Astronomik O-III surpassing expectations

[Special K]

I had a mad moment several days ago and still feel a bit guilty but I ordered a 2" O-III and have had a session or two with it so far. I was dreading looking through this as it could have been an expensive mistake to combine this with the ED80 but I am really surprised how much light this allows through! There's lots of advice not to stick an O-III on scopes below 6" or 8" which even Astronomik publish, but I have to say it's more like using an LPR filter when working at lower powers. Astronomik are reputed to be more relaxed on narrow band pass, than say the Baader, so the choice seems to have paid off.

I've been after views of large nebulae in the ED80 using the 31mm Nagler to gain a big widefield and also a decent 4mm pupil exit to address low surface brightness. The O-III comes in to trim down all that light into the spectrum I'm interested in. Time will tell if im not on the right track but I was pleased to get some results the other night by seeing the edge of SH2-273 in contrast to where the nebula finishes. This type of nebula is pretty tough from my moderate LP area and recognized I'll have to get to a dark area to get the most out of it.

What was stunning was the view of Orion's Sword with the Nagler: the Nebula was of course nicely enhanced by the O-III and suddenly suggested it's huge size, but the stars of the Sword were not muted very bad at all and I felt as though I still retained the context of the star fields of the area which gives such pleasure to view. The Trapezium, though dim, was still there and that's cool!

Perhaps my expectations were way off, so I'm pleasantly surprised. My only other experience with nebula filters is the Skywatcher UHC and that can cut light down real heavy to my eye. The other bonus is the filter screws smoothly into the 2" diagonal which leaves me free to chop and change EPs (including 1.25") at will. This came in handy with the Eskimo. I can't really compare to other brands personally, but can definitely say the Astronomik works for my combo, and seems a very nice O-III filter indeed. I'm looking forward to some projects using this guy!

Clear skies!

[John]

I've been using an Astronomik O-III for a couple of years now and I've found it a superb filter in all my scopes from 4" to 12". Before I bought it I did some research and it does have a more generous band width throughput than most O-III filters which helps when using it with smaller aperture scopes. My preference is similar to yours, I like to see background stars to set an object in context.

My favorite object with the Astronomik O-III is the Veil Nebula in Cygnus which can look stunning on a dark night with a wide field eyepiece

I've recently acquired a Lumicon O-III to compare with the Astronomik so it will be interesting to see how they stack up against each other.

[Stu]

Nice write up Special K.

I too have ignored advice not to use OIII filters in smaller scopes and have had the benefit of some lovely views.

Either in my TV85 or in particular the Genesis with a 31 nag or 21 ethos, the widefield views of the Veil or North America Nebula are stunning under a dark sky.

I do think that gaining the true benefit of these is down to good dark adaptation and dark skies. In these conditions, the views are lovely.

John, it will be interesting to see what you think of the Lumicon (which I have) vs the Astronomik. Given your preferences you may not like it as much as it has a narrower bandpass. Perhaps that's why I need darker conditions to get the benefit in smaller scopes?

Perhaps we can compare at SGLX if we get some clear skies?

[John]

....John, it will be interesting to see what you think of the Lumicon (which I have) vs the Astronomik. Given your preferences you may not like it as much as it has a narrower bandpass. Perhaps that's why I need darker conditions to get the benefit in smaller scopes?

Perhaps we can compare at SGLX if we get some clear skies?

It would be fun to do that at SGLX Stu

I used the Omega DGM NBP filter quite a lot last night and was impressed with it. It's more in the UHC class so a compliment to the O-III's rather than a replacement. 

[Special K]

Thanks guys, and if I make it to SGL X, I'll come have a look at the Lumicon too! It was the other contender I was considering. It would be quite interesting to see how these two compare in a moderate LP environment too.

One question I have is does it make a blind bit of difference to quality whether I put the filter on the diagonal or the EP? I tried both on the Nag, and think they were equal. However the Nag sticks out of the diagonal when the filter is attached by about 5mm. This is due to the Skywatcher diagonal only just accommodating the long neck of that EP.

