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Spectroscopic Analysis & Comparison of Nebula Filters

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#1 LunarFox

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Posted 10 February 2016 - 04:06 PM

So I'm relatively new to amateur astronomy, but one of the first things about this hobby that really caught my interest were nebula filters. I work in a Molecular Biology lab where we routinely use fluorescent tags to label structures of interest, so I am quite familiar with the use of filters to isolate certain wavelengths and just found it very interesting that very similar filters are being used in astronomy.

 

However, I was a bit disappointed when I noticed the lack of spectroscopic data that is available on the various filters that are sold online. A few vendors will post the bandpass of the filters they sell, but they're often simple cartoon schematics as opposed to actual scans, so it's really hard to know what you're actually buying and if it's worth the extra cost. 

 

With that in mind, I've taken it upon myself to use the UV-Vis spectrometer in our lab to take scans of any and all filters that pass through my hands in an effort to provide as much additional data as possible on some of the more common filters. This analysis is by no means final or comprehensive, but I think I have accumulated enough to stimulate some sort of discussion and hopefully answer a few of the more common questions I've seen come up on this forum. In particular I would like to provide a few insights to the following:

 

1. Which narrowband "UHC-like" filter should I buy?

2. How consistent is the coating process from one batch to another?

3. Is the performance of my "mottled / rusted" filter compromised by the apparent hazing in the coatings?

 

I will do my best to provide answers to these questions, but please keep in mind that everything I am about to say is entirely based off scans I've taken on our UV-Vis, so it's purely an analytical approach. I've spent very little time at the eyepiece with these, winters in the NW are a disaster for astronomy, and reports provided by David Kinsely and others will be much better at describing the performance of some of these filters in the field.


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#2 LunarFox

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Posted 10 February 2016 - 04:07 PM

1. Comparison of Narrowband Filters - Lumicon UHC, Orion UltraBlock, DGM VHT

 

So a common question that often comes up when someone new such as myself enters the hobby is "What should be my first filter"? And the general consensus is that a narrowband filter (green trace) isolating the H-Beta and O-III lines is usually the best all around filter to start with, as it offers noticeable improvement on most nebula

 

Broad Narrow O-III.jpg

 

And among the narrowband filters, two usually come out on top, and those are the Lumicon UHC and the DGM NPB (Narrow PassBand). Unfortunately, I haven't had an NPB in my hands just yet, but I did recently come across a DGM VHT (Very High Transmission) instead, and was pleasantly surprised at how similar its bandpass was to that of the Lumicon UHC. I also picked up a 2" Orion UltraBlock along the way and have included it for comparison as well. It's the cheapest of the three so I thought it would be good to have an "economy" filter in the mix to use as a comparison.

 

UHC UB VHT.jpg

 

As can be seen from these traces, the three are very similar spectroscopically.

 

I was slightly disappointed by the UHC, as it had an unfortunate dip in transmission right at the two O-III lines, and I was most impressed by the Orion UltraBlock. Its bandpass and transmission were nearly identical to that of the Lumicon UHC, and no dip anywhere in the bandpass itself. Additionally, the UltraBlock was the only one that completely blocked out all UV and near-IR wavelengths, truly isolating only the H-Beta and O-III lines within the visible spectrum. But the VHT also took me by surprise, I expected it to be more of a "broadband" style filter, but the trace to me places it more into the narrowband category, as it had a relatively narrow FWHM and minimal transmission in the reds compared to most broadband filters. I've heard people regard the VHT as a "jack of all trades" filter, and based on its spectral properties I would have to agree. It's almost like a narrowband / broadband hybrid.

 

Here are the stats from the above scans:

 

                         FWHM        486nm        496nm        501nm        

UHC                 ~24nm         93.3%         92.5%        90.7%

UltraBlock       ~27nm         93.6%         95.8%        95.9%

VHT                  ~33nm         94.6%         95.5%        94.1%


Edited by LunarFox, 10 February 2016 - 05:57 PM.

