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

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#1 moshen

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Posted 09 March 2019 - 05:48 PM

Huge thanks to GeezerGazer for sending me his Astrodon 5nm Ha filter so I could compare it with my Astronomik 6nm.

 

We had the first clear night in a few weeks and I was able to spend a couple of hours comparing these two filters.

 

Skies: White zone. SQM 17.8. 

Scope: 92mm refractor f/6.65 reduced 0.6x (effective f/4).

Night Vision: L3 unfilmed white phosphor in Mod3 C-mount housing.

Targets: Rosette, NGC 2174 (Monkey Head), IC 443 (Jellyfish), Horsehead nebula & region.

 

I mounted each filter in my 5 speed filter wheel and was able to switch between them very quickly. This way I didn't need to rely on memory for the comparison. I needed to slightly refocus for each filter as the filter glass thickness is different in each causing a slight difference in the focal plane.

 

Results: If both filters are well made and in spec then there is theoretically a 16% reduction in off band light filtration between 6nm & 5nm. In practice I could see this difference but it was extremely subtle. The most noticeable effect was on the Rosette. The detail in the petals via the dark areas was just slightly more contrasty. Switching from the 5nm Astrodon to the 6nm Astronomik I could see everything as with the Astrodon but there was just the slightest bit less contrast. It was subtle enough that I had to switch back and forth about a dozen times just to make sure I wasn't imagining it.

 

On threshold stars the 5nm made them slightly harder to detect. Again very subtle and required a lot of switching to determine it wasn't just due to the changing conditions or my imagination.

 

I think if I had to screw and unscrew each filter to test and rely a bit more on memory to do the comparison it would be very tough to notice the difference. The difference is far less than the 16% spec difference would appear on paper, but still noticeable.

 

In conclusion both filters appear to be well made and perform as their specs suggest. There are no surprises here and I didn't expect one but it was still interesting to verify this in practice.

 

Monday is suppose to be clear so I'll do the same comparison but with 1x viewing instead.

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Edited by moshen, 09 March 2019 - 06:24 PM.

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#2 GeezerGazer

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Posted 09 March 2019 - 07:15 PM

Nice comparison report Moshen; and thanks for taking time to post your results.  It's always interesting to me how different filters present differently to a different set of eyes, under different LP skies and with different optical systems.  There are LOTS of variables, but so often the conclusions about filters are quite similar.  The biggest surprise to me during the recent past was Gavster's not-so-subtle experience with the Chroma 5nm H-a.  There has been speculation that Chroma actually makes the Astrodon filter, but who knows for sure.  Gavster's Chroma does sound like it provides significantly more contrast than the Astronomik 6nm H-a against which it was compared.  I have wondered if the Harder intensifier tube might be more sensitive to something in the 5nm filter than our L3 tubes are... or, if there is something about the Chroma filter that DOES increase perceived contrast.  I would think that there is a greater chance of intensifier differences than a significant difference between the Chroma and Astrodon 5nm filters.  But this is purely speculation.  


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#3 Gavster

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Posted 10 March 2019 - 02:37 AM

Nice comparison report Moshen; and thanks for taking time to post your results.  It's always interesting to me how different filters present differently to a different set of eyes, under different LP skies and with different optical systems.  There are LOTS of variables, but so often the conclusions about filters are quite similar.  The biggest surprise to me during the recent past was Gavster's not-so-subtle experience with the Chroma 5nm H-a.  There has been speculation that Chroma actually makes the Astrodon filter, but who knows for sure.  Gavster's Chroma does sound like it provides significantly more contrast than the Astronomik 6nm H-a against which it was compared.  I have wondered if the Harder intensifier tube might be more sensitive to something in the 5nm filter than our L3 tubes are... or, if there is something about the Chroma filter that DOES increase perceived contrast.  I would think that there is a greater chance of intensifier differences than a significant difference between the Chroma and Astrodon 5nm filters.  But this is purely speculation.  

