Let's talk filters

[SiriusB]

17 minutes ago, John said:

I used to think that but I have found both UHC and O-III filters effective with apertures from 80mm and upwards.

I used to stick with the "generous" (bandwidth) Baader UHC-S filter with smaller scopes but having tried more regular UHC's and more recently O-III's with such scopes I find them more effective despite the smaller light grasp of the scope.

Hopefully Don will see this and explain some more.

 

Thanks John, that's interesting & sounds expensive~ thought a cheapie would be all i'd need.😉

[Louis D]

3 hours ago, SiriusB said:

Any chance you could expand on this please for the benefit of all?

I'd been given  to believe the best(bandwidth) nebula filters work best with larger scopes due to light loss? If you've only a small scope......

Thanks.

The contrast ratio remains the same regardless of aperture.  If a filter passes 94% of the OIII lines with a 15nm Full Width, Half Maximum (FWHM) bandwidth, it will pass that amount of the emission lines and reject the light outside that width around the emission lines.  As long as the passband is well centered on the emission lines, it's going to be effective at increasing contrast without reducing the OIII lines from the nebula.  Having a wider passband will make the image brighter by passing more undesirable light around the OIII lines, but this is not desirable at any aperture since the whole goal is to increase contrast by rejecting as much undesirable light as possible.

I find OIII filters applicable to any aperture.  Image brightness is determined by the exit pupil, not the aperture.  Greater aperture does allow for a higher magnification at a given exit pupil and greater resolution.  Basically, if the nebula dims too much with the OIII to be enjoyable, back off on the magnification to increase the exit pupil to brighten up the nebula.  It will appear smaller, though.

[jetstream]

5 hours ago, SiriusB said:

If you've only a small scope......

These filters work with small apertures. My 90mm likes the tight Lumicon OIII, as does the120mm fracs and the H130. I was easily observing the Flaming Star neb in Auriga with a DGM NPB and the H130 the other night.

The reason I said if I could only have one filter it would be a quality OIII is that in lighter skies it will give needed maximum contrast and under darker skies it will give stunning views. Obviously on targets that have a lot of OIII .

I do find that filter transmission matters- mind you I'm sure someone will debate this. My finding is using my eyes and not scientific formulas etc. My very best filters all have really high transmission.

[Don Pensack]

On 16/01/2021 at 15:39, SiriusB said:

Any chance you could expand on this please for the benefit of all?

I'd been given  to believe the best(bandwidth) nebula filters work best with larger scopes due to light loss? If you've only a small scope......

Thanks.

Since I've used narrow bandwidth filters with 50mm finders and even with the naked eye, I'll try to explain:

The narrower the bandwidth, the greater the suppression of non-nebular light and the greater the contrast between the nebula and sky.

Some people with smaller scopes have remarked that this makes the overall field image too dim for the small scope because it dims the stars and the sky, leaving mostly the nebula,

and the nebula is fairly faint in the first place in their scopes.

I think at least part of that, and the reason some have said the narrower filters are better in larger apertures is that they are not using the filters correctly.

There are some "parameters" of use that have to be followed for them to work well:

1. You must be completely dark adapted--at least 30-45 minutes outside away from all lights--for the sensitivity of the eye to increase to its maximum.

2. You should use low powers: a maximum of 10x/inch of aperture, or an exit pupil of 2.5mm and larger.

3. the filters don't work well in a hazy, cloudy, or otherwise not clear sky, and I KNOW a lot of observers tend to observe in those conditions, despite the poor results.

4. the nebula in question should be higher than 30° from the horizon or extinction due to the atmosphere will reduce its visibility

5. nebula filters work better if the background sky is not so bright that the improvement isn't sufficient to enable you to see the nebula.  Nothing can make a faint nebula visible in bright city lights.

6. Use the right filter on the right nebula.   An O-III filter isn't the right filter to use on a large hydrogen emission gas cloud like M42, M8, M20, M17, M16 because it suppresses the light from hydrogen emission.

 

The nebula filters work by dimming the background by 2.5 to 3 magnitudes while only dimming the nebula by about 0.05-0.1 magnitude.  A wider filter dims the background less so doesn't improve contrast to as great a degree,

Hence, the visibility of the nebula will also be less.  Yes, there will be some contrast enhancement, but the extra brightness of the background will only reduce the size of the nebula seen and reduce the details seen in the nebula.