[Stu]

I don't think it should make any difference to quality where you put it, but I normally go for the front barrel of the diagonal so you don't need to change it with varying eyepieces.

I suppose, theoretically, you are also a little bit further up the light cone so are using more of the filter at that point, but I am sure any difference is not visible

[Saganite]

At Kelling Heath, in September, I was able to see the Veil Nebula for the first time, through a club colleagues 20" Dobsonian,

with an Astronomik O-III filter, and a Televue ethos ( I don't know the focal length), and under the most superb dark sky.

An unforgetable experience.

[swamp thing]

Hi Kevin

I believe the advice offered about aperture is to do with the number of available targets rather than anything else. As you have noticed O-III filters work best with large exit pupils. This means that the scope one is using it in must have sufficient image scale, or the target object must be large to get the most out of it.

Small scopes lack the image scale required to get the most use out of line filters. Yes all line filters will work in any scope, regardless of aperture it's just the amount of suitable target objects will be small, with smaller scopes.

On the North American nebula one doesn't even need a scope to see it, simply holding ones O-III filter up against ones unaided eye will reveal this target. You cannot get a smaller aperture than that

[rwilkey]

Great report Special K, thanks for sharing.  I am always interested in this subject as I have gone for the Lumicon, and wondering if I have made the right choice.

John - I will be most interested in your comparison report!

[Special K]

Hi Kevin

I believe the advice offered about aperture is to do with the number of available targets rather than anything else. As you have noticed O-III filters work best with large exit pupils. This means that the scope one is using it in must have sufficient image scale, or the target object must be large to get the most out of it.

Small scopes lack the image scale required to get the most use out of line filters. Yes all line filters will work in any scope, regardless of aperture it's just the amount of suitable target objects will be small, with smaller scopes.

On the North American nebula one doesn't even need a scope to see it, simply holding ones O-III filter up against ones unaided eye will reveal this target. You cannot get a smaller aperture than that

Nope, you can't get a smaller aperture than that or you'd be going the wrong way!

I understand the transmission of light needs to be high for best results, but some objects which are smaller have a higher surface brightness. Taking the Eskimo Nebula as an example, I would expect the O-III to improve the view in a small scope up to a given magnification (which could be subjective). At 50x, the result was nice but at 120x, the pupil exit gets really small at .67mm which doesn't allow in much light at all. Is that what you mean by image scale? This is where the aperture comes in isn't it? If we put the large scope and small scope side by side and get them both to 120x, the larger aperture has gathered more light due to size. I think I've talked myself through that now, thanks!

[jetstream]

I read somewhere that placing a filter farther away from the EP can cause problems, but only if the filter isn't of high optical quality and may be f ratio dependent too. The Lumicon and Astronomik have no such issues - the view is the same threaded on the EP or the extension adapter. Still this is one reason I bought a 1.25" Hb....to screw on the 25mm Plossl directly- just playing the odds. It will be replaced with a 2" Astronomik soon though.

[swamp thing]

Thats pretty much it Kevin.

The number of nebulae that are large and bright enough for a small scope to give good results is limited. Don't get me wrong the O-III will work just as well in a 2" scope as a 20" scope its just the number of available targets is greater in the larger scope due to it's superior image scale. 

What most don't get is they think that the small scopes don't collect enough light to use the line filters. This isn't strictly true. Thy do collect enough light it's just when their exit pupils are large enough their image scale often isn't 

[John]

I read somewhere that placing a filter farther away from the EP can cause problems, but only if the filter isn't of high optical quality and may be f ratio dependent too. The Lumicon and Astronomik have no such issues - the view is the same threaded on the EP or the extension adapter. Still this is one reason I bought a 1.25" Hb....to screw on the 25mm Plossl directly- just playing the odds. It will be replaced with a 2" Astronomik soon though.

I went for a 1.25" Lumicon H-Beta filter. I didn't think it would get much use due to the limited range of objects that respond to the filter and I figured that I'd be using 1.25" eyepieces with it as wide fields put too many other field stars in the view when searching for the Horsehead Nebula.

