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NV filter comparison: Astrodon 5nm Ha vs Astronomik 6nm Ha

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#26 Vondragonnoggin

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Posted 21 April 2019 - 04:19 PM

To some extent I wonder if the additional imaging capabilities of the latest phones (or of a real camera), or post-processing of image series could to some extent bring up with 12 nm filters some of the extra features  (eg better contrast) previously visible only with the narrower filters, but without the downside of bandpass shift. In the interesting comparative pics of the linked thread many of the structures highlighted by the 5nm filter are present also in the 12 nm pics, just not as visible. 

Longer exposures with a 12nm might bring out similar details. I think the Samsung and some others have the ability to expose longer than iPhones.

 

the reverse is true for narrower though - less exposure time, lower iso, and some post-processing to clean up noise and scintillation could be done as well with the narrower filters I imagine. Super sensitive regular cameras like the A7S would be fine with the wider notch filters.

 

I still say for visual, it is a matter of subjective preference. Some would rather see a cleaner visual, more stars, and less nebulosity than see the higher scintillation and rolloff at the edges with a very fast optic. The telescope and focal ratio play a big part in this also. The NV Device too. Variable gain devices have the option to smooth out scintillation by turning down gain. This works with very narrow filters too. I do this with my pvs-4’s. My fixed gain devices, I just put up with the scintillation or try and get a faster, brighter image to smooth out noise by faster focal ratios. If you have a telescope with extremely flat field and can get edges to display without aberrations, you might want to see more uniform views and not go for super narrow notches that might rolloff the signal at the edge also.

 

All very subjective which is why you see mixed reviews of different filters on the forum. Consensus is an Ha filter is essential for bringing out nebulae detail. Which flavor is the very subjective part. No free lunch for either end of the decision on wider or narrower. For me, a 5nm will be bought in the near future as a compromise between 3 and 7nm effects. I can’t see myself buying a 9 or 10nm, but someone might want or prefer a compromise between 7 and 12nm effects. Harder to find the Ha between 7 and 12nm but they are out there. I think both jdbastro and cnoct have filters between 7 and 12nm.


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#27 GeezerGazer

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Posted 22 April 2019 - 01:20 AM

All very subjective which is why you see mixed reviews of different filters on the forum. Consensus is an Ha filter is essential for bringing out nebulae detail. Which flavor is the very subjective part. No free lunch for either end of the decision on wider or narrower. For me, a 5nm will be bought in the near future as a compromise between 3 and 7nm effects. I can’t see myself buying a 9 or 10nm, but someone might want or prefer a compromise between 7 and 12nm effects. Harder to find the Ha between 7 and 12nm but they are out there. I think both jdbastro and cnoct have filters between 7 and 12nm.

 

I agree with VDN that filter selection and use is very subjective.  Visual and photographic demands on H-a filters differ mainly because photos allow for longer exposure times.  The visual use of these filters with an NVD relies on the filters ability to eliminate LP while letting that small segment of the spectrum through, which your eye/brain must interpret in a millisecond.  The narrower the band, the less light is allowed to pass which puts a higher demand on the intensifier resulting in more visible noise.  As VDN says, no free lunch.  

 

Previous communications with jdbastro revealed that he most often uses 12nm, sometimes 10nm and less often, 7nm for his images.  However, Gavster now uses his narrowest band filters (5nm and 3nm) for imaging with his Android phone which allows up to 30s exposures.  iPhones now allow up to 1s exposures, but with the NightCap app, many 1s exposures are averaged to smooth out the noise caused by using higher ISO settings (which compensates for the shorter exposure).  Where Gavster may take a single 30s exposure with his Android system, I take 30 - 1s exposures with a single shutter release, that are automatically averaged.  The Android and iOS phone cameras achieve similar results in different ways.  

 

Chroma does make an 8nm H-a and can make a 10nm on special order, but they are not inexpensive.  I haven't heard of anyone here using Omega's 1nm H-a filter... probably because noise in the image would be pretty intrusive.  



#28 Gavster

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Posted 22 April 2019 - 03:07 PM

Yes filter selection is subjective, but wink.gif
I’d strongly recommend going for very narrowband (3 or 5nm) for both visual and phone imaging.
My view is that the main aim is to show as much emission nebulae as possible and that stars are secondary. For visual from a LP site, the narrowband Ha filter filters out the light you don’t want resulting in noticeably increased contrast.
But even from a dark site, the narrowband filter will filter out any light you don’t want enabling more of the nebulae to be visible. Having manual gain is really useful to adjust the view to preference and get the best contrast. I find my gen 3 night vision tube copes very well with the strong filtration and by adjusting the manual gain I can reduce/remove scintillation to suit my tast on the object I am observing.

When using scopes, putting the filter before the reducer or if using afocal the eyepiece greatly reduces any band pass impact. But even at 1x (or 3x with the afocal lens) I am happy to have some band shift to get the extra contrast (as shown by cnoct’s images on the linked thread)

For phone imaging with scopes, yes there are less stars but the sky background is darker and the contrast is better which I prefer since I want my images to show maximum nebulae and am less concerned about how many stars are visible. Longer exposure with wider filters gives lighter sky backgrounds even at dark skies which I don’t like.


Edited by Gavster, 22 April 2019 - 03:09 PM.