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#3 LunarFox

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Posted 10 February 2016 - 04:07 PM

2. Batch-to-bath Bandpass Consistency

 

Now another question that has recently surfaced a number of times has been whether the bandpasses of the various filter are consistent from one manufacturer to another, with this being used as the argument for justifying the higher cost of the premium brand filters such as the Lumicon. 

 

So what I've done here is scanned three separate filters, from completely random manufacturing batches as far as I know, and superimposed their scans on the same graph for comparison. I've had the opportunity to scan three (3) different Lumicon O-III, Orion UltraBlock, and Orion SkyGlow filters. I would have loved to include more, but this is all I've had so far. The results are shown below:

 

O-III.jpg

 

UB.jpg

 

SG.jpg

 

From these scans, we can see that in the case of the Lumicon O-III and the Orion UltraBlock, there is virtually no significant difference between the three different batches. They're pretty much virtually identical. Comparison of the data distributions by a two-sample goodness of fit test (KS2 test in MatLab) shows p-values of <0.05%, consistent with lack of statistical difference. However, one of the Orion SkyGlow filters was quite different from the rest, as it had a significantly wider bandpass of ~62nm compared to ~45nm in the other two, but also ridiculously high transmission rates. 

 

Now please keep in mind that this is still only three (3) filters that are being compared, which is a ridiculously small sample size. Additionally, we are comparing an O-III line filter with a narrowband and broadband, but again, this is all I had available. 

 

The one thing I was very pleased with personally was to see that the UltraBlock is very consistent from batch-to-batch. Given the similarity of its bandpass to the UHC and the lack of batch variability, I've officially adapted the UltraBlock as my personal narrowband of choice, as a 2" UB is far more affordable than both the UHC and NPB.


Edited by LunarFox, 10 February 2016 - 05:58 PM.

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#4 LunarFox

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Posted 10 February 2016 - 04:07 PM

3. Compromised Performance Due to Coating Failure

 

Last, but far from least, is the topic of hazed coatings (as seen below) that appear to be most often found on older style Lumicons, and how this alters the spectroscopic properties of the affected filters.

 

Hazed O-III (2).jpg

 

FullSizeRender-2.jpg

 

Now by no means is the intent of this section to discredit Lumicon as a manufacturer, all of their filters I've scanned thus far have exhibited the tightest bandpasses with the steepest transitions, indicating of high-quality coating deposition methods that alone may warrant their higher price. As with TeleVue in the eyepiece market, they've emerged as the leader in the filter arena because they innovate and deliver high quality products, and the fact that there are still Lumicon filters from the 1980's floating around speaks volumes of their dominance.

 

However, some of these old filters have become severely compromised over the years, as is often indicated by what appears to be hazing, often regarded to as "mottling" or "rusting", of the filter coatings, and when I first entered this hobby most information I found on this topic seemed to indicate that the mottling was no big deal and that the filter is still fully functional.

 

Some of you may remember a previous thread on this topic, which spawned from a transaction between myself and optinaut on this forum, in which I inadvertently sold him a compromised Lumicon O-III filter before I knew anything about them. In fact, it was this particular case that spawned my interest in scanning the filters in order to obtain their actual bandpass in order to gain insights into their performance. That transaction was fortunately resolved, but I actually stumbled upon 3 other degraded filters that I was able to scan, and generated a pretty interesting data set shown below:

 

Hazed O-III.jpg

 

So this looks pretty interesting doesn't it? It becomes obvious that these filters slowly progress from a still somewhat usable (#4) filter and begin to undergo a rather drastic broadening of the bandpass accompanied by a significant red-shift of the bandpass until they are rendered rather unusable (#3).  As can be seen above, after some time, the bandpass shifts so drastically that it entirely stops transmitting the two O-III. 

 

Now the most interesting observation I made was that there is virtually no correlation between the level of mottling that can be seen by eye and the shift in the bandpass as seen by UV-Vis. The very first image above is actually filter #4 in the graph, and the second image is filter #3. By eye, you would say that the hazing see in #3 is far worse than that of #4, when in reality the opposite is true. 