I've yet to see a darn thing with it (that I cannot already see with other filters)

It's time will come ......

[jetstream]

Yes, not many "showcase" Hb for sure... only one for me so far- the California- if we can call a cigar shaped dark thing showcase lol!   I hope to pick apart the huge IC1318 area, which will keep me busy for a while.

Another good choice made with the 1.25" John, for your purpose and conditions. The Flame neb is a good HH test and so is the California, both should be easy to see for a good chance. Thing is how good is good? I obs the Flame so many times and no HH or IC434 at all.

Compared to the Veil, NA neb, M57 etc the Hb obects are very underwhelming to say the least.

To keep on topic a bit for myself I love my Astronomik OIII, very high quality with a nice flexible passband.

[Special K]

No worries about the topic, Gerry, the more you guys talk the more I get out of it! It was hearing on these boards that the Astronomik was a little more generous, and also a higher quality filter can make a difference that swayed me. The Veil and NA have since eluded me so they're pretty high up the list!

Returning 'off topic' I thought you had had some success with the Hb on the Flaming Star Nebula and environs. IC1848 Soul Nebula might benefit as well.

[John]

The Veil with the Astronomik O-III and the Nagler 31 are a marriage made in heaven Kevin

That one object is the primary reason that I splashed out on such exotic toys. Expensive entry fee but the show is sure worth it - time and time again

[jetstream]

No worries about the topic, Gerry, the more you guys talk the more I get out of it! It was hearing on these boards that the Astronomik was a little more generous, and also a higher quality filter can make a difference that swayed me. The Veil and NA have since eluded me so they're pretty high up the list!

Returning 'off topic' I thought you had had some success with the Hb on the Flaming Star Nebula and environs. IC1848 Soul Nebula might benefit as well.

Yes I did have good luck on the Flaming Star, Tadpole nebs in Hb, I love the area. The actual Flame neb is best seen by me with no filter and using a low scatter EP really helps when we keep Alnitak just outside the FOV.

In my earlier post I may have insinuated the Flame is a good Hb test.... I meant that the Flame (unfiltered) is a good test of conditions for trying to see faint nebulae (HH).

[Special K]

Well, I've never been able to see anything where the Pacman Nebula NGC281 should go, but spotted a bit of this last night, so I'm on a roll here! 

I could detect nebulosity around the central star cluster IC1590.  After staring for a while, I thought I could determine a bit of an bowed shape to the North side, or something like a banana shape.  Had a quick sketch/scribble where those stars were (I won't abuse you with that sketch now) and pleased to see this line up with part of the blue gassy bits you'll see in photos.  I was disappointed to be unable to see this in the past, so happy now I've got a glimpse!

Clear skies, and wish you all an Oxygen fix in the very near future!

[swamp thing]

Nice one Kevin. It is quite shy if there's any LP about and can be a tricky customer.

[allen g]

I thoroughly enjoyed reading this thread.  I have the Astronomik (UHC) and Lumicon (0111) and agree both are 1st rate filters - well worth their price.  With a 31 nagler or 21 ethos the views of veil are gorgeous (in my 4" refractor) - yes a marriage made in heaven.   The notion that these filters are not suitable for small aperture scopes makes no sense - I agree with above experiences.

[Special K]

I thoroughly enjoyed reading this thread.  I have the Astronomik (UHC) and Lumicon (0111) and agree both are 1st rate filters - well worth their price.  With a 31 nagler or 21 ethos the views of veil are gorgeous (in my 4" refractor) - yes a marriage made in heaven.   The notion that these filters are not suitable for small aperture scopes makes no sense - I agree with above experiences.

Glad you enjoyed it, Allen. This time last year I drove out to Joshua Tree for an early morning look at the sky and wished I had the above mentioned gear with me then. I did have my 10x50's and got a lot out of that morning. The ambition was to make it out to Hole in the Wall camp at Mojave but that didn't materialize. Enjoy those pristine desert skies for me!