Interesting comparison Moshen. I still have my Astronomik 6nm Ha filter so I may have another comparison.

I do remember being surprised at how noticeable the difference was but I haven’t used the Astronomik since.

I recall that there was also a beneficial impact of the chroma when using my photonis intens monocular but possibly not as much as when I was using the harder. Alanjgreen has reported benefits when using the chroma 5nm with his Photonis  but from his descriptions they sounded relatively subtle.

I do think that the extra luminance gain of the harder compared with the photonis makes the harder more suited to the narrower chroma but Moshen should have also seen this with his l3 tube? Another possibility is that the out of band suppression of light of the chroma is better than the Astronomik meaning that less of the wrong light is getting through and thus making the nebulae contrast better. Either way, I virtually always use the harder chroma combo now since it’s clear to me that this gives the best results for me whether at a LP or dark site.

 

PS Moshen you do have tough skies! Even from my light polluted South West London back garden I get sqm measurements of around 18.5. Also how does you reducer work with your Stowaway and which reducer is it?


Edited by Gavster, 10 March 2019 - 02:44 AM.

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#4 moshen

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Posted 10 March 2019 - 02:56 PM

Thanks guys.

Gavin, my skies do get around 18.5 after midnight when all the city lights and cars settle down. But they are horrendously bad before then.

 

As for filter differences, there is also most likely individual filter variability in play as well on top of all the other differences mentioned. 

 

I use the 2" GSO 0.5x reducer. It's attached to the 1.25" filter wheel via T-thread to M48 adapter then my Mod3 goes on top via a 2" M48 to c-mount adapter. The spacing is such the reduction is about 0.6x. You can see it in the pic below:

 

IMG_0809.jpg

 

Along with filters I also have a 2x Barlow in one of my five filter wheel slots (It's a 2x Barlow mounted in a 1.25" filter cell).

 

I take the whole unit with filter wheel and diagonal and it works great in all the scopes (from C9.25 to 92mm Stowaway).


Edited by moshen, 10 March 2019 - 02:58 PM.

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#5 GeezerGazer

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Posted 11 March 2019 - 02:10 PM

Gavin, just a thought... IIRC, you have both a 1.25" and 2" Chroma 5nm H-a.  If we sent this 1.25" Astrodon 5nm filter to you, would you have time to compare it to your 1.25" Chroma 5nm?  That might reveal if it is the filter or the NVD that is producing extra perceived contrast at the 5nm wavelength.  I don't know of anyone who has both filters to test together that way.  


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#6 moshen

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Posted 11 March 2019 - 03:07 PM

I took some quick and dirty photos last night with my phone. First is with Astronomik 6nm and second is Astrodon 5nm. Definitely a difference here. There is a bit more bandpass shifting at the edge with the 5nm and more contrast. Visually this is MUCH more subtle than the pics indicate, but the pics don't lie. Both pics with with the exact same exposure settings.

 

At 1x I seem to notice a slightly darker background with the 5nm. Again very subtle. The threshold object was barnard's loop. I can barely make it out because I know where it is. I can detect it equally 1x with either filter but the 5nm seemed to provide just a tad bit of a darker background.

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Edited by moshen, 11 March 2019 - 03:12 PM.

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#7 Gavster

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Posted 11 March 2019 - 04:10 PM

Gavin, just a thought... IIRC, you have both a 1.25" and 2" Chroma 5nm H-a.  If we sent this 1.25" Astrodon 5nm filter to you, would you have time to compare it to your 1.25" Chroma 5nm?  That might reveal if it is the filter or the NVD that is producing extra perceived contrast at the 5nm wavelength.  I don't know of anyone who has both filters to test together that way.  

Ray, certainly I’d be delighted to. I guess an issue is U.K. customs who are from my recent experiences of importing from the USA getting more stringent. I’ll send you a pm.



