Filters

Filter Performance Comparisons For Some Common Nebulae – By David Knisely

Some Available Light Pollution And Narrow-Band Filters – By David Knisely

Specific filter passband information – By André Knöfel

Using Eyepiece Nebular Filters for Deep Sky Viewing – By Stephen R. Waldee

The Magic Horse Head Eyepiece – By Barbara Wilson. She explains how to determine the best eyepiece to use with your filter to observe the Horsehead Nebula.

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~ by Erika Rix on December 13, 2012.

3 Responses to “Filters”

  1. I have provided a number of other links to very worthwhile articles on eyepiece nebular filters and their technical performance, at the end in the “References and Web links” section of my own article on the topic: direct link to them here:
    http://freescruz.com/~4cygni/horsehead/NebulaFilters.htm#15-REFERENCES

    The main article itself, starting from the top section, does cover some other significant topics about using filters, and it has quite a few illustrations and photos.

    I admire Mr. Knisely’s dedication and effort in doing the decade-plus-long research he has compiled into his list of recommendations for many specific emission nebulae.

    Yet it must be added that I did a 4-person double-blind test of the same kinds of filters in 1989, 90 at an observing site that is more than twice the altitude of Knisely’s venues, overlooking the Pacific ocean from the Santa Cruz mountains, and in skies that ranged in NELM from better than 6th, to a bit over 7th, magnitude, my group of observers, using multiple instruments, did NOT come to the same conclusions of specific object-filter choice as Mr. Knisely’s tests recommend. I was working then for one large US astronomical products company, doing equipment tests, writing catalogue articles and instructions, and helping develop new products.

    When his article become widely distributed after the mid-2000s, I revisited our double blind test series and then also tried to see if I personally would frequently agree with Mr. Knisely’s suggestions (I have been a telescope observer since 1957, using scopes of 10 inches and larger aperture for the deep sky since 1975.) . The earlier versions of his article did not often specify the instrument used; my tests were done in the last few years with apertures from 2 to 11 inches, and in 1989 with scopes up to 17.5 inches. VERY rarely, with any of these, could I be absolutely convinced that I agreed with Mr. Knisely–there certainly are personal differences of taste, and individual preferences and goals, that must be acknowledged.

    But, even in my 4-party controlled double-blind test, rarely could I derive an absolute consensus of ‘which filter to use, or which is best’. Often two parties would cancel out the other two observers. Virtually never did all four agree exactly on the same preference of filter, and rather often we’d get one very convinced observer disagreeing with the other 3, who were equally convinced.

    Ed Ting has also conducted tests and concluded that you cannot really extrapolate from ONE observer and then decide what’s best for everybody.

    Furthermore, objects look different in large or small apertures, a factor not really dealt with by Mr. Knisely; often his results are given for ONE specific aperture–and then, not by any means the now-standard large Dobs of 15-18 inches diameter that are very common now.

    This does not mean that I think Knisely is wrong–for him. Or wrong for his circumstances, scope, and rationalizations, explanations, and preferences! I do appreciate his attempt to systematize this, but since the tests he did cover numerous years, at differing sites, with differing instruments (and filters you cannot purchase brand new NOW, as they were made in Japan in the 1980s), I do assert that one has to use some judgment in deciding how to employ his recommendations–and to adjust them to one’s own set of conditions, after perhaps doing some specific comparisons.

    Unfortunately, one of the CN moderators basically censored and banned me when I disagreed with Mr. Knisely on another topic (faintest stars detectable by eye); so I cannot present my demur in that forum. But, it’s a respectful disagreement, and is influenced by my years of working professionally in the sales, development, testing, and marketing of such gear (and by my personal benefit of having worked directly with Dr. Jack Marling of Lumicon: especially on a computer program that made calculations of object visibility using his filters, and employing his technical specifications and empirical user-derived recommendations.)

    Now, I am retired from any such business and so feel free to comment pro/con about any type or brand. The problem with forums that are hosted by telescope sellers is that one always wonders, a bit, about editorial or moderator policies and how they are influenced by the marketing goals of the company paying for the forum! Or, one may have a concern with “reviewers” who get free products, or loans: for the purpose of engendering commentaries. For the past 15 years or so, I’ve been “forced” to spend full retail bucks for all the astro gear that I own and use. And, having paid for it, I might be just the slightest bit more anxious to express discomfort if a product costing me a lot of money doesn’t live up to expectations.

    However, I have not attempted to critique filter BRANDS against one another. This is very hard to do — maybe nearly impossible, with scientific precision and repeatability, in a one party visual test. It’s perhaps better done with instruments first, and then followed by an attempt to verify (or at least understand) the performance as seen by eye, at night, in a scope.

    I do very much appreciate, therefore, Mr. Knisely’s many filter reviews and comparisons, which I’ve linked to often in my own articles. He is certainly more up to date than I am, having tried more brands and newer models.

    But, to reiterate my point of view: THERE ARE MULTITUDES OF VARIABLES in analyzing filters, and judging visual performance on specific nebulae. I think that one article by one person, cannot possibly hope to cover the insights and experiences that many OTHER careful and intelligent observers can express. There have been some good articles in S&T and Astronomy, and recently in some web review ‘zines. I have made a conscious attempt to cover some small peculiariaties of filters that few others have documented. Mr. Knisely has done a massive collection of specific one-party, non-blind (arguably not fully controlled, but no doubt VERY careful and well-expressed) tests. Other web authors like Rainer Vogel, Carlos Aguya, Maurice Gavin,and Jim Thompson, have done very worthwhile work of wide diversity of approach.

    Furthermore, though Mr. Knisely reports less than 100 nebulae in his study, I’ve collected *several hundred* reports, linked in my article, that show how specific filters might help, or hurt, the view of both emission AND reflection nebulae–as well as other objects.

