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Experience with 3.5nm H-Alpha

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#1 Jeff Morgan

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Posted 22 May 2020 - 11:24 AM

Last night I was able to use a Antlia 3.5nm h-alpha filter. I am not too wild about the quality levels I have seen in products from China (nor spending my money on them in light of recent events). But, I have been curious about obtaining something "tighter"  and I don't think 5nm is a significant jump from the 7nm I currently have.

 

It was at the end of a session cut short due to fatigue from several long observing sessions earlier in the week. Thus, I was not able to give it the work-out it deserved. We just used it on a Mod 3 and PVS-7 at 1x sweeping from Mu Cephei to Alberio.

 

What jumped out at me was a "Spotlight Effect". The first view was in someone else's PVS-7 and I thought the intensifier tube had a defect or hot spot. The center of FOV seemed quite bright in relation to the out 1/2 of field. Swapping the filter to my Mod 3 the same effect was noted, eliminating the possibility of the effect being a tube issue.

 

Then going back to my 7nm Baader, I was able to discern the same effect although it was very subtle (indeed, in nearly four years I never noticed it without specifically looking for it this time).

 

So is this normal experience with very narrow bandpass filters?

 

Does the effect only happen with faster optics, such as the ENVIS lenses we were using?

 

Or was this an issue with that particular filter sample?


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#2 Dale Eason

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Posted 22 May 2020 - 11:55 AM

My Baader 7nm filter does it too and did it with my Nikon lenses.  I don't remember if it does it with the Computar but think it does.  I don't use it often.

 

I had never realized it was filter related.  However I never use handheld without the 7nm filter.  Now I will have to experiment to see.

I thought it was some kind of reflection.  I wondered why no one here talked about it because it has fooled me into thinking a bright nebula was in view.

 

Dale



#3 Richard O'Neill

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Posted 22 May 2020 - 12:18 PM

 What you've observed is normal. When an object is viewed off axis through a very narrow band filter the pass band wavelength is shifted. The narrower the band pass the more pronounced the effect.


Edited by Richard O'Neill, 22 May 2020 - 12:19 PM.


#4 Eddgie

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Posted 22 May 2020 - 05:41 PM

Yes, totally normal and the narrower the band pass of the filter, the more band pass sift one will typically encounter.

 

Not everyone is good about publishing the maximum focal ratio of the filters, but some do.   In the 5nm to 6nm range, band shift will start off axis at about f/2.8.  Baader has "High Speed" filters that they say are good to f/2. 

 

When running at 1x, you are not really using a lens or objective, so here it is just the angle of incidence for the subject.  something at the center of the field would be 0 degrees, and something at the outside of the field would be 20 degrees.  Even using a 6nm filter Astronomic, I cannot get the entire Barnards's Loop into the narrow band portion of the field at the same time so I have to pan around the nebula to see it well and only the part that is near the center of the field will be shown at full brightness and unless I put it in the center of the field, I can't see the Anglefish nebula.

 

Astronomic says this about their 6nm (which I take to mean f/3.75 to f/15) 

 

 

The design of the transmission curve makes it possible to use the filters with instruments from 1:3.75 to 1:15

This would mean that only the light coming in from the angle of an f/3.75 light cone (or about the central 30% of the field) would be fully in band.

 

The Astrodon 3nm is rated down to f/3 so with the 1x lens, only the central 1/3rd of the field is in band. 

 

 

3 nm filters can be used on systems to f/3

 

 

Under dark skies with my binocular, the 12nm filters actually gave the best view of Barnard's Loop because while the pass was not narrow, under dark skies it is not super critical, and with the 12s, I can see the entire nebula in the field at once without having to pan.  

 

Now not everyone provides a maximum focal ratio for the field to be in band, but if they do, then you can pretty easily determine about how much of the field will be in band in your application.