When I have used an O-III filter to see the Veil Nebula with a 50mm finder scope, the field did darken, but the nebula became visible.  That's the purpose for the filters, right?  To make the nebulae more visible to the eye when looking through a telescope.  

In a really big scope, where the nebula is easily visible without a filter, paradoxically a wider filter can be used and still see a good view of the nebula.  Less contrast enhancement still works when there is aperture to waste.

Still, even there the narrower filters improve visibility more and create greater contrast.  

And contrast enhancement even works at very dark sites by suppressing sky glow.

 

So, other than price (the main motivator for the purchase of a lesser-performing filter), might there be a reason to prefer a filter with less contrast enhancement?  Maybe, if the overall field + nebula is what the observer is viewing.  NGC2359 is in a rich Milky Way field and seems to sit suspended in front of a background haze of faint stars.  The best contrast for the nebula suppresses the starlight and reduces that impression of the field of the nebula.  If studying the nebula only, the narrowest O-III filter is best.  But if looking for an overall aesthetic experience, a wider filter might be the choice.  But I can say that because I view with 32cm in a dark sky, dark enough to see the Veil Nebula with an O-III filter held up to the eye, and Barnard's Loop in Orion with an H-ß filter held up to the eye.  With a little more light pollution, the nebula would disappear, and ONLY the narrowest of filters would make it visible at all.

So, back to the small scope: if the small scope has any chance at all to see the nebula, it is by yielding the best contrast enhancement possible, and that is with a narrower filter.  But it is a matter of using the filter correctly, as I outlined.

[Ships and Stars]

On 18/01/2021 at 16:37, Don Pensack said:

6. Use the right filter on the right nebula.   An O-III filter isn't the right filter to use on a large hydrogen emission gas cloud like M42, M8, M20, M17, M16 because it suppresses the light from hydrogen emission.

Great comment Don, there is a ton of really good information, much of it I flat out didn't know or know how to explain other than pure filter wizardry

A little off topic and not doubting you that OIII isn't optimal for M42, but a few weeks ago I happened down to M42 with a 20mm APM and Astronomik OIII on the 20" at a 21.5ish dark site after looking at the Rosette and was treated to quite a different and stunning view of M42 I don't recall seeing in my short time observing.

I've seen it before with a UHC but got into the habit of generally not using a filter and concentrating around 42/43 area and not covering the full extent under dark skies, because I'm usually chasing things I can't see from home.

The overall normal extent of M42 around the Trapezium and normal swathes or 'wings' were present with the OIII. While tightly defined, it blocked a fair bit of the usual detail, but enhanced other areas. So that in itself was a mixed bag, but the thing that really made me sit up was the clearly defined, narrow ring which completely extended well past my FOV and back up to the other side of M42. I don't think I've seen this huge feature before, probably because I don't have a widefield scope (other than bins) and it was striking. The central area was completely empty of nebulosity as I recall, but the outer edge was magnificent and absolutely glowing. I thought it was some kind of stray reflection or eyepiece fogging at first, but glass and mirrors were clear. It was complete except one small area where this band necked down. 

I took the OIII off and could barely see this band in comparison, but it was still there, very faint. Didn't try UHC, don't have one in 2" (yet).

So my question, and this is for anyone really, is M42 a mixed object with different emission lines, incl HII, etc that different filters enhance at the expense of others, or does this large circular feature maybe have its own name/ID? I don't see it in photos, though that entire area is ablaze in long exposures. 

*edit I took the creative commons wiki page photo for M42 (thank you Keesscherer) and desaturated it and boosted blacks. The bottom photo is along the lines of what I was seeing with OIII, but not that striking. It was like an inverted image, the red arrow points to the dark gap I noticed. Very pronounced!

Will try the TV Nebustar II next time. 

 

Edited January 19, 2021 by Ships and Stars

[jetstream]

44 minutes ago, Ships and Stars said:

So my question, and this is for anyone really, is M42 a mixed object with different emission lines,

For me M42 is best no filter, revealing its blazing mottled green core in most of my scopes. Each filter will show the object a bit differently IMHO.

Back to my OIII recommendation-  many observe under LP skies so really only the brightest objects are on the list and the OIII will make visible many of the brighter ones. Most Hb targets are tough.....

Rob, in another thread I mentioned Hb doesnt work on dark neb- upon reflection I thought I would clarify with respect to B33, the HH. What the Hb is actually doing is framing the dark notch, allowing a profile of it to be seen.

