UHC vs OIII - a short review/comment

[Piero]

I have been comparing my Astronomik UHC and OIII filters for a while on my Tele Vue-60, and I thought about sharing some comments about them.

I apologise in advance but this is not a ‘scientific review’. The reasons for this are:

1. there are already comprehensive reviews about the topic on the internet (e.g. work by Dave Knisely);

2. exit pupil argument (I will comment later);

3. the whole subject is highly dependent on sky conditions (in particular Dark vs Light Polluted (LP) skies);

4. there is also a fairly important personal choice (in particular user preference for observing nebula extension vs contrast);

5. there are large differences in UHC filters and OIII filters between manufactures.

Comment on the 2nd point. Although I agree that the exit pupil is a much more reasonable measure than the simple magnification because the former is in a way independent on the telescope in use, I also think that there is a warning with the exit pupil too. In particular I believe that the exit pupil depends on the eyepiece in use. In my personal experience, I do not see a significant decrease in object surface brightness and sky brightness between my Tele Vue Plossl 20mm (18x, 3.3mm exit pupil) and my Tele Vue Panoptic 24mm (15x, 4.0mm exit pupil). Also, I found that the Plossl 20 shows more contrast and works slightly better with nebula filters for this reason I guess. I doubt that this is because of an increase in detail due to increase magnification as I am not actually able to distinguish between the two (15x vs 18x).

In conclusion I think that exit pupil concept works consistently if you compare eyepieces of different focal length but same optical scheme, whereas you may obtain potentially inconsistent results otherwise. In my limited opinion, I think it is worth to report both magnification and exit pupil in order to offer the reader a better idea about the observing conditions.

Coming back to the original discussion, I have tried my UHC and OIII filters a bit under a sufficiently LP sky (NCL) and moderately LP sky (CAM).

Here are my conclusions for extended nebulae (mainly Rosette and Orion nebulae) where both filters work:

(Eyepieces: Panoptic 24mm and Plossl 20mm)

1. the more the sky is polluted the more OIII seems to work better than an UHC (on the Orion nebula, whereas the Rosette was invisible);

2. under moderately LP skies (and I guess under dark skies too), I have not found important differences except for those highlighted on the 4th point above (contrast vs extension which are more user preferences to my mind). (Rosette slightly perceived with averted vision, Orion quite clear instead). Between the two, I prefer the UHC, as this filters less light (showing more stars), and it is easier to reach focus;

3. both filters work with my 60mm telescope

 

In conclusion (so far):

From my point of view, an OIII filter is an excellent filter under LP skies for most of the targets. Under moderate LP to dark skies, an UHC is more than sufficient (and in my opinion better) for most of the extended nebulae.

My suspect is that an OIII filter will make a difference on some extended nebulae (e.g. Veil nebula), and on planetary nebulae. I am comparing more nebulae (planetary and extended) in the next months and continuing reporting.

Which one to buy? I would consider the type of sky first and then the objects of interest.

Thanks for reading, and I am happy to hear your comments.

Piero

[swamp thing]

Intersting findings Piero.

I've always found the UHC worked better under LP skies for me as I find it more difficult to get the proper dark adaption that the O-III demands. I will say though I use a Lumicon O-III version which is slightly more aggressive, perhaps the Astronomik is a better in town alternative.

From darker skies I find the more aggresive filters work better for me. Funny old game filters

[John]

Interesting stuff Piero

I tend to prefer O-III filters because they make a really significant difference on some objects (the Veil and Owl Nebulae for example). Where the difference is slight I generally prefer not to use a filter although I do have the Omega DGM NBP filter which I use from time to time on M42 and one or two objects when I fancy some slight enhancement.

For a long time I was happy to use the Astronomik O-III as my only deep sky filter and it did a great job with my scopes fro 102mm to 300mm aperture.

Some interesteing targets for further comparison would be the Eskimo Nebula, the Little Dumbell Nebula, the Owl nebula and of course the Spring / Summer glories of The Ring, The Dumbell, the North American and, in my opinion, the finest of them all, the Veil Nebula complex.

I'll be interested to read your further reports

[Piero]

I agree with you Steve, filters are a funny thing! Although our telescopes are "only slightly" different , the results are opposite!

Do you prefer your OIII filters on all nebula targets? 