    So, Erika: I do hope that you will occasionally augment your very useful list, above, with some LATER web references.

    Congratulations on your fine website, and excellent astro-art.

    Sincerely yours,
    Steve Waldee
    San Jose, CA.

  2. Thank you for your reply and for the additional information. As with all equipment, there are many variables that come into play and reviews are used as guidelines. 😉

  3. Erica:

    Thanks for including the link to my article on your list; but be advised that my entire astro website was taken down around the end of 2015! Due to a confluence of factors that made it impossible to continue to maintain over 2 GB of data (including my leaving Calif. and the bay area club that hosted my website) I had vague intentions of reorganizing it from my new living and observing space in SW Utah. For some reason, your link to my OLD, offline page now redirects to André Knöfel’s article listed immediately above mine.

    I have found that an old “interim” copy of my article is still available on the Internet Archive; but unfortunately many of the necessary graphics are gone–and all of the valuable material I added in the following year is of course missing; furthermore, the links to my hundreds of observations of nebulae using filters refer only to my now-missing website and most don’t come up via the Archive. It’s almost pointless to include that unsatisfactory version so I won’t specify the Wayback Machine URL. This is, in fact, the only time I’ve explained publicly why my site went down, and the sole time I intend to do it.

    Sadly, at the end of 2015 I found that the counters on my site showed next to NO visitors! Even a hidden counter on the filter article showed no redirections, Erika, from your blog link! So, apparently nobody reading your article bothered to click on it. Furthermore: my gigantic series of articles on the Horsehead Nebula and its complete history had only ONE single viewer hit during the months of October and November, 2015! This is *most* discouraging–so at present I’ve decided that I shall not put in the effort to re-establish the site. Ergo, Erika: your own blog has one of only about a half-dozen currently extant web search links to my name and related astro-work; before I took down the site there were literally *hundreds* of links to my articles, all now excised–I’m a sort of “non-person”! (The upside of this is that now, nobody will write on CN that my website is “disconcerting” or “mlisleading” and “should be avoided”!)

    A final word about the specific opinions of the use of pertinent nebular filters for objects: I made a purchase a few months ago from one of the most prominent CN vendors and contributors, and as one item was a particular filter, I mentioned David Knisely’s tests of 93 nebulae and asked if the vendor — himself an avid and longstanding observer — quite agreed with it, or would line up somewhere nearer to my “deviant” experiences, in which I preferred nearly the opposite of what Knisely suggests, in several instances. I then received a sort of ironical laugh from the vendor, and the impression given to me, unmistakably, was that this particular (experienced and very sophisticated) amateur astronomer was not a devotee of Knisely’s general advice–though he wouldn’t say so on CN, nor elsewhere in public. So, without my specific critiques and analysis, comparing Knisely’s preferences with numerous instances of my alternative tests with other scopes and filters, we are left now with only David’s opinions plus some of the purely technical articles by other writers, dealing only with THE FILTERS and their bandpass and performance: not related to objects. For “what filter to use for object XYZ” we now have only Mr. Knisely’s no doubt very careful tests, in his lucid report. This is not unfortunate as, at the time he began his project, many amateurs hadn’t the *slightest* idea what specific visible wavelengths were transmitted by many nebulae, and most beginners hadn’t a clue about Balmer lines, bandpasses, and human dark-adapted visual frequency response. Knisely has helped fill that vacuum of ignorance over the years with his tireless, careful work…BUT I have to say that I am still convinced that it is not “the last and most comprehensive word”–at least as long as you cannot get close to universal agreement with his recommendations for specific objects and filters from all observers who are Knisely’s peers. (I have recounted dozens to hundreds of disagreements, from observers using scopes from 3 inches to 30+ inches aperture.)

    Indeed: in one memorable exchange of experiences of the Veil Nebula, in a CN discussion thread, I found that many apparently experienced observers disagreed with Knisely’s distinct, narrow choice. He prefers only the O-III; but many chose the UHC type or even felt it was a sort of toss-up, as those two filters give different kinds of representation of details in the nebular structure. I find no technical fault with Mr. Knisely’s method, but only question his lack of further tests with alternating scope types/apertures, other observers, and a wider range of exit pupils/magnifications.

    Of course I always recommend any and all of his articles, reviews, and reports and have enjoyed them for years–won’t miss them! He deserves high praise and respect, and gets this from his many enthusiastic followers.

    Yet, I feel that the holistic subject of “how a nebula looks in a telescope–filltered and unfiltered” is so gigantic, with so many (nearly countless) variables, that a simplistic reduction into one single article and list is giving the subject rather short shrift. So, Mr. Knisely’s approach is, I believe, best understood in a larger context, including all of his technical filter tests and reviews; his various telescope reviews (often involving test observations); his other astro articles; and his observing reports and commentaries. Ironically, sometimes — particularly over the passage of time — I’ve received quite a bit of opinion from him that more than subtly differs from his one article about testing 93 nebulae. So, I believe that his article–which chronicles more than a decade of observations with diverse (old) instruments of his–would benefit from a thorough revision and upgrade, made by using MODERN filters available now, which are demonstrably superior to old 1980’s first-gen Lumicons. Even the latest Lumicon nebular filters — some of which I’ve purchased in the last year — almost ‘blow away’ the very oldest ones Knisely employed, giving higher contrast and definition. And, Knisely now has better scopes, including a 14″ Dob with a superb Lockwood mirror, and a nice refractor and SCT: his experience viewing the same nebulae with *these* instruments should be added as an upgrade to his ‘antique’ observations with an old, and admittedly deficient, 10″ Coulter. Please, David: DO YOUR TEST OVER and give us all the benefit of the latest popular gear you use, and your increased experience and advanced knowledge!

    Sincerely,
    Steve Waldee

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