@Don Pensack I totally agree that we need all of the filters- OIII, Hb and UHC in my case.

[Don Pensack]

51 minutes ago, Ships and Stars said:

Great comment Don, there is a ton of really good information, much of it I flat out didn't know or know how to explain other than pure filter wizardry

A little off topic and not doubting you that OIII isn't optimal for M42, but a few weeks ago I happened down to M42 with a 20mm APM and Astronomik OIII on the 20" at a 21.5ish dark site after looking at the Rosette and was treated to quite a different and stunning view of M42 I don't recall seeing in my short time observing.

I've seen it before with a UHC but got into the habit of generally not using a filter and concentrating around 42/43 area and not covering the full extent under dark skies, because I'm usually chasing things I can't see from home.

The overall normal extent of M42 around the Trapezium and normal swathes or 'wings' were present with the OIII. While tightly defined, it blocked a fair bit of the usual detail, but enhanced other areas. So that in itself was a mixed bag, but the thing that really made me sit up was the clearly defined, narrow ring which completely extended well past my FOV and back up to the other side of M42. I don't think I've seen this huge feature before, probably because I don't have a widefield scope (other than bins) and it was striking. The central area was completely empty of nebulosity as I recall, but the outer edge was magnificent and absolutely glowing. I thought it was some kind of stray reflection or eyepiece fogging at first, but glass and mirrors were clear. It was complete except one small area where this band necked down. 

I took the OIII off and could barely see this band in comparison, but it was still there, very faint. Didn't try UHC, don't have one in 2" (yet).

So my question, and this is for anyone really, is M42 a mixed object with different emission lines, incl HII, etc that different filters enhance at the expense of others, or does this large circular feature maybe have its own name/ID? I don't see it in photos, though that entire area is ablaze in long exposures. 

*edit I took the creative commons wiki page photo for M42 (thank you Keesscherer) and desaturated it and boosted blacks. The bottom photo is along the lines of what I was seeing with OIII, but not that striking. It was like an inverted image, the red arrow points to the dark gap I noticed. Very pronounced!

Will try the TV Nebustar II next time. 

 

 

 

 

all large Hydrogen emission nebulae, like M42/43 emit most of their energy at H-α and H-ß

But, all of them also emit light from the excitation of other ions, like O-III, S-II, N-II and various Helium wavelengths.

So looking at M42, say, with an O-III filter will reveal different details than you might see in a narrowband UHC-type filter simply because the contrast on the O-III features will be boosted.

But, that will be at the sacrifice of the H-ß features.  The combination of the H emission and O emission will probably yield the largest view of the nebula.

M42 is so bright, it is a special case.  I've verified several times that it damages your night vision because of its brightness, which is why it is one of a very small handful of nebulae in which color is seen.

I usually see the most color and the largest extent of nebula in that one with a broadband filter (example Baader UHC-S) or a UHC filter with unrestrained red (examples: Astronomik UHC or DGM NPB),

but, so far, nothing has beaten the view of the nebula in my 12.5" at a high altitude site (2550m) under skies of mag.21.95 mpsas (essentially pristine--no light pollution at all) without a filter.  That night, not only were greens and reddish hues visible, but also beige-yellow and dusty rose, and bluish hues. And nebulosity was seen all the way to NGC1977, which appeared distinctly blue.

[Ships and Stars]

10 minutes ago, jetstream said:

For me M42 is best no filter, revealing its blazing mottled green core in most of my scopes. Each filter will show the object a bit differently IMHO.

Back to my OIII recommendation-  many observe under LP skies so really only the brightest objects are on the list and the OIII will make visible many of the brighter ones. Most Hb targets are tough.....

Rob, in another thread I mentioned Hb doesnt work on dark neb- upon reflection I thought I would clarify with respect to B33, the HH. What the Hb is actually doing is framing the dark notch, allowing a profile of it to be seen.

@Don Pensack I totally agree that we need all of the filters- OIII, Hb and UHC in my case.

Hey Gerry! I don't normally use a filter either on M42. The strange thing is I don't spend much time on M42 when at dark sites, because I'm pressed for time and searching for other more elusive things. It loses a lot of nebulosity at home so I should start over with M42/3 again. I edited my post and attached a B&W image with a few tweaks that kind of replicates what it looked like with OIII. The ring was bright and apparent and the dark notch was clearly visible. The centre was just black with a few stars popping though as I recall, maybe not. Perhaps this notch is visible without filters as well. 