#8 GeezerGazer

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Posted 11 March 2019 - 10:55 PM

OK Gavin, when the Astrodon gets back to me in a couple of days, I'll get it off to you.  Since it's not a purchase and you are returning it to me, there shouldn't be a problem with customs.  I'll see what I can do.  

 

Moshen, yes, there is a subtle contrast difference visible in your images.  That is just a small degree less contrast difference than I noticed when I tested the 5nm against a 7nm found in this report from a year ago but under much darker conditions (green zone) than you have in San Francisco (white zone):  

https://www.cloudyni...astronomy-r3135

That was solely a visual comparison of the two filters; but we both agreed that the 5nm did provide slightly better contrast on most H-a subjects visually with our L3 tubed Mod 3Cs, using my 140 refractor and Matt's fast 16" Dob.  

 

Moshen, taking images at the same settings was a good idea... was your SQM 17+ again?  Not bad from such severe LP and with a partial moon.  Clearly the 5nm would help under those conditions.  Gavin, maybe you can do the same, taking an image with the Astrodon and Chroma 5nm filters using the exact same camera settings.  It becomes a little more objective I think.  

 

Definitely, band shift becomes more of a concern with really fast optical systems.  We'll see where this goes.  waytogo.gif  



#9 moshen

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Posted 11 March 2019 - 11:37 PM

Thanks! I'm not sure of SQM, didn't take a reading. It's probably high 17 low 18 though. No tracking mount so it's a really poor image but it does show an objective difference.

 

I will say it's definitely not worth upgrading from the 6nm Astronomik to the 5nm for me. I'll wait and see how Gavin's comparison turns out with both 5nm and his upcoming 3nm filter.


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#10 Vondragonnoggin

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Posted 12 March 2019 - 01:57 AM

I’ll be interested to read his take on the 3nm. I don’t have a 5nm yet, but have had 2” versions of 3nm and 7nm for about 5 years now. I also have an Astronomik 12nm. I’m in Bortle 7 skies and usually the 3nm is my choice, particularly if I decide to view nebulae under anywhere from half to full moon present. There is definitely more rolloff towards edges, but I’m not usually going below F/3.2 unless I am using a camera lens and putting outside the objective. I have one 300mm F/2.8 lens (112mm objective) that is modified with a 2” telescope back so I can still use 2” nosepiece with 2” filter at the back, but it is straight through viewing because of limited back focus, so it doesn’t come up much unless I’m planning on lower targets. 

 

In each application though, there is still substantial center area that benefits from increased contrast. My inspiration to try a 3nm was the original article that pushed me towards NV Astronomy and this excerpt here:

 