 

I use a 5nm rated for f/3 in my Boren Simon, and in this scope, almost all of the field is in band, making it an excellent scope for fields where the nebula is larger than the true field of the scope. If there is band sift, it can look like the nebula does not fully cover the field and when you are under dark skies working along the Milky Way, there are many areas where the entire 2.4 degree true field of the Boren Simon shows nebula even if no catalog items are present.  Big parts of the Milky Way are simply carpeted over with nebula, but if you use a very fast lens that is too fast for the filter, you only see the nebula at the center of the field and reach the conclusion that this is all the nebula that there is, especially if you are combining that band sift with any amount of vignetting. Another reason why I like f/2.8.  It is about as fast as I can go before band-shift and vignetting start to obscure too much nebula out there at the edge of the field. There is more noise at f/2.8 than f/1.2, but I would rather have the full aperture of the filter (or close to it) in band than have a big area of it out of band and vignetted.    

 


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#5 Eddgie

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Posted 22 May 2020 - 05:51 PM

And the Astronomic 12nm is rated at f/2.8, but because the band shift is less severe when you are using a wider pass, while the fill transmission is only over about 35% of the field, the shift is not so great that it causes me not to be able to see the entire Barnard's Loop.  Barnard's Loop is about 15 degrees top to bottom so while it is a bit more than the size of the band window (about 38% of the field, the band only sifts a little bit outside of the window and I still can take in the entire loop. 

 

Now it is much better in the full binocular because of binocular summation. Even though the very outside edge of the east side is a bit outside of the band and a bit dimmer, the binoculars really punch up the view by brightening these dimmer parts up.

 

Barnard's Loop in a binocular under dark skies is one of the greatest showcase objects in the sky.  The shear size and detail of it is hard to convey.   But a 5nm filter ruins the experience because you can't see all of it at once.  



#6 Jeff Morgan

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Posted 22 May 2020 - 11:26 PM

I'm not quite getting the math here, but no matter.

 

Wish I would have tried that 3.5 in for example, my f/7 refractor.

 

While the Spotlight effect was a little disconcerting, the nebula in the central part of the field really stood out.

 

Such a filter could be handy for those really faint (and smaller) Sharpless objects.



#7 simoneb

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Posted 23 May 2020 - 02:41 PM

I have noted the same spotlight effect with my fastest setup: Borg f/3.6 + 55mm plossl (f/1.8 overall) and while using the Baader 7nm. I was looking for some nebula in my horribly polluted NYC sky and I was bothered by this sort of brighter center field. I thought about my tube being defective at first, but then I saw that without filters I could not see it. I have recently acquired a 3nm filter from chroma (Gavin’s suggestion!) but did not get the chance to try and see if the spotlight effect was visible.
Simone

#8 Mike Lockwood

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Posted 23 May 2020 - 04:42 PM

This is normal - the passband shifts when light comes in at an angle to the filter.

 

The narrower the bandwidth, the more pronounced the effect will be.  The 3.5nm Antlia seems to work quite well.

 

This was discussed some time ago, and it inspired my Nov 5, 2017 entry in my NV page, discussing where it was best to locate a narrowband filter:

  http://www.loptics.c...ightvision.html

 

The effect should be quite minimal using the filter on the bottom of an eyepiece or in a filter wheel in a telescope such as the one you have under construction.

 

I think that a curved filter (meniscus) could help reduce this effect, but I'm not sure if it's possible to make one.  It would be spectacular for use at 1X, though!

 

On a slightly different topic, does anyone know a filter vendor that they could ask about this?  I'd be interested in buying one or two if it could be done.


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#9 slavicek

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Posted 23 May 2020 - 10:53 PM

Last Thursday I finally tried for the first time my Cannon 200mm f/1.8 with Chroma 5nm Ha filter, placed between the lens and NV. To my (pleasant) surprise, it worked very well. So next time I use this lens I will use it with the Baader 3.5nm Ha filters and I will look for the "spotlight effect". I will try to move the nebulas around the FOV to see if anything changes.

I still have some "issues" with my "200mm f/1.8" set up but I am very encouraged by the initial results. Hopefully I will have separate report on it in near future. smile.gif


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#10 GeezerGazer

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Posted 28 May 2020 - 04:00 AM

Last night I was able to use a Antlia 3.5nm h-alpha filter. I am not too wild about the quality levels I have seen in products from China (nor spending my money on them in light of recent events). But, I have been curious about obtaining something "tighter"  and I don't think 5nm is a significant jump from the 7nm I currently have.

