23 replies to this topic

[mloffland]

CN Report: The DGM Optics NPB Nebula Filter
By: David Knisely

[Mr. Bill]

Hi Dave

I thought that the coatings of Lumicon filters were always oxide overcoated; can you elaborate on this? What sort of laminate are on the earlier filters and whose are we talking about?

[Starman1]

A question for David:
Are there any test reports for this filter that show whether or not the off-axis light of a short focal length reflector causes enough frequency shift in the filter to actually have one of the 3 "essential lines" get shifted slightly out of the filter's bandwidth?
A narrow bandwidth that encompasses the necessary frequencies/wavelengths is great, but does the transmission of the H-Beta line (486nm) or upper O-III line(501nm) get reduced below 90% by the angle of incidence of the light cone on, say, an f/4.5 scope?
If not, then the performance enhancement you see is legitimate and praiseworthy.
But I wonder if such a narrow bandwidth has the filter functioning more like an O-III filter, with a reduced transmission at the H-Beta line.
Do the filters come with individual test graphs?

[David Knisely]

Hi Dave

I thought that the coatings of Lumicon filters were always oxide overcoated; can you elaborate on this? What sort of laminate are on the earlier filters and whose are we talking about?

The early Lumicon filters were laminated, but I am not certain what the material was. It did not seriously affect the performance, but it may not have protected the coatings quite as much as some of the newer harder overcoatings. I have noted that some of my early filters have shown some mild degredation at the edges, which may be attributable to failures in the lamination protection. So far, the jury is still out as far as whether the newer filter overcoats do a better job at protecting the dielectric layers better than the lamination did. Clear skies to you.

[David Knisely]

A question for David:
Are there any test reports for this filter that show whether or not the off-axis light of a short focal length reflector causes enough frequency shift in the filter to actually have one of the 3 "essential lines" get shifted slightly out of the filter's bandwidth?
A narrow bandwidth that encompasses the necessary frequencies/wavelengths is great, but does the transmission of the H-Beta line (486nm) or upper O-III line(501nm) get reduced below 90% by the angle of incidence of the light cone on, say, an f/4.5 scope?
If not, then the performance enhancement you see is legitimate and praiseworthy.
But I wonder if such a narrow bandwidth has the filter functioning more like an O-III filter, with a reduced transmission at the H-Beta line.
Do the filters come with individual test graphs?

It would take a substantial shift in the filter's orientation to get the passband to move enough so as to exclude one of the lines. Any shift of an interference filter due to increasing field angle tends to move the passband slightly towards the blue end of the spectrum, so the OIII line at 5007 Angstroms would be probably the most vulnerable to any potential passband shift and not the H-Beta line. It would also take a transmission drop of 10 percent or more to be very noticable to the eye. I have not noted any variation in the response of the filter in the scope at f/5.6, but I have not used it under f/5. I have noted in my old Lumicon OIII that the intensity of the nebulosity varies slightly at f/5 when I pan the scope across a bright emission nebula, so we might be seeing that effect in that particular case. However, the passband of the OIII is much narrower than the NPB, so I think that probably the NPB's passband is still wide enough that it is unlikely to exclude either of the OIII lines. The spectral data I refer to comes from graphs of the NPB from DGM Optics (the FWHM is actually the 50% transmission level width), and the filter I got did not come with test data (but it was one of the first production models of the filter). In hearing what other deep-sky observers who have been using the NPB have said, they tend to perfer it over the Lumicon UHC by a somewhat wider margin than I do, and some of them are using some fairly fast large Dobsonians for this comparison. Clear skies to you.

[Tele]

I have been using this filter for some time in my 14" f4.8. It seems to perform equally well on H Beta and OIII type objects. On the horsehead under very dark transparent skies; ie (I could just see the hoursehead without a filter) the DGM filter made the dark nebula very apparent, while the Lumicon only slightly improved the view. In side by side tests agianst Lumicron I always perfered the DGM. But the difference was not extreme. I my buddys 22" f5.6 the difference was more pronounced.

