual work people might not need to squeeze out all the SNR they can - and pay extra for it.  When you look in the eyepiece narrower might be better than wider, but it may not be a big difference.  But if you are imaging for 20 hours you might be more inclined to spend more to get the most out of that effort.

 

But as for bandwidth and passing one or two OIII lines - I think the same SNR benefits apply to visual as they would to imaging.  The whole point of the narrow bandwidth is to block out light pollution, and if the sky is bright you really want narrow.  If the sky isn't bright you don't need narrow at all.

 

The main OIII line is I think over 3 times the strength of the secondary peak.  So whether you are better with a narrow or wide filter just depends on how bright the sky is.  Wider gets more signal - but also more background noise, and that applies to visual and imaging.

 

I looked around a bit and I think the Baader filter is around 8nm - which is still pretty narrow compared to 25nm or something.  The visual filters don't tend to be as detailed in the description in terms of things like bandwidth.

 

Frank


  • Kevin Thurman likes this

#11 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 94,712
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 01 August 2021 - 09:12 AM

With visual filters you need to maximize transmission of the band, with a photographic filter you need to maximize contrast.

 

It does seem that if a 3 nm filter had full transmission of the H-Beta band, then it could be a big help in viewing faint nebulae from light polluted skies. It would take a great deal of effort to achieve full dark adaptation..

 

Just send me your 3 nm filter and I'll give the California nebula from 18.4 mpsas skies.

 

A rough estimate is that a 14nm filter gives 3 magnitudes increase In contrast. The 3 nm filter would increase that by 1.7 magnitudes.

 

That means using the 3nm filter from my backyard would be like using a 14 nm filter from 20.1 mpsas. 

 

That's not that great, I wouldn't be seeing the Horse head, it's slimly possible I might see the California, I can already see the Veil with a 14nm O-lll.

 

I'm not sure how much it would help under my high desert 21.3 skies, the 14nm gets me to 24.3 mpsas, the 3nm would get me to 26.0 mpsas. At 24.3, I'm not sure contrast is the issue even with large exit pupils, I think my 73 year old eyes are just needing light..

 

Jon




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics





Also tagged with one or more of these keywords: Filters, Imaging, Observing, Accessories, Astrophotography, Equipment



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics