Filter placement

[Sirius Bizness]

I have a mix of 1.25" and 2" EPs. I am looking to get an OIII filter primarily to view the Veil. Due to the size of the object I would want to be using my lower power EPs which are 2" only. Therefore I feel I should buy a 2" filter (not cheap!). I have only ever attached filters directly to the EP so a 2" filter here would limit my use. I can buy an extender for my 12mm Nagler but that is more expense!

So this got me thinking and I noticed the 2" fitting on my diagonal (the part that goes into the focuser is threaded for 2" filters. Can I use the filter here thus allowing me to use it with all my EPs? Is there any downsides to this i.e. loss in performance? If the filter attachment increases the barrel length then the diagonal will stick out more than it already does. Can this set-up be used or am I missing something?

[Stephen]

Yes, the end of the diagonal is the perfect place to put the filter

[John]

Yes, the end of the diagonal is the perfect place to put the filter

Yep - thats where I keep mine

Looking at the aperture of your scopes, you may find a UHC more effective than an O-III though. Some of the O-III's do darken the starfields quite considerably in smaller aperture scopes.

[Sirius Bizness]

Thanks for the confirmation guys.

John, I am going specifically after the Veil and have read that it's the OIII that really brings this object out. I have also read that the Astronomik filter has less darkening effect than say the Baader option so that is what I am looking at. I am a little concerned I am buying an expensive filter just to view one object.

[Carbon]

I just picked up a Baader O-III filter and works well with many nebulas. Last night, under a near full moon I was able to see the Dumbbell Nebula from my back yard under polluted skies for the very first time using this filter through my 11" SCT.

[swamp thing]

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Thanks for the confirmation guys.

John, I am going specifically after the Veil and have read that it's the OIII that really brings this object out. I have also read that the Astronomik filter has less darkening effect than say the Baader option so that is what I am looking at. I am a little concerned I am buying an expensive filter just to view one object.

With your 120ED the O-III will be a killer on the veil. I have compared a UHC and a O-III filter on this object through this scope. The O-IIII is far better this was agreed by all four of us that were present.

This comparison was made under a VLM 5 sky (moderate LP). The veil was invisible without either filter.

From a better sky the NA and crescent nebula are both observable with this combo too.

Expensive maybe but it is a great combo. The O-III filter also works very well on M17, M8, M20, M16, M42/3, M97 the rosette nebula, the cone nebula the Pacman nebula......etc let's just say it works on a huge number of targets

[John]

I currently have a 2" Astronomik O-III and it is a killer filter on the Veil and a number of other objects. It's an expensive accessory (£167 new) but the optical quality is excellent and I enjoy using more than other filters I've tried. For an O-III the band pass width is on the generous side which is why I find it pleasing to use even with my 4" refractor.

The Baader O-III is also good but it's band pass width is narrower than many and, personally, I did not enjoy the views through it with smaller aperture (ie: < 8") scopes as most of the background stars were either very dim or invisible.

There are less expensive filter options than the Astronomik and they are worth thinking about. The Astronomik is the most satisfying O-III that I have used in smaller aperture scopes though and I find it provides all the DSO filtering I need.

I don't use filters very often to be honest as I prefer unfiltered views of dso's wherever practical but for a few objects the O-III really makes a big difference.

Just bear in mind that not all O-III's have the same specs which will affect the view you get.

[alan potts]

John,

Seems like a great deal of money for as you say only a few objects. I guess by the cost that Astronmik is among the better filter on the market.

Alan.

[Moonshane]

I use newts and put my 2" filters into the 2-1.25" adater so O can use it with all my 1.25" eyepieces. a word of warning though, check the nosepiece length as it may be necessary to buy e.g. a Baader 14mm fine tuning ring, to move the filter away a bit. I use Castell filters (2" UHC and Oiii) and they provide excellent visual images for the cost with my scopes from my old 90mm refractor to my 16" dob. the view of M42/3 through the UHC and 16" is jaw dropping even from my light polluted home site. I have never used an expensive filter but would like to compare some day.