[Starman1]

Thanks, David.
I should know that any review of a product from you is carefully done.
As we both know, other filter reviewers tend to evaluate solely on how things look in the eyepiece, and do not take into account bandwidth differences. So one manufacturer's LPR filter may produce better results than another manufacturer's LPR filter simply because the bandwidth is narrower than other LPR filters.
In this case, I was suspicious that the NPB filter was actually performing more like an O-III filter because of its bandwidth.
If, on the other hand, the narrower bandwidth still passes the correct wavelengths acceptably AND shows better contrast doing so, then it is a "better mousetrap".
Certainly your review has me interested in buying one and doing my own comparisons.

[Herenomore]

Excellent review, David. I have a 2" DGM NPB and I'm very happy with it. If I recall, Phil Harrington did an article on the NPB and several other nebula filters last year for Astronomy magazine and gave the NPB high marks.

Tom

[Jim Carpenter]

Hi David,
Have you evaluated this filter on the basis of your earlier filter article at http://pages.sbcglob...ash/filters.htm and if so, will you be updating that site to show the comparitaive results in that format?

Thanks,
Jim

[jmcdonald]

Thanks so much for this review David.

[reflector74]

What is the "mismatch" of the 1.25" thread??

[David Knisely]

Hi David,
Have you evaluated this filter on the basis of your earlier filter article at http://pages.sbcglob...ash/filters.htm and if so, will you be updating that site to show the comparitaive results in that format?

Thanks,
Jim

No, the article there is an early version of the one I have here on Cloudynights:

http://www.cloudynig...hp?item_id=1520

I used the UHC filter for the continuation of that study because that is what I started the study with a number of years ago. Generally when doing a scientific study with a certain set of instrumentation, one generally does not change the equipment so as to not invalidate the comparative results. The differences between the Lumicon UHC and the NPB are relatively small, so by in large, they will not generally cause a massive change in the results I cited. However, there are times when I have occasionally gone with the NPB instead of the OIII on some "OIII" objects with surprisingly good results. One night when viewing the Helix (NGC 7293), I had my Nexstar 9.25 inch SCT on it and put in the NPB at 59x. It looked quite a bit like the view I got using the OIII filter, although the contrast was still somewhat higher with the OIII. At magnifications at or beyond 6x per inch under dark-sky conditions, the NPB can occasionally be a slightly better choice than the OIII even on some "OIII" objects purely for nebula brightness alone. Indeed, at that same power, the Veil looked very nice with the NPB, although again, the OIII provided just a bit more contrast. The NPB will not replace the OIII by any means, but it does slightly reduce the size of the difference in performance between the narrow band and the line filter, at least on some objects. Clear skies to you.

[David Knisely]

What is the "mismatch" of the 1.25" thread??

As I stated in the article, the filter threads on the 1.25" model of the NPB caused a minor problem in a few of my eyepieces. For example, on my 24mm Tele Vue Panoptic, the NPB would only thread on about a third of a full rotation as I screwed the filter onto the threaded end of the eyepiece before the threads locked. This was more than enough to keep the filter in place, but less than the full extent of the filter threads should have allowed the filter to be screwed on. On a number of my other eyepieces, the filter threaded on fully all the way down to the unthreaded part of the filter's housing, so it was mainly on that eyepiece that the problem occurred. This was a minor problem and did not really affect the usefulness of the filter, although I had to keep it in mind when using the 24mm Panoptic. These threading mis-matches are more common than they need to be, and the blame can go to both the eyepiece and the filter manufacturers depending on who you talk to. One of my other filters (an old 1.25" Lumicon H-Beta) has a more severe threading problem, in that it will thread only very slightly onto all my eyepieces except for two. However, I have replaced that filter with the 2" H-Beta, and that filter has no problems threading into any of my 2" eyepieces or adapters. Clear skies to you.

[reflector74]

Well..hopefully, with my son's new eyepiece collection consisting of UO Abbe's and modified 5-element Antares Plossls (Ultimas) this won't be a problem. Also it had better not be a problem with the Denkmeier OCS thread.. If there are issues with the threading, the filter will be sent back and I'll have to live with a different manufacturer.

[reflector74]

What is the "mismatch" of the 1.25" thread??