 

The bottom line is that if we encounter a filter with their coatings degraded like this, it should not be assumed that the performance is unaffected. They may still be usable if the oxidation hasn't completely taken over, but it's virtually impossible to assess this by eye. These filters should be purchased only if full disclosure and a generous discount in price are provided before the sale, and immediately returned for a full refund otherwise! 

 

But if I can provide one potential waring sign, it's the fact that these often come in the older style blue Lumicon boxes. 3 out of the 4 mottled filters came in one of these, and the filters themselves usually have no markings on the side to indicate that it's indeed a Lumicon. So if you see one of these on eBay or in the classifieds, ask for a straight-on, full flash, strong light photo of the filter itself, as this usually exposes the coating defects quite well.


Edited by LunarFox, 10 February 2016 - 05:48 PM.

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#5 LunarFox

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Posted 10 February 2016 - 04:07 PM

So with all that laid out, please feel free to post your comments, questions, and/or suggestions!


Edited by LunarFox, 10 February 2016 - 05:51 PM.


#6 Starman1

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Posted 10 February 2016 - 05:55 PM

I have actual measurements of many nebula filters.

Send me your email in a private message and I will forward them on to you.



#7 LunarFox

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Posted 10 February 2016 - 06:02 PM

Don,

 

I've definitely seen you post a lot of yours recently, it's kind of what inspired me to make this post to be honest. I just wanted to provide a dedicated thread to the topic, as these spectra are usually buried deep within other threads that can be hard to find later on.

 

And I would love to see all of your scans. If you don't mind, I would gladly go through them and add them to this thread for future reference.

 

-Oggie



#8 jetstream

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Posted 10 February 2016 - 07:28 PM

I should have sent you my Ultrablock ...before I threw it away.


Edited by jetstream, 10 February 2016 - 07:28 PM.


#9 LunarFox

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Posted 10 February 2016 - 07:38 PM

What was wrong with it?



#10 optinaut

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Posted 10 February 2016 - 08:32 PM

Excellent study, LunarFox. Looking forward to more of your data on the subject.  :waytogo:


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#11 jetstream

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Posted 10 February 2016 - 11:13 PM

What was wrong with it?

It just didn't have the transmission, poor dim views. I eventually replaced it with a Lumicon UHC and have been very happy, I actually just came in from using it. My last views with the UHC tonight were the Spider and Fly, Flaming Star and IC410 in Auriga.. With the Ultrablock these were mostly invisible and the HH was out of the question with it whereas the Lumi gives VG views of it in my 15".

 

I just got a poor copy of the filter, thats all- I hear many of them are good though.


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#12 LunarFox

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Posted 11 February 2016 - 12:42 AM

Yeah I would have been more than happy to scan it for you. I'm beyond pleased with mine, really couldn't see any difference between it and the UHC the few times I tried comparing them. 

 

Do you remember it having any visible coating defects as seen on the Lumicons above?



#13 David Knisely

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Posted 11 February 2016 - 01:52 AM

The DGM VHT I tested was a bit broader than I would have liked, providing contrast improvement somewhere between the broad-band filters and the true narrow-band filters.  It helped over no filter at all, but for a nebula filter, in terms of a contrast boost, for me, it just wasn't doing enough.  I mainly tell people that if a TRUE narrow-band nebula filter has a transmission of over 89% at the OIII and H-Beta lines and a Full-width at Half Maxima (FWHM) bandwidth of between 23 and 29 nm, it should work just fine.  A number of commonly available narrowband nebula filters usually meet these two criteria, although every once in a while, a defective one is known to slip through quality control.  However, I have had good results with the Lumicon UHC, Thousand Oaks Narrowband LP-2, DGM Optics NPB, and Orion Ultrablock filters, so one can probably buy on price here and still get a good narrow-band filter.   Clear skies to you.


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#14 sg6

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Posted 11 February 2016 - 03:50 AM

This is usually my first reference:

http://www.astrosurf...ters/curves.htm

 

Took a copy of the measurements made by Starman1 for reference.