[swamp thing]

ATM yes, it's all I've got

I sold my UHC a while back and haven't got round to replacing it yet.

[Piero]

I hope I did not mislead in the point 2 of my conclusion. When I said that I did not find a significant difference, I meant between the two filters. There was clearly difference between using a filter or not using it at all.

@John: Thank you for your interesting comment. I am very curious to try the filters on the objects you mentioned If I manage to spot them with my 60 of course!    

When I was in NCL (LP sky) I tried the Ring nebula with my UHC at 15x, 18x, 51x but was not able to see it. I suspect though that this happened due to the LP sky, as I spotted the ring with my previous 15x70 one time in Devon (without any filter). Never tried with OIII filter though.. 

Dumbell nebula spotted in NCL, with both bins and refractor at 15x without filters. I will try it again with the filters to see whether it improves.

My feeling is that the OIII filter works better on planetary nebulae and therefore it should make a difference on a few targets you listed.

I will try these targets with the blinking technique, otherwise I think I might be fooled by the very low magnification. 

I am going to buy a bike soon, so that I can go to the countryside when I have time, and see under fairly dark sky. And then report! 

[swamp thing]

What I find is the extra contrast offered by line filters can give more definition to fine structure and detail within a nebula.

Take M8. I find an UHC gives more spread of gas if you like, but the line filter really makes the dark areas stand out. It kinda pulls detail forward.

Its best to have both filters IMHO plus an H-Beta. For me its a question of cash flow ATM (it's all flowing the wrong way )

[Piero]

I think you (Steve) said an important point when you said "more definition to fine structure and detail within a nebula. " Your instrument is able to show inside the nebula, whereas mine is limited to the external shape. 

Therefore, I believe you are right about your better experience with your OIII filter, as your aim is to increase those fine details. In my case instead, it is about bringing the object alive and with my limited experience, I found (so far) that the type of sky can suggest the type of filter to use. 

Having said this, I believe that there are objects which can be very difficult to see with an UHC or in any case can be easier with an OIII filter even with my 60mm telescope. 

I want also to highlight that I am not biased toward one or the other. I think there is room for both. I suspect that behind the user preference towards one or the other one, there are multiple factors. At least: 

1. interest in contrast vs extension. This is affected by telescope aperture: for a large telescope, an user can see into it, whereas for a small telescope an user focuses more on the overall shape. 

2. sky conditions (dark sky vs LP skies)

3. target preferences. Again, this is somehow a bit related to aperture. My 60mm will only spot few of the planetary nebulae whereas your >10"s are able to investigate in detail.

4. ... 

The fact that people report very different experiences show that a specific choice of filters is a tricky subject. For the limited targets I tried them, I can say that both work. However, for "working" I mean that improved / revealed a nebula w.r.t. no filter. This was very evident. Orion nebula showed the outside border clearly and the whole shape increased 3 times. Rosette nebula was slightly detectable with averted vision. With none of them I was able to see complex details inside the nebulae, as I was looking at 15x or 18x! 

[YKSE]

I've used both UHC and OIII filters, though not any comparison between the two, have mostly just followed the general guideline that OIII for some specific nebulae and UHC for others. David Knisely's review is certainly the most comprehensive summary, also comments above are vaulable for many, I'd like to make comments only based the characters of these filters:

1. Lights visible to our eyes is in wavelength about 390nm to 700nm, these filters block lots of light in the visual spectrum, UHC only lets through lights between 470nm and 520nm with some loss(no 100% in the range), no wonder the view gets dimmer, and thus some what larger exit pupil required. The narrower the filter bandwidth, the larger exit pupil should be used.

2. OIII filter is narrower band filter than UHC, only around 501nm, slight bandwidth difference can have quite big impact, e.g. Astronomik OIII is characterized as broader band OIII, it has 12nm range,

http://www.teleskop-express.de/shop/product_info.php/info/p6623_Astronomik-ASO32-OIII-Filter-12-nm---2-inch-mounted.html

while baader 10nm OII is said to be narrow band OIII

http://www.teleskop-express.de/shop/product_info.php/info/p489_Baader-2--O-III-filter---10nm-Narrowband---more-contrast.html

3. Because of the difference in bandwidth, assuming the same good transmission,  It might be just possible that one filter works better than the other in some nebulae, not the others, or in extreme case,  for some large nebulae, it may depend on which part of the nebulae we're looking for details.