”We have concentrated on a very tiny portion of the visual spectrum which, interestingly, gives us two great advantages, first, the tiny portion of spectrum which is Hydrogen-Alpha carries the vast majority of detail information in the luminance portion of an astrophotograph. Most of your expert astro-photogropher's use narrow Hydrogen-Alpha filters to bring out the detail in their nebula pictures. Look at almost any nebula shot by Russell Corman, Robert Gendler, or Ken Crawford, just to mention a few-they all use narrow Hydrogen-Alpha filters to capture the luminance detail in the image. The wispy, cloudy detail in virtually all nebula are principally in the Hydrogen-Alpha region. Putting this filter in front of a 25 dB gain block gives you amazing detail, as well as allowing you to see portions of the nebula you've never seen before. Then there is the improved signal-to-noise factor that you benefit from simply by reducing the bandwidth. Signal-to-noise is inversely proportional to bandwidth. It takes a much larger signal to give you the same signal-to-noise ratio in a wideband reception situation as it does in a narrowband situation. It would require the slide-rule-types to figure out exactly what that improvement equates to, but-there are several dB to be gained simply by narrowing the band pass of light. There is also a small price to pay in that your intensifier tube is starting out with a lower level of protons to work with, and is thus working closer to the noise floor than it was before. One more factor comes into play here, and this one is one that you would not ordinarily think of. The I3 has an automatic gain control, or as the tekies call it, an AGC. What this means is that when a strong signal enters the band pass of the I3, such as a bright star, the gain automatically throttles down to keep it from being over driven. What this does is reduces the amount of gain available for all of the information within the eyepiece, therefore, the much lower level information is also reduced by the same amount. Lower gain, less wanted information. The Hydrogen-Alpha filter has an enormous roll to play here, if you recall, the people among us that like to look at the sun use a very narrow Hydrogen-Alpha filter system to observe the sun directly, in most cases a small fraction of a nm in width, sometimes as little as 0.05 nm or less. The sun puts out almost no energy in the Hydrogen-Alpha region of the spectrum. That is why these very narrow Hydrogen-Alpha filters are used for solar observation allowing you to use fairly large aperture telescopes to look directly at the sun. which viewed without the filter would damage your eye and even damage the scope. What does this have to do with the system? I'll tell you. First off, when you look at a nebula through the system, the stars (suns, if you will) are attenuated by as much as 20 dB or more and thus the gain of the I3 is ramped up, compensating for the lack of bright stars in the field to the point that the nebulosity comes booming through. If you have ever looked at the black and white Hydrogen-Alpha photographs on the web or in magazines, you will notice something else you may not have paid attention to before. The number and size of the stars in the image are dramatically reduced. The reason should now be obvious, they are not there as bright or in as large a number because they have been attenuated by the Hydrogen-Alpha filter. This unique combination of light gain, filtering and manipulating of the AGC all working together to give you an image that simply cannot be replicated in any other way, with the exception of long exposure photography.”

 

 

Excerpt from Cloudy Nights article - Collins I3 “The System”, by Lynden - May 10th 2007


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#11 Gavster

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Posted 12 April 2019 - 10:29 AM

Ray very kindly shipped his 5nm astrodon to me and this week I managed to do a comparison with my 5nm chroma. Please see two images attached, first one is astrodon, second chroma. This is the crescent nebula with an exposure time of 20 seconds and iso 50. They were taken in my back garden in London with my c11 edge reduced to f7 and then a 55mm plossl giving around f3.3. Visually and from the images below, I think it’s a dead heat!

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Edited by Gavster, 12 April 2019 - 10:30 AM.

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#12 moshen

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Posted 12 April 2019 - 05:32 PM

Thanks for the testing Gavin. 

 

Based on this, I don't think it's worthwhile to go from an Astronomik 6nm Ha filter to the Chroma 5nm Ha filter. If I do decide to get another filter I would look into a 3nm Ha filter instead.


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#13 GeezerGazer

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Posted 12 April 2019 - 11:36 PM

Thanks Gavin for taking time to test the filters and take the images.  They are pretty close, but the Chroma looks a tad sharper to me.  I suppose it could be a slight difference in focus, but they are so close, it's hard to tell.  I can certainly see why the 3nm would be useful for those observing under severe LP.  


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#14 careysub

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Posted 14 April 2019 - 02:12 PM

For someone who does not have any H-alpha filters, but needs to get one to go with his Gen-III tube (on order) what advice would you give.

 

Take the financial hit and go for a 3-3.5nm, putting all the filter money into this one purchase?

 

Something else?

 

Also, about those low F-ratio filters - worth it/useful or no?

 

With camera lenses you can go down to very low F-ratios, and I have wide angle telescopes with ratios from F/3.7 and up.


Edited by careysub, 14 April 2019 - 02:15 PM.

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#15 Vondragonnoggin

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Posted 14 April 2019 - 02:24 PM

For someone who does not have any H-alpha filters, but needs to get one to go with his Gen-III tube (on order) what advice would you give.

 

Take the financial hit and go for a 3-3.5nm, putting all the filter money into this one purchase?

 

Something else?

