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David Knisely
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Reged: 04/19/04

Loc: southeastern Nebraska
Re: Forbidden Lines: Mini [OIII] Filter Shootout new [Re: Shaky]
      #3500342 - 12/14/09 12:47 AM

Quote:

Gene,

Thanks for sharing your experiences with Baader. What impressed most about Baader was the baseline as seen in my post above. I looked at the German report also and saw that my results were in line with what they reported. For that matter, even the Televue peak near 700nm and red noise in Lumicon/spectral shapes are similar to what I obtained in my experiments.

I am not sure if the Baader spectral shift would be a whopping 32A (3.2nm for metric lovers) in real world. Even with the lower transmission, Baader OIII should be an excellent AP filter because of the clean baseline, could anybody else weigh in?




The Baader works pretty well visually, but its narrowness can cause problems. The weaker 4959 angstrom line often gets attenuated at normal incidence, although with off-normal light, the passband might shift just enough to let more of it in. Since the 5007 angstrom line with most nebulae is stronger to begin with, the reduction in intensity might not be as great a factor for some nebulae. The other factor is peak transmission for the visual Baader OIII which is around 92% (around 90% for the 5007 angstrom line and anywhere from 32% to 49% for the 4959 angstrom line). The Baader CCD OIII has a somewhat lower transmission peak of 85.5%, but is pretty near zero transmission everywhere outside its primary passband. Clear skies to you.


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Shaky
member


Reged: 06/29/08

Loc: Mason, OH, USA
Re: Forbidden Lines: Mini [OIII] Filter Shootout new [Re: Shaky]
      #3502598 - 12/15/09 10:44 AM Attachment (78 downloads)

I did some quick numbers for maximum spectral shifts expected in Baader based on incident light cone angle s shown in figure below

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Shaky
member


Reged: 06/29/08

Loc: Mason, OH, USA
Re: Forbidden Lines: Mini [OIII] Filter Shootout new [Re: Shaky]
      #3502635 - 12/15/09 11:05 AM

I used the formula described in the scientific article

Lambda (shift) = Lambda (max) x [1 - (Beta squared/ 4 x n squared)]

and cone angle sin-beta = 1/2N0 where N0 = aperture (f/5)

where Beta = maximum angle of incidence and n = refractive index of glass (typically 1.4 but I used maximum 2.0 with filter coatings)

For refractive index n = 2.0,
10" Newtonian f/5 with maximum cone angle = 5.71 degrees
Maximum spectral shift = 503.90 [1 - (0.09949/16)]
= 503.58 nm
Peak wavelength shift = Half the value it would shift in collimated light at the cone's most off axis angle
Baader peak wavelength shift in a 10" f/5 Newtonian = 503.74nm


Similarly for a 20" f/4 Newtonian

Baader spectral shift = 503.56 nm

Thus the spectral shifts, though mathematically significant are not much to change the peak transmission dramatically.

Comments?


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Shaky
member


Reged: 06/29/08

Loc: Mason, OH, USA
Re: Forbidden Lines: Mini [OIII] Filter Shootout new [Re: Shaky]
      #3502641 - 12/15/09 11:09 AM

Sorry for the typo, the shift in the larger Newtonian in my above post is actually 503.83nm and very close to 503.90nm obtained in the lab.

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Scanning4Comets
Markus
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Reged: 12/26/04

Loc: Ontario, Canada
Re: Forbidden Lines: Mini [OIII] Filter Shootout new [Re: Shaky]
      #5688308 - 02/19/13 04:21 AM

Very interesting thread!!!

Cheers!


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Forbidden Lines: Mini [OIII] Filter Shootout new [Re: Scanning4Comets]
      #5695376 - 02/22/13 06:45 PM

I've been evaluating filters from Lumicon, TeleVue and Baader in the field for the last few months.

Though I generally agree with David's conclusion regarding bandwidth and effectiveness in contrast enhancement, I am not so tied to having my filter be the optimum filter for the lines observed.

I'll explain.

On a lot of objects, my 12.5", in dark skies, has an adequate light grasp that the nebula is quite visible without a filter. Adding a filter increases the contrast, allows me to see details in the nebula, and makes the view that much more profound.

But, the narrower the bandwidth of the filter, the darker the overall field is.
Stars get dimmer or disappear, and while the nebula is definitely improved, the esthetic view is somewhat diminished. After all, in a photo, you get the nebula AND the stars.

And, though I certainly agree that the most aggressive filter usually produces the best view of the nebula, it doesn't always produce the best view.

For instance, though the Lumicon UHC filter definitely revealed some fainter structures in the Orion Nebula, the much wider Baader UHC-S (almost a broadband, it's so wide) revealed a more esthetic view of the overall scene, and allowed a lot more reds to be viewed in the nebula (not surprising, given the strong red transmission of the UHC-S). In this case, the view with no filter was magnificent, but the UHC-S gave a prettier overall scene.

On NGC2359, Thor's Helmet, the Baader O-III gave a much darker image than the Lumicon O-III, but with similar nebula enhancement, while the TeleVue O-III did not provide quite as much nebular enhancement but still gave the "brightest and prettiest field" of the 3.
And, the nebula was definitely strongly enhanced over the no-filter view.

One can argue the purpose of a nebula filter is to view the nebula, and that one which produces the best results is the best filter. But I, and probably others, enjoy the overall view when viewing an object. I even enjoy the field of NGC2359 with no filter at all. And the TeleVue O-III yields an image that is definitely enhanced and which is also a very pretty field. In already dark skies, my wife even prefers a broadband filter for most bright nebulae because she wants to see a lot of stars in the image with the nebula.