As I stated in the article, the filter threads on the 1.25" model of the NPB caused a minor problem in a few of my eyepieces. For example, on my 24mm Tele Vue Panoptic, the NPB would only thread on about a third of a full rotation as I screwed the filter onto the threaded end of the eyepiece before the threads locked. This was more than enough to keep the eyepiece in place, but less than the full extent of the filter threads should have allowed the filter to be screwed on. On a number of my other eyepieces, the filter threaded on fully all the way down to the unthreaded part of the filter's housing, so it was mainly on that eyepiece that the problem occurred. This was a minor problem and did not really affect the usefulness of the filter, although I had to keep it in mind when using the 24mm Panoptic. These threading mis-matches are more common than they need to be, and the blame can go to both the eyepiece and the filter manufacturers depending on who you talk to. One of my other filters (an old 1.25" Lumicon H-Beta) has a more severe threading problem, in that it will thread only very slightly onto all my eyepieces except for two. However, I have replaced that filter with the 2" H-Beta, and that filter has no problems threading into any of my 2" eyepieces or adapters. Clear skies to you.

I just went to get my eyes checked yesterday afternoon at the optomitrist for my annual visit. They're as sharp as a razor (perfect vision for a human being). Also, my dark adapted pupils are ~7.5mm - not bad for a 32 year old. I was told something interesting in that direct vision capability drastically affects the peripherary or averted abilities in people with limited eyesight capacity. We concluded our conversation with the differences of human scoptic and photopic eyesight.

It would be interesting to conduct your test as a "blind test" on say 100 people under the most controlled conditions possible. There was a similar test by Mike Palermiti at ITE in Florida a while back regarding the NPB Filter.

I would dare to say that a good O-III Filter, in my experience, is the single most valueable filter for most planetary nebulae available to the amateur for visual purposes, so long as it is as close to a "line" filter possible, isolating 496nm and 501nm.

It is worth some consideration to have more observer's data rather than just one individual. The graph alone if accurate, should give the best preliminary idea of how the filter would perform.

If anyone wants to see that throughput graph of the NPB Filter or learn directly about this filter from Dan, President of DGM Optics as well as employee of Omega Optical, please visit his website AND/OR give him a call a speak to him directly.

[David Knisely]

Well..hopefully, with my son's new eyepiece collection consisting of UO Abbe's and modified 5-element Antares Plossls (Ultimas) this won't be a problem. Also it had better not be a problem with the Denkmeier OCS thread.. If there are issues with the threading, the filter will be sent back and I'll have to live with a different manufacturer.

If you send the NPB back, you may end up living without one really decent filter (perhaps the best narrow-band filter out there). As I said, the thread mis-match happened on only one or two of my 14 eyepieces, and was not enough to prevent successful filter use on *all* of my eyepieces. The 1.25" NPB threads on nicely into the front end of my Burgess binoviewer. However, unless you have a rather long travel with your focuser, you may be working at a bit higher power than the recommended optimal power range for filter use (3.5x to 9.9x per inch). In my NexStar 9.25 SCT, I can go no lower than about 94x on the Burgess using my 25mm Plossls, which is just above the recommended power range for filter use. For deep-sky observing of faint extended nebulae, I recommend no binoviewer be used and the power level be kept fairly low for proper filter use. Binoviewers are wonderful for the moon and planets, and will work to some extent for deep-sky observing, but when working to "the edge" of visual observing, it is best to go with just one eyepiece. Clear skies to you.

[reflector74]

There are of course, objects which required only one eyepiece..

My low power is about while 60x binoviewing. Binoviewers I agree are not something to use on the faintest DSO's and that' juat good common sense.

I'll just keep my fingers crossed on the threading.


Michael

[David Knisely]

reflector74 (hi Mike!) posted:

I would dare to say that a good O-III Filter, in my experience, is the single most valueable filter for most planetary nebulae available to the amateur for visual purposes, so long as it is as close to a "line" filter possible, isolating 496nm and 501nm.

The OIII really is a great filter to have when hunting down the smaller planetary nebulae that seem to "hide" in rich starfields at low power. Using the "blinking" technique, finding these little guys is like shooting fish in a barrel! You just pass the OIII filter between your eye and the eyepiece and watch for the star in the field that does *not* get dimmed. Then, you can lock onto the nebula and increase the power. I rarely use my OIII filter at high power because the higher power already does the job of reducing the skyglow fairly effectively, although I have used the OIII to "tone down" the brighter central stars to make the nebulosity a little easier to see. Two such objects that need central star "elimination" are NGC 2392 (Eskimo), and NGC 1514 (Crystal Ball).