I think I know what you mean wit an Hb on B33 - it's enhancing the contrast between IC434 which has a slight glow, enhancing the backlighting behind B33? That makes sense, if something is effectively a black cloud of carbon etc, a filter isn't going to do much  

1 minute ago, Don Pensack said:

all large Hydrogen emission nebulae, like M42/43 emit most of their energy at H-α and H-ß

But, all of them also emit light from the excitation of other ions, like O-III, S-II, N-II and various Helium wavelengths.

So looking at M42, say, with an O-III filter will reveal different details than you might see in a narrowband UHC-type filter simply because the contrast on the O-III features will be boosted.

But, that will be at the sacrifice of the H-ß features.  The combination of the H emission and O emission will probably yield the largest view of the nebula.

M42 is so bright, it is a special case.  I've verified several times that it damages your night vision because of its brightness, which is why it is one of a very small handful of nebulae in which color is seen.

I usually see the most color and the largest extent of nebula in that one with a broadband filter (example Baader UHC-S) or a UHC filter with unrestrained red (examples: Astronomik UHC or DGM NPB),

but, so far, nothing has beaten the view of the nebula in my 12.5" at a high altitude site (2550m) under skies of mag.21.95 mpsas (essentially pristine--no light pollution at all) without a filter.  That night, not only were greens and reddish hues visible, but also beige-yellow and dusty rose, and bluish hues. And nebulosity was seen all the way to NGC1977, which appeared distinctly blue.

Thanks Don, yes the OIII completely blocked a lot of the nebula, it's fun to play with and reveals a new side but definitely a lot of the character gets lost in translation.

I think part of the reason I haven't studied M42 as much as I should have, is because I'm often trying for the HH when in that part of the sky and as you say, trying to keep my dark adaptation. I'm missing out on one of the brightest, most impressive spectacles in the night sky, to see the polar opposite, a black shape set on a slight less black background  

Your account from this mountain spot or elevated pass sounds unforgettable! If I ever get anywhere near those conditions, I'll skip the filter and the HH and settle in on M42/3 for awhile.

Thanks Gerry and Don 

[jetstream]

5 minutes ago, Ships and Stars said:

I think I know what you mean wit an Hb on B33 - it's enhancing the contrast between IC434 which has a slight glow, enhancing the backlighting behind B33?

A good way to practise B33 is just trying to see IC434 no filter, which is do able in smaller scopes than one might think. This object frustrated me for quite a while actually- I should have started observing dark nebs, like in Aquila first to get a handle on them.

Have you tried the even more frustrating Cone nebula?

With my 24" I've settled into observing bright objects strangely enough as well as unique ones like Hickson 55 and the PNs.

[Ships and Stars]

37 minutes ago, jetstream said:

A good way to practise B33 is just trying to see IC434 no filter, which is do able in smaller scopes than one might think. This object frustrated me for quite a while actually- I should have started observing dark nebs, like in Aquila first to get a handle on them.

Have you tried the even more frustrating Cone nebula?

With my 24" I've settled into observing bright objects strangely enough as well as unique ones like Hickson 55 and the PNs.

I've had hints of IC434 in binoculars from a really dark site, but that was very optimistic. I think I saw the Cone a while back on a really good night, that was late 2019 I think from a hilltop site near me. Near Hubble's Variable Nebula? I'll have to go back through reports, if it's that challenging, I may not have seen it?? I got the HH from home the other night in the 12" but it was very, very subtle with the faintest glow from 434. The flame that night was actually ok from my LP locale. Friday or Sat night.

I tried for HCG55 last night from home with the 20", NGC3735 next to it was an easy one, but things started frosting up about that time. Still, don't think I'd be able to bag it from here. I'll try again on that and HCG57, and on M42/3 again if I get a chance, setting quickly now! 

 

[jetstream]

1 minute ago, Ships and Stars said:

I may not have seen it??

What did you see?

[Ships and Stars]

24 minutes ago, jetstream said:

What did you see?

Definitely Hubble's variable nebula, need to go back through reports to see if I was confident on the Cone. I had the 20" that night and conditions were really good, but don't recall much about the Cone! Suggets I may have passed on it. We're in full lockdown here, you can travel for outdoor recreation, but it's not recommended. Not sure how the police would react with me carting the dob around at 3am. Probably ok going to my local site, but who knows...