I could probably get the same view with a Lumicon UHC filter, if I read David's argument right, and use an O-III filter when I need more enhancement on a particular nebula, at the O-III lines part of the spectrum. Instead of a wider O-III filter.

I don't disagree with that, but I also find there are circumstances where the maximum enhancement isn't necessary. I have had a lot of people have a hard time holding their heads in the right place to see an image through a filtered eyepiece because the view was dark, and though the nebula was less enhanced, a wider filter was easier to use for everyone sitting down at my scope.

I see both points of view.

Of course, if the nebula is faint, and I'm looking for details in the nebula, the narrower bandwidth (that doesn't exclude the line or lines) is the one I grab. And it would likely be the one I recommend unless some other factor overrides that.......like seeing reds in M42, for example.


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Jeff Morgan
Postmaster
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Reged: 09/28/03

Loc: Prescott, AZ
Re: Forbidden Lines: Mini [OIII] Filter Shootout new [Re: Starman1]
      #5696212 - 02/23/13 09:12 AM

Quote:

I've been evaluating filters from Lumicon, TeleVue and Baader in the field for the last few months.

Though I generally agree with David's conclusion regarding bandwidth and effectiveness in contrast enhancement, I am not so tied to having my filter be the optimum filter for the lines observed.

I'll explain.

On a lot of objects, my 12.5", in dark skies, has an adequate light grasp that the nebula is quite visible without a filter. Adding a filter increases the contrast, allows me to see details in the nebula, and makes the view that much more profound.

But, the narrower the bandwidth of the filter, the darker the overall field is.
Stars get dimmer or disappear, and while the nebula is definitely improved, the esthetic view is somewhat diminished. After all, in a photo, you get the nebula AND the stars.

And, though I certainly agree that the most aggressive filter usually produces the best view of the nebula, it doesn't always produce the best view.

For instance, though the Lumicon UHC filter definitely revealed some fainter structures in the Orion Nebula, the much wider Baader UHC-S (almost a broadband, it's so wide) revealed a more esthetic view of the overall scene, and allowed a lot more reds to be viewed in the nebula (not surprising, given the strong red transmission of the UHC-S). In this case, the view with no filter was magnificent, but the UHC-S gave a prettier overall scene.

On NGC2359, Thor's Helmet, the Baader O-III gave a much darker image than the Lumicon O-III, but with similar nebula enhancement, while the TeleVue O-III did not provide quite as much nebular enhancement but still gave the "brightest and prettiest field" of the 3.
And, the nebula was definitely strongly enhanced over the no-filter view.

One can argue the purpose of a nebula filter is to view the nebula, and that one which produces the best results is the best filter. But I, and probably others, enjoy the overall view when viewing an object. I even enjoy the field of NGC2359 with no filter at all. And the TeleVue O-III yields an image that is definitely enhanced and which is also a very pretty field. In already dark skies, my wife even prefers a broadband filter for most bright nebulae because she wants to see a lot of stars in the image with the nebula.

I could probably get the same view with a Lumicon UHC filter, if I read David's argument right, and use an O-III filter when I need more enhancement on a particular nebula, at the O-III lines part of the spectrum. Instead of a wider O-III filter.

I don't disagree with that, but I also find there are circumstances where the maximum enhancement isn't necessary. I have had a lot of people have a hard time holding their heads in the right place to see an image through a filtered eyepiece because the view was dark, and though the nebula was less enhanced, a wider filter was easier to use for everyone sitting down at my scope.

I see both points of view.

Of course, if the nebula is faint, and I'm looking for details in the nebula, the narrower bandwidth (that doesn't exclude the line or lines) is the one I grab. And it would likely be the one I recommend unless some other factor overrides that.......like seeing reds in M42, for example.




+1

A little perspective. I'm not doing science and do not need to measure the expansion rate of the shell by seeing the last arc seconds extent of a tendril. I want an aesthetic view, and that means a filter that will not snuff the background stars. Indeed, one of the attractions of planetary nebula is their typical location in rich Milky Way star fields. Take that away and I'm losing half of the information.

And of course filter ownership is not an either/or proposition. The Lumicon O-III is on my toy list, just not in the Top 5 (yet). Until then the much-maligned Tele Vue filter is doing a great job for me.

Edited by Jeff Morgan (02/23/13 09:33 AM)


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deepskydarrell
member


Reged: 03/09/08

Loc: Abbotsford, BC Canada
Re: Forbidden Lines: Mini [OIII] Filter Shootout new [Re: Jeff Morgan]
      #5696734 - 02/23/13 02:36 PM

+2

I must agree with Don and Jeff. I'm so glad I finally bought my Bandmate OIII. It makes the filter experience so much more enjoyable. I save my Thousand Oaks OIII possibly for some ultra faints and definitely the ultra smalls for Blinking, but the best looking OIII views come through my Bandmate.

Thanks for the research Shaky.

DSD.


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David Knisely
Postmaster
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Reged: 04/19/04

Loc: southeastern Nebraska
Re: Forbidden Lines: Mini [OIII] Filter Shootout new [Re: Starman1]
      #5697786 - 02/24/13 05:07 AM

Again, while the Tele Vue Bandmate OIII may "work", it isn't a well-designed filter. Its bandwidth is just too broad for a decent OIII. Indeed, all you would need to get a better filter is to get the DGM Optics NPB filter which would give you identical performance *plus* the ability to enhance objects which have either a balance of OIII and H-Beta or purely H-Beta emission. The Bandmate OIII can't do that due to the poor positioning of its passband. If you want to get the most bang for your buck, the choice is simple: avoid the Tele Vue Bandmate OIII and get a DGM NPB. Then, you will get both the "aesthetics" of a view that lets in a little more starlight, and the dual line species performance that a good narrow-band nebula filter will provide. Clear skies to you.

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