A few planetaries are, however, *not* best seen using the OIII filter. M27 (Dumbell) is best in a narrow-band filter, as the object has some H-Beta emission as well as the OIII lines. It looks distinctly larger when using either the NPB or the UHC, and shows the fainter outer "wings" off the side of the dumbell better than when the OIII is used. Another one is NGC 40, which again is a bit better in a narrow-band filter like the NPB than it is in the OIII. Indeed, one planetary nebula is best seen in the H-Beta filter: Campbell's Hydrogen Star (Cygnus). I hear from friends using very large instruments that there are a few other planetaries that are not purely OIII targets, so I have a lot more "hunting" to do. Clear skies to you.

[Starman1]

The rule that nebula filters should all be used at less than 10X per inch is a nod to the realities of the apertures used by most amateurs.

I have experiential reasons to know that nebula filters can be used and have positive effects at magnifications higher than this IF the light loss that comes from magnification does not reduce the brightness of the nebula significantly.

I've seen the difference a filter makes in the appearance of the Saturn Nebula (NGC7009) at both 750X and 1050X in a 28" reflector, and the effect an O-III filter had on NGC1514 at nearly 1000X in a 60" reflector.

Whereas I generally agree that David is correct in recommending that nebula filters be used at lower powers, there are many specific nebulae in which the combination of aperture and object brightness allows much higher powers to be used successfully with Narrowband and line filters.

The last time I was out, my best overall view of the Dumbbell nebula (M27) was at 140X with a UHC filter, and that was 11.2X per inch in my scope. There were also some small planetaries in Cepheus that stood out from the field at 203X (16X/inch) with an O-III filter that were essentially invisible at lower powers.

So David's "Rule" isn't hard and fast. Experimentation is desirable. Chances are you'll agree about low powers being best on most objects.

[David Knisely]

The rule that nebula filters should all be used at less than 10X per inch is a nod to the realities of the apertures used by most amateurs.

I have experiential reasons to know that nebula filters can be used and have positive effects at magnifications higher than this IF the light loss that comes from magnification does not reduce the brightness of the nebula significantly.

I've seen the difference a filter makes in the appearance of the Saturn Nebula (NGC7009) at both 750X and 1050X in a 28" reflector, and the effect an O-III filter had on NGC1514 at nearly 1000X in a 60" reflector.

Whereas I generally agree that David is correct in recommending that nebula filters be used at lower powers, there are many specific nebulae in which the combination of aperture and object brightness allows much higher powers to be used successfully with Narrowband and line filters.

The last time I was out, my best overall view of the Dumbbell nebula (M27) was at 140X with a UHC filter, and that was 11.2X per inch in my scope. There were also some small planetaries in Cepheus that stood out from the field at 203X (16X/inch) with an O-III filter that were essentially invisible at lower powers.

So David's "Rule" isn't hard and fast. Experimentation is desirable. Chances are you'll agree about low powers being best on most objects.

Yes, it isn't a hard and fast rule that you should always use low power, but much of the time (and *especially* for those just starting out with filters), it is the single best filter use "guideline" to follow. I note that for many diffuse nebulae, using much over 6x per inch may start to make them more difficult to see using certain filters over others, so the difference tends to show up the higher you go with power. I have successfully used the OIII and UHC filters at some rather high powers, but in many cases, the changes seen didn't really add that much unless one has some *serious* aperture to play with. For example, on the Eskimo, the area between the inner and outer shells tends to "fill-in" with the OIII filter, so the outer shell appears somewhat less distinct than it does without the filter. I recall one gentleman who at the Nebraska Star Party was trying and failing to find the Veil in his 8 inch NexStar with an OIII filter. I looked in and he had the Veil running right down through the field of view, but only a section of it, as he was running a little too much power for a proper view (and he wasn't experienced enough to really know how to use averted vision). Once he got over to an 6 inch f/4.5 running the OIII at a much lower power, he saw the Veil immediately. Clear skies to you.

[LateViewer]

Excellent review.

Ordered a 2" NFB today.

[Sgt]

Anyone know if this is available in the UK? Thanks.

[Mikael Joe]

Does anyone know where i can get the 2" version of the NPB Nebula filter?

[Starman1]

http://www.omegafilt...vlet/StoreFront