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OIII Filter Choice

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#51 David Knisely

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Posted 24 September 2012 - 12:51 AM


For the Owl, a 'true' OIII filter like the Lumicon or Thousand Oaks would probably be best. The NPB will help as well, but perhaps with not quite as much contrast as the narrower OIII filters. As for Tele Vue's Bandmate OIII, that one is *WAY* too wide (nearly as wide as the NPB), so I would pass on getting that filter. As for "100% transmission", no commonly manufactured filter has that despite whatever passband plots you will see. The most one might expect for most production filters like these is in the high 90's (perhaps 97% or so at one wavelength). In any case, the human eye cannot easily detect a change in brightness of less than 10%, so if an OIII filter has a transmission of around 88% or more at one or both of the OIII lines (and has a Full Width at Half Maximum bandwidth of less than 15 nm but more than 8.5 nm), it should work just fine for visual use. Clear skies to you.


David,
I think I am finally catching on!
Bandpass means alot also :)
I appreciate the info as you have had practical and in the field experience with several filters.
But, can you give me a quick definition of FWHM ?
Does it have something to do with the 496 nm and 501 nm OIII lines? Which have a difference of 5 nm.
I am confused about the 8.5 nm and 15 nm wavelength positions.

Thanks


FWHM is the full width of the filter at half of its maximum transmission level. Most narrow-band nebula filters have FWHM values in the 23 nm to 28 nm range, while good OIII line filters will have FWHM values of between 8.5 nm and about 15 nm. Generally, the filter passband of an OIII filter will be centered at a point in the visual spectrum that is roughly halfway between the 495.9 and 500.7 nm wavelengths with a peak transmission at that point. There is some slight fall-off in transmission away from that peak transmission point, but most line filters have more than 88% transmission at the two OIII lines, so there is little to worry about there. The Baader OIII is a little on the narrow side (8.5 nm FWHM passband width), as the 495.9 nm OIII line is transmitted at only between 35% and 50%, while the other line comes through at over 90%. However, typically, the 495.9 nm OIII line is weaker than the 500.7 nm line, so the Baader filter still works fairly well. Clear skies to you.

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#52 rick-SeMI

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Posted 24 September 2012 - 10:49 AM

FWHM is the full width of the filter at half of its maximum transmission level. Most narrow-band nebula filters have FWHM values in the 23 nm to 28 nm range, while good OIII line filters will have FWHM values of between 8.5 nm and about 15 nm. Generally, the filter passband of an OIII filter will be centered at a point in the visual spectrum that is roughly halfway between the 495.9 and 500.7 nm wavelengths with a peak transmission at that point. There is some slight fall-off in transmission away from that peak transmission point, but most line filters have more than 88% transmission at the two OIII lines, so there is little to worry about there. The Baader OIII is a little on the narrow side (8.5 nm FWHM passband width), as the 495.9 nm OIII line is transmitted at only between 35% and 50%, while the other line comes through at over 90%. However, typically, the 495.9 nm OIII line is weaker than the 500.7 nm line, so the Baader filter still works fairly well. Clear skies to you.


David,
Thank You.
Excellant explanation and graphic.
I believe I now understand OIII filters much much better from when this thread was started.
These light filters act much the same as SSB and CW filters :)
Of course with these filters, in the newer rigs, you can vary them at will.
Wonder when the astro filters will become tunable, such as those in a few of the solar scopes ?

TNX
73, Rick - N8XI

#53 Starman1

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Posted 24 September 2012 - 12:14 PM

One further note: the transmission bandwidth moves in the spectrum as the angle of light incidence changes. The light transmission FWHM figures are for light entering the filter perpendicular to the glass. The bottom and top wavelengths change slightly with small angle changes.
So if you are looking at an object 1 degree wide, the edges of the object do not see exactly the same "window" of wavelengths the center of the object does.
So:
--to examine details, bring the target to the center of the field

Some very inexpensive filters' housings can result in slight tilts in the glass in the housing. That will impact the bandpass.

Because these are basically interference filters, off-axis light may see a completely different spectrum of transmission. You can see that will some narrowband filters which appear blue-green when looking straight through, and pink when looking through at an oblique angle.

If you have a scope with a truly wide true field (like 4-5 degrees), this can actually have an impact on what you see in the field in different parts of the field.

This is a logical reason why the bandwidth of a filter has to be wider than the minimum necessary to be sure to transmit the desired wavelength everywhere in the field.
Otherwise, a 15nm bandpass could transmit H-Beta, and both O-III lines. But, most narrowband filters have bandpasses more like 25nm to be sure slight off-axis views won't get "clipped".