 

This one for some internal camera filters:

http://kolarivision....r-transmission/

 

Part of the problem is that the curves manufacturers give are from the design parameters and so the "ideal" but theoretical operation, what the actual operation is can well be different.



#15 Scanning4Comets

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Posted 11 February 2016 - 04:11 AM

Very well done sir.

 

:grin:



#16 jetstream

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Posted 11 February 2016 - 10:02 AM

Yeah I would have been more than happy to scan it for you. I'm beyond pleased with mine, really couldn't see any difference between it and the UHC the few times I tried comparing them. 

 

Do you remember it having any visible coating defects as seen on the Lumicons above?

The thing looked perfect and I did shine a flashlight at many angles of its surfaces- no hazing or rusting. After this filter I tried Astronomik and Lumicon filters and am happy with them, the Lumicon OIII seems a bit tighter than my Astronomik judging from the views of the Veil, including the "thin thread" etc.



#17 LunarFox

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Posted 11 February 2016 - 11:35 AM

The DGM VHT I tested was a bit broader than I would have liked, providing contrast improvement somewhere between the broad-band filters and the true narrow-band filters.  It helped over no filter at all, but for a nebula filter, in terms of a contrast boost, for me, it just wasn't doing enough.

Nice to hear you chime in here David, your detailed descriptions of the various filters in the field has been most helpful to me when I first started out. 

 

So from your experience, what would you say is the optimal application of the VHT? I never got a chance to actually use it, but the bandpass looked awfully similar to the UHC and UBlock to be considered a light pollution filter in my opinion. Of course, actual use ultimately trumps these scans, so I'm curious to hear what you would recommend the VHT be used for (if anything).



#18 David Knisely

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Posted 11 February 2016 - 02:55 PM

 

The DGM VHT I tested was a bit broader than I would have liked, providing contrast improvement somewhere between the broad-band filters and the true narrow-band filters.  It helped over no filter at all, but for a nebula filter, in terms of a contrast boost, for me, it just wasn't doing enough.

Nice to hear you chime in here David, your detailed descriptions of the various filters in the field has been most helpful to me when I first started out. 

 

So from your experience, what would you say is the optimal application of the VHT? I never got a chance to actually use it, but the bandpass looked awfully similar to the UHC and UBlock to be considered a light pollution filter in my opinion. Of course, actual use ultimately trumps these scans, so I'm curious to hear what you would recommend the VHT be used for (if anything).

 

 

They might be useful to someone with a small scope who has never used a filter before, but otherwise, I would have to pass on getting the VHT.  If I want to see stars with a modest amount of nebular contrast improvement, I will use a true "broadband" filter.  However, for a true "Nebula" filter, I will want one that only lets through the major nebular emission lines and little or nothing else.  That requirement would exclude the DGM VHT filter in favor of the true narrow-band filters like the DGM NPB or Lumicon UHC.  Clear skies to you.



#19 Live_Steam_Mad

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Posted 20 February 2016 - 03:16 PM

This is usually my first reference:

http://www.astrosurf...ters/curves.htm

 

 

I like this one here too ;-

 

http://www.reinervog...Filter_UHC.html

 

I am looking for the best, most aggressive filter for galaxies (I am hoping to use IR in addition since galaxies emit a lot of IR and I have an LN300 camera with the internal IR filter removed and the ExView 2 sensor).

 

Regards,

 

Alistair G.


Edited by Live_Steam_Mad, 20 February 2016 - 03:18 PM.

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#20 havasman

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Posted 20 February 2016 - 03:59 PM

Having "one of us" unattached amateurs with access to appropriate test equipment and knowledge to use it and interpret the results take time to distribute this information is a treat. Thank you. Very interesting. 

 

Are you interested in expanding your library of results by testing more filters? I have a couple of filters I do not carry in my normal kit that I would be happy to loan you for testing: Starguy UHC $ Celestron UHC/LPR. If you are interested and it goes well, maybe I could send along my front line filters (Lumicon UHC 2" and 1.25", Thousand Oaks O-III 2" and 1.25", DGM NPB and Thousand Oaks H-Beta) at different times for your use in testing to increase your database of results. Others may also be so inclined. PM me if you'd like to try the Starguy and Celestron.



