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#1 EdZ

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Posted 24 October 2011 - 03:54 PM

another of my waking up early mornings. This time I took outside my GO Signature 22x85 and my Fujinon 16x70s. the 16x70s were fitted with a pair of nebula filters. I compared all the objects to see which views seemed best.

Once again, early morning, mostly between 4:30 and 5:30, skies maybe mag 5.2. transparent.

First, I will say, two nebula filters on a pair of 16x70s really really makes the view dark. For a while I tried it with both filters, but found the view so dark I decided to take one out and view with just one filter. perhaps I'll really put some effort some time into detailed comparisons with one filter vs both, but most of what follows here is 16x70 with one barrel filtered, vs 22x85 unfiltered.

I viewed M42 first. No contest. 22xx85 put up a better view. The nebula is so bright didn't seem to improve from the filter.

next I tried M78 and then M1. Not sure I noticed any improvement on either one using the filter. In both cases the slightly larger image of the 22x85 seemed more pleasing.

The previous morning I had spent some time finding the Eskimo nebula. It's very small, but can be identified as not starlike in the 22x85. Tried this filtered and unfiltered. Once again the 22x85 was my choice. Actually this nebula was too small to see a diffuse extended object, but was bright enough to see it wasn't a star. Probably not a good choice for this comparison.

But this last one was different. I moved down just below Gemini right near the Christmas Tree cluster and searched for Hind's Variable Nebula. I found it using the 22x85s. It would blink in and out. Well, I thought this one was better thru the nebula filter. 16x70s won this one.

I tried both binoculars on the Rosette nebula but couldn't see it with either one.

So, for the most part, I found I preferred the view with the larger unfiltered image scale. But there was that one instance where the filter seemed to make a difference.

I'll have to try those objects again with two filters vs one filter.

edz

#2 Mark9473

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Posted 24 October 2011 - 04:18 PM

Very intersting report, Edz! I've been toying with the idea of a front-mounted filter. Could you specify which brand/type filter you used?

#3 EdZ

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Posted 24 October 2011 - 05:04 PM

Kasia Narrow Band Super Nebula Filters, made to fit under the removeable eyecap of the Fujinon binocular. They are extremely thin, fit inside a recess under the Fujinon eyecaps and the eyecaps screw back on and do not reduce the eye relief at all. The Kasia filters are wafer thin and have no threads so can't be screwed onto just any binocular. AFAIK, they are made for the Fujinon.

I tried screwing my Orion Ultrablock filter onto the threaded eyepiece of the Signature 22x85, but that stuck out so far it reduced the already short eye relief by about 5-6mm. In addition, it now was a metal filter ring up against my eyeglasses. I found that completely unacceptable.

edz

#4 drshr

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Posted 25 October 2011 - 01:44 AM

A pair of 3D glasses with the old lens material removed and replaced with a shaped pair of 2" broadband filters (some cheapo Celestrons or Orbinars). No more fiddling.

#5 The Ardent

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Posted 25 October 2011 - 07:57 AM

Can you provide a link or dealer info please?

Kasia Narrow Band Super Nebula Filters, made to fit under the removeable eyecap of the Fujinon binocular.



#6 EdZ

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Posted 25 October 2011 - 08:47 AM

Sorry, I'm affraid I don't have that info.

Look for Fujinon nebula filters.

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#7 RichD

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Posted 25 October 2011 - 10:56 AM

Do you mean Hubble's variable nebula EdZ? I think Hind's is in Taurus...

If it is Hubble's, i'm kind of surprised that it responded to nebula filters as it's a reflection nebula - normally star light is dimmed through that type of filter.

#8 EdZ

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Posted 25 October 2011 - 11:59 AM

I think you are correct that it is Hubble's var neb. It was in Mono.

It would figure the one I thought most affected is actually the one that should be least affected. ???

edz

#9 GlennLeDrew

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Posted 29 October 2011 - 02:25 PM

I first assumed the filters were of the broad-band type. I take it that the Fuji filters are equivalent to the UHC, Orion Ultrablock, etc.? If so, the view will indeed be dark, with sky surface brightness (and stars) reduced by about 1.6 magnitudes, or just over a factor of four. But I would hardly call this "really really dark". I use Ultrablocks on my RA 21.8X60, and even with this smaller exit pupil the glow of the sky is well enough seen under rather darker 6.4m skies. Of course, this set-up is for emission nebulae-only observing.

#10 faackanders2

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Posted 29 October 2011 - 04:32 PM

Both Hubble's variable and M78 are reflection nebula, so narrow band filters won't work, and wide band may or may not work either. Filters work best on emission and planetary nebulas, as well as super novas.

#11 Rich V.