 

It was at the end of a session cut short due to fatigue from several long observing sessions earlier in the week. Thus, I was not able to give it the work-out it deserved. We just used it on a Mod 3 and PVS-7 at 1x sweeping from Mu Cephei to Alberio.

 

What jumped out at me was a "Spotlight Effect". The first view was in someone else's PVS-7 and I thought the intensifier tube had a defect or hot spot. The center of FOV seemed quite bright in relation to the out 1/2 of field. Swapping the filter to my Mod 3 the same effect was noted, eliminating the possibility of the effect being a tube issue.

 

Then going back to my 7nm Baader, I was able to discern the same effect although it was very subtle (indeed, in nearly four years I never noticed it without specifically looking for it this time).

 

So is this normal experience with very narrow bandpass filters?

 

Does the effect only happen with faster optics, such as the ENVIS lenses we were using?

 

Or was this an issue with that particular filter sample?

Jeff, all of your questions have been answered.  But let me add that I have a 2" Antlia 3.5nm H-a filter and have used it attached to one of my 50mm Computar lenses, when using my Mod 3C and Micro as binoculars (with a 2" 7nm filter on the other monocular).  I have also used it with my f:7 refractor where it seemed to perform really well.  As noted in previous posts, the faster the optical system, the more band shift will show up.  I did use the filter in afocal as well, with a Russell 50mm, with very good results at f:3.5 using the same refractor, but I was not looking for the effects of band shift.  Overall, my experience with this narrow band filter has been positive.  I have used it only visually; I have yet to use it for NV phonetography.  When I compared the 3.5nm H-a views to those from a 7nm filter, I found the H-a subject much brighter visually with the 3.5, with only a slight increase in scintillation and star attenuation.  Next time I'm out, I'll try to take some photos while it.  


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#11 Mike Lockwood

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Posted 12 June 2020 - 11:42 AM

I used the 3.5nm Antlia filter last night and compared with a 7nm Baader at 1X.

 

It definitely improved contrast, and didn't change scintillation that much.  I just sat out on my south-facing deck last night after about 11:30 pm with the Astrophotography Sky Atlas and had fun picking off objects in Scorpius and up the Milky Way, which looked great at 1X with no filter.

 

In particular, the very large Sh2-27 in southwest Ophiuchus was much more conspicuous at 1X with the 3.5nm than the 7nm.  This was from my deck, in a rural area with mild to moderate light pollution, SQM used to be about 21.0 on a good night at zenith.  I hope to get some photos through both fairly soon.



#12 PEterW

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Posted 12 June 2020 - 12:51 PM

Interesting! One of the first reports of sh27... its a big one, but rarely noted by anyone!

Peter
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#13 Mike Lockwood

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Posted 12 June 2020 - 02:33 PM

That's what made me notice it - I was surprised at how large it was and how visible.  Reminded me of Sh2-264 at the top or Orion, but bigger and with a less defined edge.


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#14 Jeff Morgan

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Posted 12 June 2020 - 02:55 PM

Sometimes referred to as the Zeta Ophiuchi Nebula. Huge, fills half the field in my Canon 50mm lens.

 

Using a 7nm I find it to be mostly amorphous, with loosely defined lanes near Zeta. It sounds like an excellent candidate for a tighter filter.


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#15 GeezerGazer

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Posted 13 June 2020 - 01:02 AM

I think we should capture Sh2-27!  

 

Here is a great long-exposure AP image of it taken with a 50mm lens (bigger FoV than we have with NV):  

https://project-nigh...t/print054.html


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#16 Mike Lockwood

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Posted 13 June 2020 - 12:04 PM

I can see Sh2-27 faintly in some images that I shot last night, as well as the neighboring Sh2-1, and RCW 129.  This was with the 3.5nm filter.  Not sure if it showed up with 7nm, I will have to dig through some images.



