OIII Filter Question

[NeilW]

Hi all,

I have an 8" Dob, and I recently bought an OIII filter hoping that I would see some improvement in views of nebulae (Orion, Eta Carinae, etc.) I must say I have been a little disappointed. The filter just seemed to make the overall view darker with a slight blueish tinge. Was I expecting too much, or is my scope not up to producing significant results with the oIII ?

Thanks,

[astro744]

What filter do you have? What eyepiece/magnification were you using? You should have noticed a significant increase in contrast with the nebula showing white against black background. Depending on the filter make you will have more or less of a greenish tinge to the white. 8" aperture is more than enough for an O-III but as always bigger equals brighter.

[hickny]

I have an OIII filter that I use in my 13.1" Dob and the increase is significant. My scope is however f4.5 which is relatively fast. Your 8" Dob could be f6 or f8 somewhat slower. This may b the reason for the lack of improvement in viewing. Can you verify the quality of the filter? Were you viewing from a dark sky?

[AstroGuy]

Was it a narrow band or a broad band filter? broadband will let you see more, where as narrow band less and is really only suited for astrophotography longer exposure image acquisition.

Vitamin A and phosphor rich foods in your diet help night vision and, will eventually improve the detection of fainter objects and make them stand out more. :thumbsup:

[ausastronomer]

Hi,

Most filters are designed to work well from about F4 to F15 so an F6 to F8 scope really is the "sweet spot" for just about all filters.



It is neither of these. He already said it was an OIII filter which is in fact known as a "line filter".

I am "guessing" his problem was he was observing under light polluted skies where the filter wont help a lot.

Cheers,
John B

[AstroGuy]

Hi John,

The reason I asked is because there are more than one variation of the OIII filter.

Some manufacturers use a very narrow bandpass (steep ramp and roll off)
peaking at the OIII spectral wavelength. This form of filter allows very little other wavelengths on either side of the OIII spectral line to get through.
This type of filter is only recommended for imaging and not recommended for common/casual visual use. Many humans do not have eyes sensitive enough to see just the OIII spectral line on it's own (especially with small aperture). This is why long exposures with the use of photo imaging are required to see good acceptable results.

Other manufacturers create broadband bandpass OIII filters (non steep ramp and non steep roll off). Yet the bandpass envelope still peaks at the OIII spectral line. They are still called OIII filters with the difference that, these type of filters allow more of the wavelengths from either side of the OIII spectral line to get through. Image at the eyepiece seems like it is brighter with more to see because, more light gets through from either side of the OIII spectral line. These type of OIII filters are the filters that are recommended for common/casual visual use.

Then there is broadband OIII with H-beta enhancement with Nebula peaks such as the GSO version. Again, this is a another filter with multiple bandpass characteristics, and different from the others. They also come in narrow band for extremely accurate filtering. This result is very accurate rendition on film and in digital images of the wavelengths mentioned in their specifications. I have seen these filters being flogged off and classified as OIII by some sellers just because OIII happens to be the buzzword at the time.

The point I'm still trying to make is that narrow band filters which allow only one very precise/narrow spectral band are not very good for common/casual visual use. Of course, if you had huge aperture it would be a different story. This is why you hear and read about people with large aperture scopes, getting better results than those with smaller aperture using narrow band filters. Keep in mind, they do vary from manufacturer to manufacturer, therefore the bandpass envelope differs, primarily due to the individual manufacturers' processes.

I think If NeilW could inform us a little more as to the exact manufacturer, make and model of the OIII filter he is using, we might be able to answer his questions with more accurate data that could explain his dilemma.



Regards...

[NeilW]

Hi Guys,

I was using an Astronomik OIII (1.25) with a Bintel SuperView 20mm eyepiece. I was about 100km north of Adelaide, so the skies were reasonable dark.

[troypiggo]

Pretty sure that's the narrowband filter meant for imaging. It's only 12nm bandwidth?

[NeilW]

According to Bintel's description of this filter on their website:

"From the start, the Astronomik OIII filter has been very specifically designed for the visual observation of gaseous and planetary nebulae. The extremely narrow pass band of the two OIII lines brings a substantial contrast gain to these lines even under best observation conditions. On faint super nova remnants and faint planetary nebulae the Astronomik OIII filter will often make the difference as to whether the object can be seen or not!"