#54 Scanning4Comets

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Posted 29 January 2013 - 01:54 PM

After reflecting on this subject and owning the Celestron, (Baader OIII), and Lumicon OIII filters, I have come to the conclusion that the Lumicon is the way to go for the OIII filter of choice.

The Celestron / Baader OIII is indeed too dark at 8.5nm.

Cheers,

#55 photiost

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Posted 01 February 2013 - 11:51 PM


Here is a site that has some of the current OIII filters transmission curves:

http://www.astroamat...ilter/oiii.html

I still like the Lumicon OIII (if you can get one), but the DGM Optics OIII or Thousand Oaks OIII are pretty good as well. Clear skies to you.


I recently got the Burgess Optical (2in) OII filter as part of a deal. The previous owner could not tell me more than he bought it from Burgess about 4 yrs ago.

Any comments/info on this filter is appreciated.
.

#56 Scanning4Comets

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Posted 15 February 2013 - 02:40 AM

My comment two above will now seem somewhat hypocritical because I just purchased yet "another" O-III filter like my last one. My last one was a Celestron and this one is a Baader O-III.

I wanted to try the Baader to see how much alike they really are! There was also a DGM O-III I just missed out on as well. David Knisely: What is the DGM O-III like?

Cheers,

#57 Ava

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Posted 15 February 2013 - 04:32 AM

I have the DGM O-III filter, bought new around 6 months ago. Unfortunately skies have been pretty bad since october basically so I have not used it a lot under the stars. It does give good sharp views, it is certainly a lot narrower than my Astronomik UHC (that one is rather wide) and hightens contarast well for some objects. If I remember correctly, it should be a few nm wider than the Lumicon O-III (around 14-15nm).

#58 David Knisely

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Posted 15 February 2013 - 01:52 PM

My comment two above will now seem somewhat hypocritical because I just purchased yet "another" O-III filter like my last one. My last one was a Celestron and this one is a Baader O-III.

I wanted to try the Baader to see how much alike they really are! There was also a DGM O-III I just missed out on as well. David Knisely: What is the DGM O-III like?

Cheers,


The one I got for review here on Cloudynights is actually a tad wider than my Lumicon OIII:

CN REPORTS: DGM Optics OIII Filter

Otherwise, it works fairly well. Clear skies to you.

#59 Starman1

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Posted 15 February 2013 - 05:48 PM

David,
Since you are still logged in to this thread, you might be interested in my notes from last Saturday night in which I viewed NGC2359, "Thor's helmet",
with both the Lumicon O-III and TeleVue O-III filters.

As you would expect, the narrower bandwidth of the Lumicon displayed more nebulosity--3 distinct "tails" away from the central bubble, striae within the bubble, and some 'clouds' away from the main body a bit. A very spectacular view of the nebula.

The TeleVue filter did not show the level of contrast--the background was lighter, and the 3rd extension from the central bubble was hard, and the outer 'cloud' of nebulosity wasn't seen.

BUT, the number of stars in the field of view with the TeleVue filter was similar to the non-filter view--very rich with stars.

Esthetically, I couldn't say which I preferred the most. The 12.5", under very dark skies (SQM21.7) showed a lot of stars in the field even with the Lumicon filter. But the overall impression of the field with the TeleVue filter was most impressive, even if the very faintest nebular details weren't as obvious. And the TeleVue also showed M46 and its associated planetary better than the Lumicon (the stars were a lot brighter).

Comparing a lot of filters recently on a number of nebulae, I'm coming to realize that there is room for different filters to be used on different objects.
M42, for instance, was far more colorful to me with a Baader UHC-S than with the Lumicon UHC. The red wavelengths transmitted and the slightly broader main passband of the Baader really matched well with that object.
On M27, though, the Lumicon was definitely preferred, revealing ropy tendrils in the outer nebulosity far better than the Baader.

I think it is up to guys like you and me to point out the bandwidth issues of the filters that are out there, but also to ask what it is the observer is hoping to accomplish. Not everyone is going to prefer exactly the same filter choice under all conditions and on all nebulae.

#60 Scanning4Comets

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Posted 15 February 2013 - 07:05 PM

What about this filter ? :

Omega O-III Nebular filter # 144918

#61 Starman1

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Posted 15 February 2013 - 07:56 PM

Markus,
I think David linked to a review of that very filter a couple posts ago.
DGM filters come from Omega.

#62 Jon Isaacs

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Posted 15 February 2013 - 08:03 PM

I think it is up to guys like you and me to point out the bandwidth issues of the filters that are out there, but also to ask what it is the observer is hoping to accomplish. Not everyone is going to prefer exactly the same filter choice under all conditions and on all nebulae.



