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... so is there any way of finding out if the filter bandwith actually has changed or whether the 12nm figure it mentions is just a webpage mistake?

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This is an often-quoted myth and is definitely *not* true. As I have stated in the past, I have used the OIII in apertures of 50mm and larger with good results. Indeed, one of my favorite views of the North America Nebula comes in my 100mm aperture f/6 refractor at 15x and 25x using the OIII filter, so it definitely does work in smaller scopes. The larger the aperture, the more objects (and detail) will be visible that the OIII will help with, but this does not mean that its use is limited to apertures larger than six inches. It would be nice if Astronomik would not continue to perpetuate this myth. Clear skies to you.

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turtle86 wrote:

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No sarcasm taken, but I was actually talking about the quality of the stars being seen, not the quantity. I used to have one of the older Lumicon OIII filters, but the stars were bloated or halated.

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Those were the very old laminated filters. Lumicon no longer uses that method of protection. In addition, Lumicon's OIII no longer has the "red leak" secondary passband, so the images of stars tend to be not terribly "bloated" or having red secondary star images anymore (at least mine doesn't). Clear skies to you.

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David--

About what year did Lumicon make the switch? My Lumicon O-III is late 80s/early 90s vintage, and it seems to have considerably degraded over time to where I can no longer get pinpoint stars through it.

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The point to be made here is that a slightly narrower bandwidth would enhance the contrast more, but at the expense of the stars. So which do you use the nebula filter for--nebulae, or views of stars? I'm not being sarcastic because there really is a difference in preference involved.

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Thanks for all the good info.
It helps me decide which OIII to choose.
But, if I were to use the waveform graphs in my post #5432706, I would choose, according to the graphs.
Using O-III Group A because the filters in Group B transmission characteristics bottom out around 90%.
1) Televue - because it allows 496 and 501 nm at 100% transmission and almost flat towards the violet and red spectrum.
2) Astronomik - 496 just below 97% and 501 99%
3) Lumicon - 496 at 99% but clips 501 around 90%

I have the Orion Ultra Block and DGM NPB (Nebula) filters.
The graphs for these two are similiar to a couple of the OIII filters

So, anyone want to take on the question of which would be better for M-97, the Owl Nebula ?

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This is an often-quoted myth and is definitely *not* true. As I have stated in the past, I have used the OIII in apertures of 50mm and larger with good results. Indeed, one of my favorite views of the North America Nebula comes in my 100mm aperture f/6 refractor at 15x and 25x using the OIII filter, so it definitely does work in smaller scopes. The larger the aperture, the more objects (and detail) will be visible that the OIII will help with, but this does not mean that its use is limited to apertures larger than six inches. It would be nice if Astronomik would not continue to perpetuate this myth. Clear skies to you.

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Same here. I have an older (late '90's) Lumicon OIII. With my 105mm f6 refractor and an appropriate eyepiece I get a gorgeous view of the Veil.

David, how do the newer Lumicons compare to an older one like mine? The label on the back of the plastic case says:

<<< PREMIUM
Nebula Line Transmission %
Oxygen - III 496nm 94
Oxygen - III 501nm 95
Hydrogen-Beta 486nm 0.3 >>

The label is pre-printed with the exception of the transmission % numbers at each bandwidth and the word Premium which are hand written in ink. I assume they tested each filter and sold as premium those with the better actual measurements. My Deep Sky is similarly labeled with an additional line for transmsision at 491nm handwritten in.

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The newer ones are a little better than those sold in the late 1980's and into the 1990's that still used the laminated protection. The 2" one I have was purchased in the mid 2000's and has fairly good star image quality and no significant secondary passbands outside of the primary one (transmission values of around 94% to 96% depending on which OIII line you are looking at). Clear skies to you.

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This is an often-quoted myth and is definitely *not* true. As I have stated in the past, I have used the OIII in apertures of 50mm and larger with good results. Indeed, one of my favorite views of the North America Nebula comes in my 100mm aperture f/6 refractor at 15x and 25x using the OIII filter, so it definitely does work in smaller scopes. The larger the aperture, the more objects (and detail) will be visible that the OIII will help with, but this does not mean that its use is limited to apertures larger than six inches. It would be nice if Astronomik would not continue to perpetuate this myth. Clear skies to you.

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Same here. I have an older (late '90's) Lumicon OIII. With my 105mm f6 refractor and an appropriate eyepiece I get a gorgeous view of the Veil.

David, how do the newer Lumicons compare to an older one like mine? The label on the back of the plastic case says:

<<< PREMIUM
Nebula Line Transmission %
Oxygen - III 496nm 94
Oxygen - III 501nm 95
Hydrogen-Beta 486nm 0.3 >>

The label is pre-printed with the exception of the transmission % numbers at each bandwidth and the word Premium which are hand written in ink. I assume they tested each filter and sold as premium those with the better actual measurements. My Deep Sky is similarly labeled with an additional line for transmsision at 491nm handwritten in.

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The newer ones are a little better than those sold in the late 1980's and into the 1990's that still used the laminated protection. The 2" one I have was purchased in the mid 2000's and has fairly good star image quality and no significant secondary passbands outside of the primary one (transmission values of around 94% to 96% depending on which OIII line you are looking at). Clear skies to you.

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Thx.

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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.

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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

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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.

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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

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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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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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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

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

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

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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,

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