#17 Dale Eason

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Posted 13 June 2020 - 08:47 PM

Wow,  I'm glade you guys pointed out sh2-27.  I was out in the south facing back yard the other night hand held and saw a large nebula that I could not identify.  I forgot about the astrophotography atlas.  Now I think that i what I saw.  Will have to take a look again.  I had identified the usual targets like lagoon, swan, trified and friends but the other one was not shown on SkySafari that I could isolate it.



#18 phonehome

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Posted 13 June 2020 - 09:03 PM

I can see Sh2-27 faintly in some images that I shot last night, as well as the neighboring Sh2-1, and RCW 129.  This was with the 3.5nm filter.  Not sure if it showed up with 7nm, I will have to dig through some images.

 

Should also be visible with the 7nm.  Some friends and I were gawking at it down at TSP last year and it also took us awhile to identify it as the Zeta Ophiuchi Neb.  It is amazingly large which highlights one of the unique features of NV at 1X - you get a sense of scale, especially with both eyes open, that is all but impossible from a scope or binoculars or even AP...


Edited by phonehome, 13 June 2020 - 09:10 PM.

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#19 Jeff Morgan

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Posted 13 June 2020 - 11:04 PM

I think we should capture Sh2-27!  

 

Here is a great long-exposure AP image of it taken with a 50mm lens (bigger FoV than we have with NV):  

https://project-nigh...t/print054.html

 

Lanes! OMG, that is a face smirking back at me!

 

Yes, NV photos needed. I have an extra dovetail plate, I'll get my 50mm mounted on it for some pics. An 85 telephoto might be ideal, but I don't have one at the moment.


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#20 GeezerGazer

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Posted 14 June 2020 - 10:45 AM

Lanes! OMG, that is a face smirking back at me!

 

Looks like an Ork to me.  lol.gif  Perhaps Humpty-Dumpty before he fell!


Edited by GeezerGazer, 14 June 2020 - 10:47 AM.


#21 Mike Lockwood

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Posted 14 June 2020 - 11:35 AM

Here you go, Sh2-27 from two nights ago, 1X TNV-14 (PVS-14 from TNVC), 7nm Baader H-alpha filter used for the left image, 3.5nm Antlia H-alpha filter for the right image.  Both are 2" filters over the front of the 1X lens.  Shot from my deck.

 

Unprocessed except for some in-camera noise reduction and resizing artifacts.  I changed the NV gain and camera (Canon G15) settings for each filter to properly expose the image.  Camera was set to 1s exposure and ISO and f/# may have varied.  This is fairly faithful representation of the visual view.

Attached Thumbnails

  • Sh2-27_small_7nm_3p5nm.jpg

Edited by Mike Lockwood, 14 June 2020 - 04:23 PM.

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#22 Dale Eason

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Posted 14 June 2020 - 12:47 PM

Yes that is what I saw again last night at 3X hand held.  It is huge.  Much bigger than the others in the neighborhood.  So glad you guys figured out what it was.  For me the south view from my back yard is a struggle of power lines, trees and neighbors security yard light.  Which was on last night giving me a challenge to keep it out of the NV objective.  The nebula was just above a bush when the light was out but later was on when it had climbed higher.  So then it was lost in the glare of that darn light.

 

Great example of the difference of the filters.  Shows that the 3.5 has advantages under the right conditions and object.  I had not thought of getting one before this.  Thanks.



#23 PEterW

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Posted 14 June 2020 - 03:02 PM

Great images and great report!
Peter

#24 Dale Eason

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Posted 17 June 2020 - 01:01 AM

I have to confess  what thought I was seeing as sh2-27 turned out to be Lagoon.  Tonight I had the 10  inch F3 out and not till I came inside and reviewed my images of it did I discover what it really was because in the same field of view was the Trifid Nebula.  Shucks so I have yet to see sh2-27.  Sorry for the noise.  Just not use to the NV view yet. Lagoon is so much bigger than I remember. 


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#25 Dale Eason

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Posted 18 June 2020 - 09:40 PM

Last night I made sure I knew where I was looking and definitely could not see sh2-27 with my 7nm HA on my 10 F3 in my light pollution.  I have ordered the 3.5 maybe it will help.  I'm guessing I have too much LP.




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