#21 Cames

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Posted 20 February 2016 - 06:45 PM

So with all that laid out, please feel free to post your comments, questions, and/or suggestions!

 

Your work is breaking new ground in our understanding of filter performance. Thank you for doing it. 

 

One area of this science that interests me is filter transmission when the incoming wavefront encounters the filter substrate at an angle.

For example, considering the light cone of an f/4 telescope, the off-axis angle of attack can be something like 7° and even steeper in faster focal ratios.

 

Can your equipment gather reliable data if the sample is tilted a little?  I'd be interested to see what effect the tilting of the filter might have on transmission, if any. When I hold a filter in my fingers up to the sky on an overcast day, I see a color change in the transmitted light as I rock the filter back and forth.

--------

C



#22 Starman1

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Posted 21 February 2016 - 12:04 PM

Of course.  Multiple layers of coatings have different interference effects when light passes through at oblique angles.

The primary effect with a more normal +/- 1 degree of angle is to move the bandwidth back and forth in wavelength.

For instance, a 476-505nm bandwidth may become 481-510nm 2° off axis.

It is the primary reason why filters cannot be made with bandwidths too tight around the necessary wavelengths.  It might be possible for off-axis light to actually move the bandwidth laterally far enough to miss one of the lies that is desirable to be transmitted.



#23 precaud

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Posted 21 February 2016 - 12:16 PM

So with all that laid out, please feel free to post your comments, questions, and/or suggestions!

 

Yes! Thank you! I love it!



#24 Scanning4Comets

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Posted 21 February 2016 - 02:12 PM

 

So with all that laid out, please feel free to post your comments, questions, and/or suggestions!

 

Your work is breaking new ground in our understanding of filter performance. Thank you for doing it. 

 

One area of this science that interests me is filter transmission when the incoming wavefront encounters the filter substrate at an angle.

For example, considering the light cone of an f/4 telescope, the off-axis angle of attack can be something like 7° and even steeper in faster focal ratios.

 

Can your equipment gather reliable data if the sample is tilted a little?  I'd be interested to see what effect the tilting of the filter might have on transmission, if any. When I hold a filter in my fingers up to the sky on an overcast day, I see a color change in the transmitted light as I rock the filter back and forth.

--------

C

 

 

Excellent points!

 

I have seen the change in color when tilting the filter as well.



#25 LunarFox

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Posted 22 February 2016 - 05:02 PM

 

So with all that laid out, please feel free to post your comments, questions, and/or suggestions!

 

Your work is breaking new ground in our understanding of filter performance. Thank you for doing it. 

 

One area of this science that interests me is filter transmission when the incoming wavefront encounters the filter substrate at an angle.

For example, considering the light cone of an f/4 telescope, the off-axis angle of attack can be something like 7° and even steeper in faster focal ratios.

 

Can your equipment gather reliable data if the sample is tilted a little?  I'd be interested to see what effect the tilting of the filter might have on transmission, if any. When I hold a filter in my fingers up to the sky on an overcast day, I see a color change in the transmitted light as I rock the filter back and forth.

--------

C

 

That's an interesting point I haven't thought of. It's definitely something I can measure, but not very reproducibly at this point. I would pretty much need to hold the filter at a slight angle, which would be far from quantitative or repeatable from one measurement to the next. 

 

But you've certainly given me a good idea. I'll bring in one of my filter tomorrow and see if I can take a few quick measurements with the filter placed at different angles to see if there is any difference in transmission to begin with. Then I can start thinking about an adapter I could 3D print to allow for the filters to be mounted at angles that simulate light cones of varying scope designs.

 

I actually suspect there won't be any difference, and that what we see when we angle the filter is just an enrichment of the normal reflective properties of the coatings due to increased visible area of the reflective surface. In either case, a test is in order. I'll report my finding soon :)




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