Maybe I just needed a darker sky. :(

[AstroGuy]

Astronomik Filter Specs

Photographic OIII
-----------------
http://www.astronomik.com/en/astronomik_oiii-ccd_filter.html

Visual OIII filter
---------------
http://www.astronomik.com/en/astronomik_oiii_filter.html


It's still possible you may be using the wrong one NeilW. Does it say anything on the filter housing?

[AstroGuy]

mmmmmmmm.......I understand but mistakes can be made even by sellers. It's a possibility that they slipped you the wrong one (by accident of course).

From what I've seen the photographic ones are more expensive, so you may have received more than you bargained for. ;)

All things mentioned in this thread are valid. Light pollution, eyepiece used, eye condition, OIII filter and scope type etc...all these factors may or may not contribute to your situation. It may also be the case that all these factors are contributing to your situation. At least now you have something to go on. Good luck with it and hope you get it sorted out. :) Please let us know how it turns out.



Regards...

[ausastronomer]

Hi Neil,

I own the Astronomiks OIII filter in 1.25" and 2" format, amongst a host of other visual filters. It is an excellent filter and works very well on a number of emission nebula, including eta carina, the Orion nebula and the Tarantula Nebula. I would have suggested the DGM Optics NPB (which I also own) as a better "all round" visual filter for an 8" telescope, over the Astromiks OIII, but the Astronomiks OIII is still a very good choice. I reckon darker skies will help, if your skies aren't that dark. If your skies are "reasonable" that filter should have provided a noticeable effect.

Cheers,
John B

[M54]

Maybe the Astronomik UHC filter would be better in an 8" scope???:shrug:

[Suzy]

Hi Neil,
I too have this filter and it works brilliantly. Although my dob is 2" larger than yours, it really brightens up the Orion Nebula. Perhaps you have a faulty filter or they gave you the wrong one? From what I can remember, it didn’t make this nebula look blue- it made it white.
Just for future reference, note that UHC and OIII filter work differently on different objects. Orion Nebula as an example is better seen through a UHC and planetary nebulas generally in an OIII. But regardless of which one, both would definitely have given you a much brighter image.


Here’s a very helpful link which you should find interesting regarding filters.
http://pages.sbcglobal.net/raycash/filters.htm

[AstroGuy]

Not trying to brag or anything like that but...there have been times I have been able to see the Orion Nebula with the naked eye, even better through my scope. When I asked my friend if he could see it he could not. I must admit, and I kid you not, I had been taking cod liver oil for weeks prior. I think that made a big difference!

[NeilW]

Thanks Suzy, excellent link showing relative observations for several objects using different filters. I'll keep trying with the OIII and see how I get on. Thanks to all for your replies.

[ZeroID]

How well dark adapted were your eyes at the time ? Even though posters are saying 'significantly' brighter you are still observing faint objects and the actual light value is still very small. It would be more a contrast improvement I would think. My OIII is on the way and it is Astronomiks so I can't speak from experience yet but I work from LP skies in Auckland where on a good night I can just see the Orion Neb with the naked eye. It will be interesting to compare when the filter arrives ..... with inbuilt clouds probably :sadeyes:

[NeilW]

I'd say they were pretty well light adapted, but it seemed to me that I could see more detail without the filter than with. Let me know how you get on when yours arrive and the clouds clear! :rolleyes:

[ZeroID]

Finally got a chance to check out my O III and CLS filters on a hazy sort of night. Eta Carina up near zenith, high humidity and high wispy clouds ( killed all viewing by about 8:00pm ).

O III filter darkens the viewing considerably but does make a significant difference to the Nebula dust. Without it I could barely see some discolouration around the central region but the O III defined it quite clearly out towards the edges and thes sweeping arches on either side. Some visible lanes within the dust and If I'd had time to get better light adapted probably could have seen more.

CLS Filter surprised me in firstly not dimming the view very much at all and then by also showing up the nebula dust. Not as clear or defined as the O III but a lot brighter overall. Again more time would have been nice but the clouds were not very cooperative.

I used the 25mm plossl with no filter, 20 SV with CLS and 15 SV with O III so was able to swap around quickly to compare. Switched the filters around a little also but the general comments on performance stand as above. Low cloud also gave a lot of reflected glare off local street lights, could just about read the paper under it. And it is very warm and humid, I was just out in my shirt sleeves.

Verdict. I am happy with what I've got. Gives me some options for visuals and photography later. Shame I didn't get a chance to look at Orion Neb but it's almost gone now by evening. There's always next year though.