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Posted 29 October 2011 - 07:52 PM

For those who haven't had a chance yet, here's a worthy CN reference article on filter use by David Knisely:

FILTER PERFORMANCE COMPARISONS FOR SOME COMMON NEBULAE

Rich V

#12 Dan McShane

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Posted 19 December 2012 - 09:12 PM

This post caught my attention as I recently setup a pair of my VHT`s as over-the-aperture filters for a CN member. They were ordered for their Kowa Highlander 30 x 82 mm binos. Yup ... big filters.

They initially wanted to go with the NPB, (UHC type). However I stumbled onto another thread where the common comment was how dark the field was with UHCs and binos.

So I suggested it may be a good idea to at least consider the VHT or even GCE. I also proceeded to conduct some tests with a cheapo, but decent pair of Tasco 10 x 50`s. Since they did not have removable EPs my only option was putting a couple of 2" filters over-the-aperture.

Here`s what I found from my fairly dark skies here in southern NH.

NPB: While it certainly enhanced M42, background stars were wiped out. Not literally of course, but very dimmed. It did help a bit on the Dumbbell, but overall I thought the dimming detracted from the aesthetics alot. So the verdict was that as good as the NPB is with even a small scope, I didn`t find it helped with binos, at least a set of 10 x 50s.

VHT: Much better. Good contrast on the same objects and while still dimmed, the overall view was quite nice. I found it was especially so when I allowed my eyes 15-20 seconds to adjust to the filtered view. Verdict on the VHT was pretty good with small binos.

GCE: The best view without a doubt. Minimal impact on star brightness. M42 was nicely enhanced with the background stars of the Belt preserved. Also, I was very impressed with Andromeda. The very low power-per-inch seemed to work the best I`ve seen with a galaxy and the GCEs. The Pinwheel also presented a nice view, and a bit later M81 & M82. So at least for small binos the GCE was the clear winner.

For obvious reasons I didn`t test the OIII`s. I would think you would have to have super big binos to get a satisfying view with such a low optical throughput filter.

#13 GlennLeDrew

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Posted 19 December 2012 - 11:54 PM

Danman,
Filter performance does not depend on aperture, but rather it's the exit pupil diameter which is the first concern. And if course the nature of the target. All aperture does by itself is impose a limit to the smallest objects observable. Why, even the unaided eye looking through such a filter as an H-beta can bring out something like the lambda Orionis cloud.

Testing filters on such bright targets as M42 is not really useful. It's the dimmer stuff that many filters were made to better bring out of the the sky glow. In the case of emission nebulae, in order to work most efficiently filters *must* of necessity highly attenuate any continuum sources like stars, and of course the offending sky glow. A dark, seemingly near starless view is just what the doctor ordered.

I've attached 2" UHC filters to the front of commercial 10X50s and enjoyed awesome views of many nebulae, such as the California, the Heart and Soul, North America and Pelican, to name just a few. For my current right-angle bino (50mm and 60mm interchangeable objectives) I use even H-beta filters.

I stress again, aperture is no impediment to the use of narrow-band filters as long as you observe suitable targets, size wise.

#14 Dan McShane

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Posted 20 December 2012 - 01:17 AM

Danman,
Filter performance does not depend on aperture, but rather it's the exit pupil diameter which is the first concern. And if course the nature of the target. All aperture does by itself is impose a limit to the smallest objects observable. Why, even the unaided eye looking through such a filter as an H-beta can bring out something like the lambda Orionis cloud.

Testing filters on such bright targets as M42 is not really useful. It's the dimmer stuff that many filters were made to better bring out of the the sky glow. In the case of emission nebulae, in order to work most efficiently filters *must* of necessity highly attenuate any continuum sources like stars, and of course the offending sky glow. A dark, seemingly near starless view is just what the doctor ordered.

I've attached 2" UHC filters to the front of commercial 10X50s and enjoyed awesome views of many nebulae, such as the California, the Heart and Soul, North America and Pelican, to name just a few. For my current right-angle bino (50mm and 60mm interchangeable objectives) I use even H-beta filters.

I stress again, aperture is no impediment to the use of narrow-band filters as long as you observe suitable targets, size wise.


Hi Glenn,

I`ve often heard that sentiment with regards to filter performance vs. aperture, but I`m not in total agreement.

And while I understand the relationship between aperture and limiting magnitude, it`s important to remember that optical interference filters exclude a tremendous amount of light.

And in truth the whole effect of contrast and enhancement with nebula/LPR filters is pure trickery to the eye. Even a strong emission object is actually slightly dimmer, as are all objects viewed with filters. The more narrow the filter bandwidth, the lower the optical throughput, and the dimmer the object, and background stars.

There`s just no getting around it.