[Pixies]

8 minutes ago, Ships and Stars said:

Definitely Hubble's variable nebula, need to go back through reports to see if I was confident on the Cone. I had the 20" that night and conditions were really good, but don't recall much about the Cone! Suggets I may have passed on it. We're in full lockdown here, you can travel for outdoor recreation, but it's not recommended. Not sure how the police would react with me carting the dob around at 3am. Probably ok going to my local site, but who knows...

The cone is the 'fairy' on top of the Christmas Tree cluster, isn't it? For ages I thought it was the Christmas tree itself, and wondered why I couldn't find it (the Christmas tree, that is)

Edited January 19, 2021 by Pixies

[Louis D]

2 hours ago, Don Pensack said:

at a high altitude site (2550m)

The Chula Vista Trailhead parking lot at Mt. Pinos, perchance?  If I'm ever in CA with a telescope, I'll have to give it a try based on your ceaseless praise of that site.

[SiriusB]

Thankyou all for your replies on passbands~ & especially for your comprehensive reply Don.

Genuinely learned something from the responses. 👍

[Merlin66]

If you Google Orion nebula spectra....

https://www.google.com/search?q=M42+spectra&tbm=isch&chips

It shows the main emission lines and relative intensities.

Typical spectra shown below (from Steve Broadbent)

 

Edited January 20, 2021 by Merlin66

[Louis D]

13 hours ago, Merlin66 said:

If you Google Orion nebula spectra....

https://www.google.com/search?q=M42+spectra&tbm=isch&chips

It shows the main emission lines and relative intensities.

Typical spectra shown below (from Steve Broadbent)

 

That makes me feel better I wasn't losing my mind when I found using an OIII filter on it useful.  Based on what Dan said below, he made it sound like OIII emission was minor.

22 hours ago, Don Pensack said:

all large Hydrogen emission nebulae, like M42/43 emit most of their energy at H-α and H-ß

But, all of them also emit light from the excitation of other ions, like O-III, S-II, N-II and various Helium wavelengths.

So looking at M42, say, with an O-III filter will reveal different details than you might see in a narrowband UHC-type filter simply because the contrast on the O-III features will be boosted.

But, that will be at the sacrifice of the H-ß features.  The combination of the H emission and O emission will probably yield the largest view of the nebula.

 

[jetstream]

12 minutes ago, Louis D said:

That makes me feel better I wasn't losing my mind when I found using an OIII filter on it useful.

Me too, and under lighter skies the OIII provides the best views IMHO. Merlin66 knows much about spectra.

[Don Pensack]

On 19/01/2021 at 12:34, Louis D said:

The Chula Vista Trailhead parking lot at Mt. Pinos, perchance?  If I'm ever in CA with a telescope, I'll have to give it a try based on your ceaseless praise of that site.

Yes.  A # of things make and keep the site popular:

--tall pine trees surrounding the parking lot which block all horizon light up to about 20° or so.  It's amazing how much difference that makes for the suppression of peripheral light in your eyes and in the scope.

--high altitude  The air, as a result, is usually very transparent, even when the sky darkness is not great.

--convenience of getting to the site.  You just drive up the road and it dead-ends in a parking lot big enough for a couple hundred scopes (assuming few motor homes).

 

The site will die as a dark site fairly soon because a large number of houses have been approved to build only a few miles away (over 20,000 homes).  Fortunately, that will take several years, but the site, which has averaged 21.45 at night for the last 15 years will be growing lighter with each passing month.  Several of us are actively looking for other sites (all, unfortunately, farther away).

 

[Louis D]

15 hours ago, Don Pensack said:

The site will die as a dark site fairly soon because a large number of houses have been approved to build only a few miles away (over 20,000 homes).  Fortunately, that will take several years, but the site, which has averaged 21.45 at night for the last 15 years will be growing lighter with each passing month.  Several of us are actively looking for other sites (all, unfortunately, farther away).

That's too bad about the housing development.  When I moved into my house in Texas, there were no more than 50 homes in a 2 mile radius.  Now, there are about 20,000 homes.  However, it wasn't the homes that were the worst.  It was the commercial development and 6 lane tollways and interchanges that were built nearby that killed my dark skies.  We now have car dealerships, strip malls, sports fields from elementary to professional levels, amphitheaters, etc.  It all dropped my skies several Bortle levels in most directions.  My best dark skies are about a 2 to 3 hour drive to the west or moderately better 1 hour east.