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#29 GeezerGazer

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Posted 22 April 2019 - 10:31 PM

Yes filter selection is subjective, but wink.gif
I’d strongly recommend going for very narrowband (3 or 5nm) for both visual and phone imaging.
My view is that the main aim is to show as much emission nebulae as possible and that stars are secondary. For visual from a LP site, the narrowband Ha filter filters out the light you don’t want resulting in noticeably increased contrast.
But even from a dark site, the narrowband filter will filter out any light you don’t want enabling more of the nebulae to be visible. Having manual gain is really useful to adjust the view to preference and get the best contrast. I find my gen 3 night vision tube copes very well with the strong filtration and by adjusting the manual gain I can reduce/remove scintillation to suit my tast on the object I am observing.

When using scopes, putting the filter before the reducer or if using afocal the eyepiece greatly reduces any band pass impact. But even at 1x (or 3x with the afocal lens) I am happy to have some band shift to get the extra contrast (as shown by cnoct’s images on the linked thread)

For phone imaging with scopes, yes there are less stars but the sky background is darker and the contrast is better which I prefer since I want my images to show maximum nebulae and am less concerned about how many stars are visible. Longer exposure with wider filters gives lighter sky backgrounds even at dark skies which I don’t like.

The 3 or 5nm filter may perform well at f:1.5 or even f:4, but when using a slower optical system, the benefit of narrower filters are increasingly offset by photon starvation and increased visual noise.  So tradeoffs are still a factor.  My 5.5" refractor at f:7 does not perform as well visually as my 8" Newt at f:2.8.  My interpretation is that slower optical systems do put a strain on the intensifier, and the elimination of additional photons by using narrower band filters can push the intensifier beyond a useful threshold.  My experience tells me that the focal ratio does play a part in satisfactory filter selection... or vice versa, and so does the surface brightness of the H-a subject.  I found with my refractor, using a 2x barlow (effectively f:14), a 5nm filter was mostly unsatisfactory, causing so much visual noise that many targets (especially planetary nebulae and dim H-a subjects) simply disappeared in the noise.  But using the same refractor afocally with a 40mm Plossl and a .8x reducer (effectively +/- f:3.6), the 5nm filter worked fine, albeit, at a smaller visual scale.  These are my findings, using my equipment under my skies, but as always, YMMV.  When I have time, I will try to take a few short exposure images to replicate the noise I see visually at different focal ratios.  I know of no other way to clearly demonstrate this issue.  


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#30 Gavster

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Posted 23 April 2019 - 01:07 AM

The 3 or 5nm filter may perform well at f:1.5 or even f:4, but when using a slower optical system, the benefit of narrower filters are increasingly offset by photon starvation and increased visual noise.  So tradeoffs are still a factor.  My 5.5" refractor at f:7 does not perform as well visually as my 8" Newt at f:2.8.  My interpretation is that slower optical systems do put a strain on the intensifier, and the elimination of additional photons by using narrower band filters can push the intensifier beyond a useful threshold.  My experience tells me that the focal ratio does play a part in satisfactory filter selection... or vice versa, and so does the surface brightness of the H-a subject.  

Ray, you make a correct and very important point. F ratio is clearly important when selecting a Ha filter. I don’t observe nebulae at more than f3.5 now due to the noise and photon starvation and hence my NV tube can deal with the narrowband filters fine. If I want more image scale, I use a bigger aperture scope rather than Barlow up. But I agree, if you are using f4 or more then 7 or 12nm may well work better.


Edited by Gavster, 23 April 2019 - 01:08 AM.

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#31 careysub

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Posted 11 May 2019 - 03:22 PM

I have just received a 4.5 nm H-alpha filter from bjomejag on eBay:

 

https://www.ebay.com...872.m2749.l2754

 

with the following nominal specs:

CWL 656.3+-1nm, HBW 4.5+-2nm, Transmission >80% at CWL, Blocking Avg 10e3 from 200 to 1000nm.

 

Looking at the transmission curve supplied with the filter, evidently a scan analysis of the actual filter shipped, its transmission peak is about 84%, and the curve width at one-half of this value is 5 nm. The transmission is thus above the nominal spec, and the curve width is much closer to spec than I expected.

 

Since the Baader 7 nm is $200, and the 3.5 nm is $300, this is a good deal at $130. 

 

For visual use transmission is extremely important, for NV use it is the rejection ratio that is most important, you can always adjust the gain a little.


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#32 Vondragonnoggin

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Posted 12 May 2019 - 07:30 AM

I have just received a 4.5 nm H-alpha filter from bjomejag on eBay:

 

https://www.ebay.com...872.m2749.l2754

 

with the following nominal specs:

CWL 656.3+-1nm, HBW 4.5+-2nm, Transmission >80% at CWL, Blocking Avg 10e3 from 200 to 1000nm.

 

Looking at the transmission curve supplied with the filter, evidently a scan analysis of the actual filter shipped, its transmission peak is about 84%, and the curve width at one-half of this value is 5 nm. The transmission is thus above the nominal spec, and the curve width is much closer to spec than I expected.

 

Since the Baader 7 nm is $200, and the 3.5 nm is $300, this is a good deal at $130. 

 

For visual use transmission is extremely important, for NV use it is the rejection ratio that is most important, you can always adjust the gain a little.

That’s great news. I’ve had my 3nm a long time now. He is really responsive on emails and exchanges also. Easy to work with and great prices. It’s no Astrodon or Chroma quality, but it’s also not 3x or more the price and if a person isn’t even sure if they’ll like a certain width Ha, that’s a lot to gamble. Much easier to part with a smaller amount for a little less transmission. 

 

If you find its your favorite, you can upgrade to a chroma 5nm later on for $460 or Astrodon at $380 and get a much higher transmission spec.




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