[John]

.... I have never used an expensive filter but would like to compare some day.....

There may not be a lot, if any, difference Shane - I've heard a lot of positive reports on the Castell filters. Good value too.

[Moonshane]

I suppose another iss ueis that the different bandwidths of different filters mean that direct comarisons are a bit tricky

[John]

I found a band pass width comparison chart on the 365 Astronomy website which is quite interesting:

[Sirius Bizness]

Thanks for that Shane. I will look at that placement option once I get the filter and see which method works best.

John, that chart is very interesting, thanks for pointing it out. One of the main reasons I was going to go for the Astronomik OIII filter was due to the fact that I had read reports stating that the wider band pass gave a brighter view of the surrounding starfields. I have read that the Baader option blocked out most of the stars. This chart confirms that but now I can see that the Castell OIII is even wider. And its £44 for a 2" filter! Thats significant;y less than even the Baader option. In terms of visual experience what does the wider band pass do to the nebula? I like the idea of the brighter starfields but does this make the nebula dimmer?

Considering its price I may just get a Castell OIII and see how I get on. Thanks for the reccomendations.

[John]

.... In terms of visual experience what does the wider band pass do to the nebula? I like the idea of the brighter starfields but does this make the nebula dimmer?.....

I guess the effect will be that the contrast between the nebulosity and the background sky will not be quite as great with the filter that has a wider band pass width.

It's about finding a balance that you find appealing I think. I've no doubt that the Baader O-III does it's job really effectively but my personal preference is to see some background stars even with my 4" scope so I'm happy to loose a little in nebulae contrast in return for that and the Astronomik is the best balance that I've used - so far !

[Moonshane]

you won't be disappointed with the Castell. I have used it at both good and bad sites and it's always useful. A wider bandpass means that more wavelengths of light are 'allowed'. this means that the nebula might possibly not be as contrasty (unless it happens to emit more wavelengths allowed than 'average') but for me the differences are likely to be very slight and I actually think you can sometimes see more detail. e.g. I much prefer M42 with UHC than Oiii as it has more tendtils of detail seen with the former.

[Josh Wilson]

I just picked up a Baader O-III filter and works well with many nebulas. Last night, under a near full moon I was able to see the Dumbbell Nebula from my back yard under polluted skies for the very first time using this filter through my 11" SCT.

What a coincidence! I also viewed the Dumbbell during the full moon and I was very clearly able to see the Dumbbell shape of it. Showed my dad, too and he was able to see the dumbbell shape of it even though he has worse eyes than me.

[Josh Wilson]

Can't edit my post above but what I was going to state after that was that the filter really demonstrates the power of the O-III filter

[Carbon]

Can't edit my post above but what I was going to state after that was that the filter really demonstrates the power of the O-III filter

You can say that again!

I'm very happy with mine. I'll be travelling to a very dark site next weekend and I expect it to exceed my expectations now that I have some aperture with it.

[John]

Under darker skies you may find you don't need the O-III filter so much. My back yard is quite dark and I find the unfiltered view of the Dumbbell and Ring nebulae excellent and, to my eyes, preferable. I can spot the Dumbbell with 8x40 binoculars in fact. For the Owl and Veil nebulae I find an O-III almost mandatory to get satisfying views though. Once I've located the Owl I often view it again without a filter so that I can see the galaxy M108 in the same wide field of view - a filter renders the galaxy all but invisible.

It's interesting to view these objects with a without a filter and see what you prefer however - tastes in views can vary !

[Carbon]

Thanks for that bit of information John. I'll report back next week!

[Moonshane]

I had excellent views of the Veil last night from a reasonably dark site (Glan Conwy) where the MW was visible with direct vision. The Oiii filter really made the bridal veil section stand out remarkably well. I could also fit the whole of the section in which made a nice change. This was with a 6" f5 newt.