Don:

I use an O-III filter when I want to maximize the nebulosity, the Orion Ultrablock when I want to balance the nebulosity and the starfield. My one nighter with the TV O-III leads me to believe the TV filter is more of a UHC filter than an O-III filter.

Jon

#63 David Knisely

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Posted 16 February 2013 - 12:53 AM


I think it is up to guys like you and me to point out the bandwidth issues of the filters that are out there, but also to ask what it is the observer is hoping to accomplish. Not everyone is going to prefer exactly the same filter choice under all conditions and on all nebulae.



Don:

I use an O-III filter when I want to maximize the nebulosity, the Orion Ultrablock when I want to balance the nebulosity and the starfield. My one nighter with the TV O-III leads me to believe the TV filter is more of a UHC filter than an O-III filter.

Jon


The Tele Vue is a narrow-band filter in terms of raw FHWM bandwidth, but the location of that passband is centered on the OIII lines and not between the OIII and H-Beta lines as is done with a true narrow-band nebula filter. I still prefer the DGM NPB to the Tele Vue OIII, as it has a similar bandwidth, sharper edges, and, in addition to passing the OIII lines, lets through the H-Beta line at a high level of transmission rather than excluding it as the Bandmate "OIII" does. For a true Oxygen III nebula line filter, I would definitely prefer the Lumicon OIII or Thousand Oaks LP-3 OIII filter. Clear skies to you.

#64 David Knisely

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Posted 16 February 2013 - 01:17 AM

David,
Since you are still logged in to this thread, you might be interested in my notes from last Saturday night in which I viewed NGC2359, "Thor's helmet",
with both the Lumicon O-III and TeleVue O-III filters.

As you would expect, the narrower bandwidth of the Lumicon displayed more nebulosity--3 distinct "tails" away from the central bubble, striae within the bubble, and some 'clouds' away from the main body a bit. A very spectacular view of the nebula.

The TeleVue filter did not show the level of contrast--the background was lighter, and the 3rd extension from the central bubble was hard, and the outer 'cloud' of nebulosity wasn't seen.

BUT, the number of stars in the field of view with the TeleVue filter was similar to the non-filter view--very rich with stars.

Esthetically, I couldn't say which I preferred the most. The 12.5", under very dark skies (SQM21.7) showed a lot of stars in the field even with the Lumicon filter. But the overall impression of the field with the TeleVue filter was most impressive, even if the very faintest nebular details weren't as obvious. And the TeleVue also showed M46 and its associated planetary better than the Lumicon (the stars were a lot brighter).

Comparing a lot of filters recently on a number of nebulae, I'm coming to realize that there is room for different filters to be used on different objects.
M42, for instance, was far more colorful to me with a Baader UHC-S than with the Lumicon UHC. The red wavelengths transmitted and the slightly broader main passband of the Baader really matched well with that object.
On M27, though, the Lumicon was definitely preferred, revealing ropy tendrils in the outer nebulosity far better than the Baader.

I think it is up to guys like you and me to point out the bandwidth issues of the filters that are out there, but also to ask what it is the observer is hoping to accomplish. Not everyone is going to prefer exactly the same filter choice under all conditions and on all nebulae.


If I want to see stars with a little nebulosity, I will use a good broad-band LPR filter like the Orion Skyglow. It is a tad narrower than the Lumicon Deep-sky and will provide a bit more contrast in emission nebulae without hurting the stars much at all. In fact, I prefer that filter for reflection nebulae like the Merope Nebula or the Iris Nebula. However, for emission nebulae, I want to see nebulosity and not just stars. For that, clearly, a narrower filter is needed. The Tele Vue OIII has the narrower passband (about 240 angstroms FWHM) but it isn't centered properly. Its transmission of the H-Beta line is only 28.8%, and the profile of the passband is more of a Gaussian form than something sharp. The DGM NPB has about the same FWHM passband width, so its effect on stars should be similar to that of the Tele Vue OIII. However, its transmission at both the OIII and H-Beta lines is around 94%, so it will enhance nebulosity that happens to have either a balance of OIII and H-Beta or just H-Beta. For my money, as a single purchase, the DGM NPB is a much more effective filter than the Tele Vue Bandmate OIII. In the case of true Oxygen III filters, stars are again not an issue. When making the choice to use an OIII filter, you are going after the maximum contrast, so you want a filter that lets through only the OIII lines and little else. Indeed, when under some skyglow, on some nebulae like the Dumbell, I will often switch to an OIII rather than use my narrow-band filters just to be able to see the object with more contrast. For that, again, the Tele Vue Bandmate OIII is an also-ran. It is just too much of a compromise in design. Clear skies to you.






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