 

Also, about those low F-ratio filters - worth it/useful or no?

 

With camera lenses you can go down to very low F-ratios, and I have wide angle telescopes with ratios from F/3.7 and up.

If you don’t want to pay the very high price for a 3nm, 3.5nm, or 5nm premium brand filter, but are ok with spending less on a 1.25” 4.5nm Omega filter with filter graph, I have used filters from this seller (who has over 5000 sales and 100% rating) with great results.

 

I have had a 3nm 2” for over 4 years in use and prefer it to my Baader 7nm 2” which was kind of expensive. The 3nm definitely provides better contrast in my light polluted zone over the 7nm.

 

1.25” 4.5nm Ha - https://www.ebay.com...D-/311627142689

 

$129.50 for the 1.25”

 

Check out his other items as well. You’ll see he sells all kinds of machine Vision and ccd filters and the Ha filter is kind of a rare bird among them. He also sells solar observing Ha extreme narrow filters. He has a good return policy also and very communicative if you have questions.


Edited by Vondragonnoggin, 14 April 2019 - 02:32 PM.

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#16 careysub

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Posted 14 April 2019 - 03:51 PM

I bought a "Optical Interference Filter 656.3 H Alpha <0.1nm Pair" for building a solar scope six years ago from this guy I realize, upon viewing the link, (still have it, and the achromat assembly I bought for it awaiting getting to this project).

 

Yes, I'll look at his offerings, thanks.



#17 Jeff Morgan

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Posted 14 April 2019 - 11:49 PM

For someone who does not have any H-alpha filters, but needs to get one to go with his Gen-III tube (on order) what advice would you give.

 

Take the financial hit and go for a 3-3.5nm, putting all the filter money into this one purchase?

 

Something else?

 

Also, about those low F-ratio filters - worth it/useful or no?

 

With camera lenses you can go down to very low F-ratios, and I have wide angle telescopes with ratios from F/3.7 and up.

 

I have been very happy with the 12nm and 7nm. There are times when 7nm is just too much. Having a 7nm only would be a mistake.

 

A 3nm .... would probably be best as a second or third acquisition.

 

By low f/ratio filters, I assume you are referring to the Baader HiSpeed. They have been tried unsuccessfully for the NV use. Search the forum.


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#18 Vondragonnoggin

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Posted 15 April 2019 - 12:01 AM

I have been very happy with the 12nm and 7nm. There are times when 7nm is just too much. Having a 7nm only would be a mistake.

 

A 3nm .... would probably be best as a second or third acquisition.

 

By low f/ratio filters, I assume you are referring to the Baader HiSpeed. They have been tried unsuccessfully for the NV use. Search the forum.

Baader makes a non-high speed 3.5nm also, but yes cnoct tested the high speed filters and wasn’t pleased at all with results. He recommended against them.

 

My take is the opposite. I have the Astronomik 12nm which never gets used, the Baader 7nm used somewhat, and the Omega 3nm which is primarily used. The narrower the filter, the higher the contrast on Ha as well as more scintillation. For Bortle 7 skies and very fast f ratios, I find the 3nm the most pleasing. If the f ratio goes up, then 7nm or if it exceeds F/6 then 12nm or even 35nm.

 

There could be an aesthetic reason for the wider filters also. If you want more stars visible, the wider notch will show more stars. Sometimes it’s fun to view both but much less contrast on Ha nebula like sweeping around with the 640nm longpass. Still a lot of Ha regions viewable but let’s  a ton more stars come through and virtually no scintillation.

 

Variable gain devices can help with a very narrow filter too. Turning down gain to reduce scintillation and smooth out the noise while retaining the higher contrast of the narrower notch.

 

Its definitely a personal subjective preference because answers on favorites are pretty varied.


Edited by Vondragonnoggin, 15 April 2019 - 12:16 AM.

