I purchased an Orion Deep Sky filter in preference to a Lunicon UHC following reading some reviews, and I am very happy with it. I am looking for some similar view on OIII filters.

I can across this site, which provides an interesting comparison between the OIII filters available from various suppliers:

http://www.astroamateur.de/filter/oiii.html

Other than concluding that the Custom Scientific is the "purest"filter (ie you probably wouldn't see any stars at all), they all appear comparable, despite the differences in price.

Anyone have any view on the relative merits of various OIII filters? I suspect that the differences are probably very subtle, and specific to the individual.

any comments?

Phil Harrington wrote a nice piece on filters in the August '05 edition of Astronomy (Secret Weapons, p.82). OIII filters from Astronomik, Baader, Lumicon, Meade, TeleVue, and Thousand Oaks were tested and results compared. They all scored very similar marks. Harrington writes:

His only caveat came in noting the Lumicon offering darkened the field of view to a larger degree than the others.

More experienced observers than I might weigh in with their two cents, but I'd imagine it pretty hard to go wrong with any of the above. I've been quite happy thus far with my TeleVue Bandmate OIII.

Well, I'm afraid that that article was flawed in a number of ways, so I would take Harrington's comments with a huge grain of salt. The Lumicon OIII probably made the view darker because it was designed to yield as narrow a passband width as possible without excluding either of the OIII lines. For an OIII, this is the best condition possible as you want to exclude *everything* but the OIII lines so that no background skyglow interferes. This yields the highest possible contrast for those nebular objects which emit primarily in the Oxygen III lines. The TeleVue Bandmate OIII is not a true OIII, as it has a much broader bandwidth than either the Lumicon or the Astronomik models (both of which I would highly recommend). The Bandmate might be more similar to the narrow-band UHC filter, but it probably won't yield the contrast which the narrower OIII line filters do. Clear skies to you.

__________________
David W. Knisely KA0CZC@navix.net
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

It's important to consider what kind of telescope you have when choosing a filter. A big reflector has more light to work with, so you can use a filter with a narrower passband. The Televue, not surprisingly, is designed with small refractors in mind, and so lets more light through.

Actually, a line filter like the OIII *can* be used quite successfully on a relatively small aperture *provided* that the right power range is used (3.5x to 9.9x per inch of aperture). I have used my 2" OIII stuck in the front end of my 9x50 RACI Orion finderscope to see both sides of the Veil Nebula at the same time. My best view of the North America Nebula comes with my OIII and my four inch f/6 refractor at around 20x or so. Even the much maligned H-Beta filter can be used this way in my 4 inch to get a great view of the California Nebula, the Seagull Nebula, or segments of Barnard's loop. Indeed, at my dark sky site, I have just held up my H-Beta to my eye and seen much of Barnard's loop as well as the large faint diffuse nebulosity surrounding Lambda Orionis, and even the California Nebula, all without any other optical aid. If one wants to use a broader bandwidth for smaller scopes, I would recommend the Lumicon or Astronomik UHC filters or the DGM Optics NPB filter. However, the Tele Vue Bandmate should not have been given any association with the OIII designation, as it clearly lets through too wide a range of wavelengths to be an effective OIII filter. Clear skies to you.

__________________
David W. Knisely KA0CZC@navix.net
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

I'll second what David said about low magnifications, small apertures, and OIII and H-Beta filters. I use a Lumicon OIII (or Orion UltraBlock) with my 80mm f/5 Orion ShortTube 80 and 101mm f/5.4 Tele Vue refractor rather often. I also view the California Nebula regularly with the 101mm using a friend's 2" Lumicon H-Beta filter.

Dave Mitsky

__________________
Chance favors the prepared mind.
De gustibus non est disputandum.
Never attribute to malice that which can be adequately explained by stupidity.

David,

Thanks for the caveat, it's much appreciated.

Is there something in print you could point the readers of this discussion toward, or (not meaning to put you on the spot), describe why Harrington's article is flawed? I'd be most curious to better understand the subject in question as it's one that comes up from time to time, and would gladly study up on anything you could provide.

Thanks again in advance.

Well,this is going to get a little long I'm afraid. The article isn't exactly horrid by any means (and is somewhat useful to those who have never used filters), but it does have a few problems, primarily with the comparison of the various filters. Although many of the filters of similar design get similar ratings in his chart on page 83, some of his results and his comments are puzzling when he compares specific filters. Basically, it mostly comes down to a lack of specifics. First, despite Harrington's statement to the contrary, a few of the major filter producers were excluded from the article (Sirius Optics is one which comes to mind). Second, some of his statements in his article do not correspond to the results obtained by a large number of experienced observers who have been using these filters for many years. One example on p. 84 reads, "Although broadband filters make their biggest impact under severe light pollution, narrowband and line filters enhance the view of emission and planetary nebulae no matter where you are." In reality, broadband filters can get somewhat saturated under severe light pollution, rendering them considerably less effective. They tend to work better under mild light pollution or even at a fairly dark sky site, as they kill off a lot of the mild residual skyglow. Indeed, on galaxies, they only help slightly and even then, the benefit mainly comes with larger more diffuse galaxies under somewhat darker sky conditions where the fainter detail may have been only be slightly compromised by the weak skyglow. As for using nebula filters under dark skies, they tend to perform *better* under dark skies, and may not seem to perform quite as well in severe light pollution at least partially because of the difficulty of the observer in getting and staying properly dark adapted.