And, you are correct, nice views of selected objects can be had with very small aperture and narrow-band filters, but I think it`s also a personal preference thing too. Some people just don`t like the aesthetics with significantly dimmed stars, some don`t mind, in the quest for maximum contrast. It`s all good if we`re happy with the view... :)

best regards,
Dan McShane

#15 Tony Flanders

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Posted 20 December 2012 - 07:19 AM

The whole effect of contrast and enhancement with nebula/LPR filters is pure trickery to the eye. Even a strong emission object is actually slightly dimmer, as are all objects viewed with filters. The more narrow the filter bandwidth, the lower the optical throughput, and the dimmer the object, and background stars.


There are three statements here. The first is false, the second is true, and the third is false.

The increase in contrast is not trickery; it's real. Contrast is the ratio between the object's brightness and the background brightness. A narrowband filter decreases the background brightness much more than it decreases the brightness of an emission nebula; therefore, it increases the nebula's contrast.

That's true both in theory and in practice. I doubt that anybody could see the California Nebula naked-eye or through small binoculars without using a filter. Holding an H-Beta filter in front of each eye, it's immediately obvious.

Filters do indeed decrease everything's brightness, because no nebula shines 100% in emission lines -- nor do all the lines make it through any given filter. However, the decrease is quite small in many cases.

It is not true that the narrower the bandwidth, the dimmer the object. For a strong O III emitter, any filter that encompasses the O III line will make the object similarly bright. Cut the bandwidth in half and the O III line still comes through just as strongly, though the extraneous light is halved.

The background stars are indeed dimmed, and that's often the whole point of the exercise. For instance, the main obstacle to seeing the North America Nebula clearly is the fact that it merges with some very rich star fields. Get rid of the stars, and the nebula stands out.

This is also often the case for clusters surrounded by nebulosity, like M16. Or for planetary nebulas where the nebula is significantly fainter than the central star.

#16 RichD

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Posted 20 December 2012 - 10:21 AM

Yes, all true Tony. It's definitely a real contrast effect, not an illusion.

It's good fun touring the sky and seeing how different objects respond to different filters. The very aggressive OIII filter is a nice one to experiment with, certain nebulae - particularly planetaries and SNR - just pop in a good OIII filter. M97 and the veil being memorable ones.

#17 Dan McShane

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Posted 20 December 2012 - 11:48 AM

The whole effect of contrast and enhancement with nebula/LPR filters is pure trickery to the eye. Even a strong emission object is actually slightly dimmer, as are all objects viewed with filters. The more narrow the filter bandwidth, the lower the optical throughput, and the dimmer the object, and background stars.


There are three statements here. The first is false, the second is true, and the third is false.

The increase in contrast is not trickery; it's real. Contrast is the ratio between the object's brightness and the background brightness. A narrowband filter decreases the background brightness much more than it decreases the brightness of an emission nebula; therefore, it increases the nebula's contrast.

That's true both in theory and in practice. I doubt that anybody could see the California Nebula naked-eye or through small binoculars without using a filter. Holding an H-Beta filter in front of each eye, it's immediately obvious.

Filters do indeed decrease everything's brightness, because no nebula shines 100% in emission lines -- nor do all the lines make it through any given filter. However, the decrease is quite small in many cases.

It is not true that the narrower the bandwidth, the dimmer the object. For a strong O III emitter, any filter that encompasses the O III line will make the object similarly bright. Cut the bandwidth in half and the O III line still comes through just as strongly, though the extraneous light is halved.

The background stars are indeed dimmed, and that's often the whole point of the exercise. For instance, the main obstacle to seeing the North America Nebula clearly is the fact that it merges with some very rich star fields. Get rid of the stars, and the nebula stands out.

This is also often the case for clusters surrounded by nebulosity, like M16. Or for planetary nebulas where the nebula is significantly fainter than the central star.


Hi Tony,

I know contrast is the relative brightness between an object and the background, I was using the term "whole effect of contrast and enhancement" more as a figure of speech. Like when you look at a bright nebula with, and without a filter. The "wow factor" if you will. Indeed contrast as you define, and I agree, is increased.

However the nebula is still slightly dimmer, regardless of the strength of the emission lines. This is just hardcore optical interference filter reality. You can`t increase the emission line strength, you can only decrease undesirable wavelengths, and strive for the greatest possible TX% for the desirable wavelengths.

Regarding bandwidth. If the light we see from strong emission lines within nebula was the only spectral content, then it would be true that bandwidth would not dim the object, as long they were inclusive within the passband. But there is at least some periphreal spectra, which is reduced as bandwidth narrows. So again less light.