[Paul73]

Good report Piero

Here are a couple of my random thoughts on the two filters (extremely unscientific):

I use the OIII a lot for finding Planetary Nebula which can look pretty starlike at low mags. But, pop in the OIII and the stars dim and the PN jumps out. The UHC isn't bad for this either, but the OIII is significantly better.

The UHC's more "Natural" view is a lot more pleasing to a lot of folks.

On the Orion Nebula - One shows a greater extent of the Nebulosity, the other shows more detail of the brighter parts. This is one target that I normally do naked (sans filter).

The OIII kills some nebula, where as the more subtle UHC helps. Eg the Flame Nebula through my 10" dob. It is likely that this finding may be reversed with Big aperture and really dark sky.

The Veil - worth owning a OIII for this alone. Although you can get some OK views with a UHC.

Paul

[John]

Paul - your filter experiences are uncannily similar to my own

[Piero]

Thanks very much Paul! Your finding are very interesting! 

I am really curious to try those filters on planetary nebulae and as you (and many other in SGL and CN) said, I also suspect that an OIII will make a significant difference. 

@John: I started looking for the Eskimo nebula. I need to study a bit better the area though, as I suspect this might be a challenging target for my 60mm. I will let you know. 

Yesterday, I watched Orion nebula again with filters (30x, 2.0mm e.p., 2.1deg). Although it is nice without it too, I have to say that with a filter in this scope became gorgeous! 

I tried again the Rosette Nebula (15x, 4.0mm ep). Completely invisible with UHC filter. With OIII you could detect there was something there. Something like a grey patch without a defined border and very confused with the sky behind. I could detect it by moving slightly the telescope and see that this patch followed the cluster. I don't know if I really managed to see it, but this patch was not there with the UHC filter. 

Question: 

NGC2264 (Cone nebula). Is it possible to see a tiny patch just below 15 Mon with a UHC or OIII filter? For tiny in my scope, I mean something like 2-3 times 15 Mon, so really tiny but with fainting borders. It seems like a dim light. Have I imagined it or do you see it too? 

[zubenel]

Interesting that OIII filter is better for Rosette nebula than UHC.  I saw quite an improvement with UHC filter (Omega DGM NBP) using Skywatcher 200/1200 dob. Maybe it is another reason to get OIII too.

As for NGC2264 (Cone nebula) I doubt you saw it. As I read it seems that this object is even harder than Horsehead. Personally, I haven't tried seeing it but looked at cluster several times. Nice Christmas tree. Didn't noticed nebula but wasn't looking for it. Though at low magnification it could be that some close stars in the cluster could coalesce and look like patch? Or maybe some reflection from the star.

[Piero]

Thank you for sharing your experience and welcome to SGL, Zubenel.

With my telescope (which is quite different from yours) I think the OIII works better under mild LP or LP skies. I have not tried a comparison between the two filters under dark skies. 

Please, consider my findings as very preliminary though. I consider myself a beginner despite the number of posts that I have. And on filters I am a novice! There are people in SGL ways more expert than me, particularly on these subjects. I invite you to ask them as well or to start a new post before buying. 

I have not seen the Cone nebula. I saw a bright area similar to a tiny patch without clear structure just underneath 15 Mon, using the OIII filter. As you said, it could be that that was the result of a cluster of close stars and at low magnification this seemed a patch. Considering the difficulty of seeing that nebula, I also doubt I saw a little bit of it. Certainly I didn't see the whole nebula though! 

Have you tried your UHC filter on that region? 

Piero

[zubenel]

Piero,

I haven't tried UHC on Cone nebula region. Also without filter I observed this region with Celestron Ultima ED 20-60x80. Cluster was visible but nebula wasn't. On filters I am myself beginner. Just have this one nebula filter (UHC) and used it on some nebulae. But what I noticed that of all nebulae I tried Rosette was the one which improved the most. Without filter it was barely visible and with filter I could see almost all the bublik around the cluster. Quite an improvement!   Maybe I'll try it on NGC2264 too. See how it changes the view. Just need a clear night which is rare this time of the year.

[Piero]

Wow! that's interesting!   Curious to hear from you whether you see something on ngc2264! 

Clear skies to you!