In his numerical rating of the Lumicon H-Beta filter, he rates the performance on M42 as a "5", which is questionable, as the nebula appears both larger in the extent of nebulosity visible as well as somewhat brighter in either the UHC or the OIII filters. Indeed, the huge outer loop that extends southward from M42 and is visible with the UHC filter almost vanishes in the H-Beta filter. Using a similar rating scale from zero to five, I rated the H-Beta merely a "3" when used on M42 for this reason. On the comma-shaped M43, the H-Beta is indeed a bit better choice, but this is *not* M42. This clearly calls into question what specific factors which Harrington may have been using to make his judgements.

By contrast, for my comparisons, I have three specific items I generally look for: 1. the extent of the nebulosity (i.e. how much is visible, and over how wide an area it is seen), 2. the brightness of the nebulosity, and 3. the overall contrast of any detail that is seen. From this, I find that generally on diffuse emission nebulae, a UHC-style "narrowband" filter usually will show a somewhat larger area of nebulosity and often at a slightly higher brightness level than the OIII "line" filter, but the OIII will often provide a greater level of contrast in what detail is visible, along with showing dark detail more prominently. Neither filter may necessarily be "better" in this case depending on the object and what the user wants to see, so it is important to state what factors are being used in judging various filters. Harrington does not appear to do this in a clear manner.

Second, the methodology of the "comparison" may have been a bit flawed in the way one filter needs to be compared with another. Specifications can tell a lot about the way a filter will behave, but no specifications of significant usefulness were really provided (i.e. maximum in-band transmission percentage, full width at half maximum bandwidth, secondary passbands or out-of-passband transmission, etc.). Even a quick look using a hand held spectroscope and a broad band light source can be revealing when comparing filters. Often, differences in filter performance come from slightly different specifications. The Orion Ultrablock, for example, has a slightly narrower and more rounded passband than the Lumicon UHC, but it does not have quite the total light through put that the UHC does. Thus, the UHC tends to show just a bit larger area of nebulosity at a slightly higher brightness level. However, the overall difference in performance between these two filters is very slight, and when asked about them, I can easily recommend either one (or fall back on my best advice which is "let the price make the decision"). With the OIII filter, Harrington states that, "The Lumicon filter darkened the field of view more than the rest.", which might mean something good or something bad (he neglects to say which). More than likely, the Lumicon OIII's passband width was narrow enough to kill off the skyglow properly as it should. When I want to see OIII, I don't want to see *any* significant skyglow, so a darker sky may mean better contrast. Here, it would have been nice to be able to see how much various filters resembled each other in their optical transmission characteristics.

In addition, the way filters are compared and, more importantly, the choice of *objects* used to do the comparison are critical. For example, Harrington apparently tested the narrow-band and line nebula filters on objects which the filters were *never* designed to work on, like the open cluster M37 or the spiral galaxy M81 (as a rule, only the broad-band filters tend to be the only ones to help these objects and even then, the improvement is often marginal at best). Nebula filters like these are almost universally known to hurt objects other than nebulae, so I can't understand why he rated them on non-nebular objects in his chart on page 83. Harrington did not also provide enough detail about exactly how he did his comparison. Did he have some sort of slide to provide instant comparison? Did he just hold two filters up over the eyepiece? Did he just rely on his memory of how things looked and take time to replace one filter with another? What f/ratio was being used? How much power was being used? Each of these items has its own set of problems which can easily affect how filters are judged comparatively. When I compare filters, I put them in my Lumicon multi-filter selector so I can do an instant "A vs B" comparison, and I use a power which close to the middle of the "optimal power range" for nebula filters (3.5x to 9.9x per inch of aperture). I also do testing using at least two different telescopes so that larger objects can be better compared in a lower-power wider-field instrument, as well as in a somewhat higher power setting in a larger instrument.

The greatest benefit of these filters on nebulae comes with their ability to enhance their faintest detail, so it is critically important in *comparatively* rating these filters against each other to be fully dark adapted and observing under dark-sky conditions so that one is viewing where this fainter detail might be visible (*not* in a light polluted backyard with a 5th magnitude limit as Harrington did). In urban settings, it is difficult to get dark adapted enough to get the full benefit of nebula filters, which is one reason some inexperienced urban observers sometimes don't report huge benefits from filter use. Under the dark sky, (ZLM 6.0 or fainter), the differences in filter performance become much more noticable and less subjective.

The *number* of objects used for Harrington's comparison may have been somewhat limited, at least from what his chart shows. Different objects are helped to different degrees by different filters. I did an extended survey of 90 bright emission and planetary nebulae over a 2-year period with the entire Lumicon filter line to judge which objects worked best with which filters. The differences I uncovered in this survey were amazing and showed how some common beliefs about use of a particular filter type or brand on certain objects are often inaccurate. This sets a baseline for object selection. The selection of which filter to use and how it improves the object is *very* important when comparing different filters. Harrington probably should have used *at least* six well-chosen diffuse emission nebulae and six *large* planetary nebulae for the comparison. It is not all that clear whether he did this kind of extensive testing or not. Instead, he lists the results for one diffuse nebula (M42), one supernova remnant (M1), four planetary nebulae, and a cluster and a galaxy. I would have probably used M42, M8, M20, M16, M17, NGC 2237, and NGC 6992 for my emission nebulae choices, and M27, M57, M76, NGC 7293, and NGC 1360 for the planetary nebulae, omitting any other non-nebular objects except when rating the broadband filters against each other. Harrington's choice of M1 was not exactly a good one, as this object emits much of its light in a broadband "synchrotron radiation" mode (probably best seen in narrowband filters except for when you want to see the faint filaments, where it is best to use an OIII filter and a large aperture). If a supernova remnant was desired, the Veil would have been a much better choice.

Again, the article isn't useless, but Harrington could have done a better job if he had just taken a little more time and dug in a bit more deeply. Clear skies to you.

__________________
David W. Knisely KA0CZC@navix.net
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/