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#19 careysub

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Posted 15 April 2019 - 05:44 AM

If you don’t want to pay the very high price for a 3nm, 3.5nm, or 5nm premium brand filter, but are ok with spending less on a 1.25” 4.5nm Omega filter with filter graph, I have used filters from this seller (who has over 5000 sales and 100% rating) with great results.

 

I have had a 3nm 2” for over 4 years in use and prefer it to my Baader 7nm 2” which was kind of expensive. The 3nm definitely provides better contrast in my light polluted zone over the 7nm.

 

1.25” 4.5nm Ha - https://www.ebay.com...D-/311627142689

 

$129.50 for the 1.25”

From the listing (and looking at the graph he provides) it looks like the nominal spec is a bit loose:

"CWL 656.3+-1nm, HBW 4.5+-2nm"

 

However, the price is very good, and it looks like a place to start with a "narrowish" filter. You expect to give up something for a low price.


Edited by careysub, 15 April 2019 - 05:45 AM.


#20 Solar storm

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Posted 15 April 2019 - 05:20 PM

I totally impulse bought the 4.5nm ha filter smile.gif .   It will be fun to try out.  Could be a good compliment to my 12 and 7.


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#21 GeezerGazer

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Posted 20 April 2019 - 09:40 PM

For someone who does not have any H-alpha filters, but needs to get one to go with his Gen-III tube (on order) what advice would you give.

 

Take the financial hit and go for a 3-3.5nm, putting all the filter money into this one purchase?

It really depends on:

1. How severe your light pollution is

2. How sensitive you are to the effects of band shift... specifically when using a very fast FR objective

3. How sensitive you are to star attenuation in the FoV

4. How much contrast is sufficient for your eyes under your observing conditions, using your scope/optical system

5. How sensitive you are to noise/scintillation at the occular

 

If you are observing from a dark site, your needs may be entirely different than when observing from downtown L.A.  There is no question that a very narrow band eg. 3nm H-a filter will provide additional contrast, it comes at a cost.  This is why different observers are satisfied with filters that are unsuitable for others.  

 

If you are buying new filters, make sure the vendor has a good return policy.  You might even talk with the vendor, explain that your want to buy two filters with an understanding that one may be returned.  If you are buying used filters, it gives you the opportunity to try them and re-sell without much loss if they are not suitable for your needs.  Better yet, try to find someone locally with filters that you can try in your own scope.  Good luck. 


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#22 careysub

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Posted 20 April 2019 - 09:50 PM

Thanks. What is "band shift"? I have run across this term several times here, but never with an explanation of what it is. Not even Vondragonnoggin's glossary mentions  it.


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#23 Vondragonnoggin

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Posted 20 April 2019 - 10:02 PM

Thanks. What is "band shift"? I have run across this term several times here, but never with an explanation of what it is. Not even Vondragonnoggin's glossary mentions  it.

This thread here explains it with pictures using 12nm vs 5nm. The whole thread is a good read and shows contrast boost for narrower but at the expense of filter effectiveness rolloff towards the edges of the FOV because of bandwidth shift.

 

https://www.cloudyni...nm-h-a-filters/


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#24 DMala

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Posted 21 April 2019 - 03:17 PM

This thread here explains it with pictures using 12nm vs 5nm. The whole thread is a good read and shows contrast boost for narrower but at the expense of filter effectiveness rolloff towards the edges of the FOV because of bandwidth shift.

 

https://www.cloudyni...nm-h-a-filters/

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. 


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#25 Jeff Morgan

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Posted 21 April 2019 - 04:09 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. 

 

To do processing, you would need flats and darks all of which need to be calibrated and stacked. Then adjusted. Complicated process.

 

I tried with some of my iPhone shots and came to the conclusion that it crosses the line from easy snap shots to traditional imaging, best done with dedicated imaging equipment and techniques. 


Edited by Jeff Morgan, 21 April 2019 - 04:09 PM.

  • DMala likes this


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