But I also very much agree that the loss, at least within the passband, is very slight. And I`m not sure most people could even distinguish between a filter with 99% TX vs. say 95% TX.

best regards,
Dan McShane

#18 Dan McShane

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Posted 20 December 2012 - 12:23 PM

It's good fun touring the sky and seeing how different objects respond to different filters. The very aggressive OIII filter is a nice one to experiment with, certain nebulae - particularly planetaries and SNR - just pop in a good OIII filter. M97 and the veil being memorable ones.


Rich, are you talking about with binos? I`m very much a bino greenhorn as far as "what can you see" with a given aperture. What can you see deep sky-wise with a pair of 10 x 50`s, with or without various filters?

#19 GlennLeDrew

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Posted 20 December 2012 - 02:27 PM

Dan,
In the case of emission nebulae, for all intents and purposes the *only* light we see is that emitted at the specific wavelengths passed by the filters we use. From a visual perspective (especially when dark adapted) we can eliminate the deep red H-alpha; our eye is so insensitive to it that it can never contribute more than a couple of percent to the visible light (except for a couple of very unique objects.) To our eyes, the vast bulk of nebular light is O-III and H-beta emission. Depending on the relative intensity of each emission component, we either employ an O-III, H-beta or 'UHC'-like filter, depending on whether we wish to pass one, the other or both.

In any case, because the emission lines are effectively of sub-nanometer width, the narrowest we can humanly make the passband the better, for then we increase contrast the most. And no desired emission light is lost.

A good example of this is an H-alpha solar filter. In order to boost contrast sufficiently so as to see the prominences, we must make (expensive) etalons of sub-nanometer bandpass.

Now, in the case of nebulosities posessing a reflection component, a nebular filter will definitely reduce the intensity of this near-continuum source. But then a two-pronged approach is appropriate; no filter--or perhaps a broad band-pass LPR filter--for the reflection component, and a narrow bandpass filter for the emission component.

#20 RichD

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Posted 20 December 2012 - 05:01 PM

Dan, i'm talking telescopes mainly. I use a UHC and OIII with 5" and a 12" as I find the best effect with filters for me occurs with at least 100mm of aperture or more. I don't have very dark skies here so a pair of nebula filters on my 16x70s or 10x50 don't work too well (I tried briefly once).

#21 Dan McShane

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Posted 20 December 2012 - 06:09 PM

Dan, i'm talking telescopes mainly. I use a UHC and OIII with 5" and a 12" as I find the best effect with filters for me occurs with at least 100mm of aperture or more. I don't have very dark skies here so a pair of nebula filters on my 16x70s or 10x50 don't work too well (I tried briefly once).


As far as the 10 x 50 mm binos, I didn't care for the NPB (UHC type) view either, although from what others are saying, I should give it another try with some more challenging objects.

#22 Dan McShane

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Posted 20 December 2012 - 07:53 PM

Dan,
In the case of emission nebulae, for all intents and purposes the *only* light we see is that emitted at the specific wavelengths passed by the filters we use. From a visual perspective (especially when dark adapted) we can eliminate the deep red H-alpha; our eye is so insensitive to it that it can never contribute more than a couple of percent to the visible light (except for a couple of very unique objects.) To our eyes, the vast bulk of nebular light is O-III and H-beta emission. Depending on the relative intensity of each emission component, we either employ an O-III, H-beta or 'UHC'-like filter, depending on whether we wish to pass one, the other or both.

In any case, because the emission lines are effectively of sub-nanometer width, the narrowest we can humanly make the passband the better, for then we increase contrast the most. And no desired emission light is lost.


Glenn, you could actually demonstrate that theory. This is an 03 modeled with a thin film design program. It has a bandwidth of about 15 nm. At 0 AOI the oxygen lines transmit over 90% TX. If you tilt the filter about 20 degrees the average 03 will shift short about 8nm, nearly completely suppressing 505 and knocking 501 to around 30% TX. So you would suppose an oxygen rich object would dim, or even disappear, if there is no significant energy outside the emission lines.

Posted Image

#23 GlennLeDrew

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Posted 20 December 2012 - 10:14 PM

Quite correct, Dan. Light entering an interference filter too far from perpendicular becomes 'de-tuned.' However, for even a fast scope of f/4, marginal rays enter a little over 7 degrees from perpendicular. A visual nebula filter can well handle this.

#24 Dan McShane

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Posted 20 December 2012 - 11:08 PM

Quite correct, Dan. Light entering an interference filter too far from perpendicular becomes 'de-tuned.' However, for even a fast scope of f/4, marginal rays enter a little over 7 degrees from perpendicular. A visual nebula filter can well handle this.


Glenn, I was actually talking about tilting the filter about 20 degrees to shift the the bandpass shorter. Just as a kind of a "what if" to test the emission line hypothesis. But you`re correct about cone angle. It is non-problem because visual filters are so insensitive to AOI, unless you get over about 15 degrees, or an f/2 system ... yikes!






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