Astronomik 2 inch OIII Filter in a small scope

[John]

I’ve owned a couple of 1.25” OIII filters in the past, one made by Baader and one branded Celestron although the looked, and seemed to perform, identically to the Baader and I strongly suspect that the Celestron name on the label was the only difference.

I’ve not found the views through OIII filters in the past that satisfying to be honest. While the nebulae, especially planetaries, are made to stand out more, this has been at the expense of the background stars which were heavily dimmed by the Baader and Celeston filters. Personally I prefer to see a DSO in the context of the starfield it’s in so I have stuck with a succession of UHC-type filters up to now.

Since parting with these earlier OIII filters I’ve learned that the Baader OIII (and therefore the Celestron if my theory is correct) has one of the narrowest band passes of the OIII’s on the market for visual use – which explains why the stars were dimmed so much of course. I was rather surprised by this because the Baader UHC-S filter which I have also owned, has a rather wider band pass than other UHC filters – the UHC-S is really a halfway point between a Narrowband UHC and a Broadband filter which explains why it is effective even in small aperture scopes.

Realising that I should probably give OIII filters another chance and having now graduated to a 12” aperture scope, I’ve been looking out for another type to try, this time in the 2 inch fitting as my eyepieces in the 13mm to 31mm focal lengths are all the larger fitting now. So when a 2 inch visual OIII filter from the well regarded German Astronomik company came on the market at a reasonable price, I pounced.

The filter duly arrived and, of course, the clouds arrived with it. But by 1:00 am the clouds had broken up and I had a nice period of clear skies for an hour. It was too later to set my 12 inch Lightbridge dobsonian up so I decided to use my 4 inch Vixen ED refractor to have a peek at some suitable DSO’s with the Astronomik OIII. I was not expecting much because Astronomik’s own advice is that this filter is intended for 6 inch and above aperture scopes.

The Astronomik OIII is a very well finished product, IMHO its build quality is a bit better than other filters I have owned. The Astronomik is stored in a very robust plastic case and sits firmly on a bed of protective foam when the case is shut.

I concentrated on 4 DSO’s that should benefit from an OIII filter, all planetary nebulae:

The Ring Nebula (M57) in Lyra

The Dumbell Nebula (M27) in Vulpecula

The Owl Nebula (M97) in Ursa Major

The Veil Nebula (NGC6992 / NGC6960) in Cygnus

As a comparison filter I used my 2 inch Orion Ultrablock which is classed as a Narrowband UHS-type filter. The eyepieces used were Naglers and an Ethos from 31mm to 5mm in focal length giving a range of powers between 21x and 133x and an exit pupil range of 4.77mm to .77mm.

I was pleased and pleasantly suprised at the performance of the Astronomik OIII in the 4 inch scope. The views of all 4 of the DSO’s listed were enhanced, but some a little more than others.

The contrast of M57 (the famous Ring Nebula) against the background sky was noticably improved when using both the Ultrablock and the Astronomik OIII with the inside of the ring looking darker than the unfiltered view. The OIII was very marginally better I felt but, with a compact object such as M57, the differences between the 2 filters were very slight indeed. The surprise came in that many background stars were still visable when using the OIII and they were more sharply defined than in the Ultrablock or other filters that I had previously tried. I do like to see DSO’s against some background stars and the Astronomik OIII impressed me in this respect contrary to what I’d experienced using the Baader and Celestron OIII filters a while ago. The fact that the stars remained really sharp as well with the Astronomik was another bonus.

The contrast improvement when viewing M27 (the Dumbell Nebula) was more marked between the Ultrablock and the OIII with the OIII showing noticably more than the “apple core” of nebulosity that the ulfiltered view of this nebula shows. M27 is a much more extended object than M57 which seems to contribute to the edge in performance that the Astronomik OIII showed.

M97 (the Owl Nebula) is often a tough object to find with a 4 inch scope from my back garden despite the fact that it’s high in the sky most of the time. On a good night I can find it without using a filter because I know where to look but using a decent UHC (eg: the Ultrablock) makes it “jump out” and I found the Astronomik OIII had the same effect only a bit more so. At the SGL4 Star Party in April this year a few of us had fun trying to spot the dark “eyes” of the Owl Nebula under a great sky with a 12” dobsonian and using the Ultrablock filter – the best we could do was to see vague hints of one of the eyes. I suspect the Astronomik OIII might be able to do a tiny bit better than that – I can’t wait to try it !.

In the case of the Veil Nebula, the brighter portion is normally is either only barely visable or not visable at all with the 4” scope from my back garden needing a UHC filter such as the Ultrablock to pick it out with any certainty. With the Astronomik OIII I could see both of the main componants of the Veil – the fainter of which, known also as the Witches Broom (NGC 6960), still needing some degree of averted vision. Using the 31mm Nagler eyepiece and the OIII filter I can see both portions of the Veil in the same field of view – it’s a lovely sight on a dark, transparent night. Other sections of the Veil complex such as Pickerings Triangular Wisp have so far eluded me with the 4” scope even with the filters in. I’m hoping that my 12” dobsonian and a good, dark Autumn night will provide a glimpse of this ellusive object.

In summary I’m very pleased with the performance of my 2” Astronomik OIII filter. I delivers improved contrast over the UHS-type filters I have used in the past while not cutting out so much bandwith that the stars dissapear in smaller aperture scopes. Optically it seems excellent quality and keeps the stars looking like pinpoints more than other filters that I’ve tried. The Astronomik OIII filter is an expensive item at around £160 new, but it’s build and peformance seem excellent to me.

John

[Doc]

Great report John which made a very interesting read.

I must admit until Saturday I had never used a O111 filter or infact a UHC either. They take some getting used to and like yourself much prefer to see the desired object in it's natural enviroment. The O111 filter blocks out so many stars and adds this green tint it does make you wonder if it's worth it.

I never tried the blinking technique explained to me by Carol so with this in mind I cannot wait for the next clear night. What I was doing was trying to locate the nebula with the O111 already in place. of course this made it hard as I did not know what was the neb and what was the star as the filter does not block out every star only the faint ones.

Blinking the filter will show you which one is which.

For me the jury is still out, I still say a dark sky and a widefield is so important in this game.

[Beulah]

Good to get another review of the O111 - I too am waiting for darker skies, although I was pleased with the viewing I had from my filter. Planning to add a UHC to my collection soon.

[John]

Thanks Mick and Sam.

One of the things I have learned over the past couple of years is that there is quite a bit of variation within the OIII and UHC filter "niches". It is worth looking at the bandpass graphs for each makers filter as they do vary which has a noticable effect when you view with them. The Astronomik OIII seems to have a slightly wider bandpass that the Baader OIII which probably explains why the stars are dimmed more by the Baader. Under dark skies and with a large aperture I guess the Baader should show a little more contrast on nebulae than the Astronomik so I guess you can, with some care, select a filter which matches your scope and observing conditions.

John

[Astronut]

I've had the Astronomik UHC filter for some time now and the best views I've had of the Veil nebula were with this filter attached to a Pentax 30mm XW and my 12" dob but it seems to work better at a dark site. I saw both eyes of the owl nebula at the Salisbury bash 2 years ago but one of them needed averted vision.

I've been thinking of getting the Astronomik OIII to complement it and after reading this it's at the top of my list as I suspect it would work better than the UHC at light polluted sites such as my backyard.

[Andrew*]

Superb report John,

You've prompted me to realise that I only have a 1.25" OIII filter - I must get a 2" for my 2" eyepieces. I have a soft spot for OIII filters, thanks to some mind-boggling views of the veil nebula. I don't much mind losing somee stars if I get better detail in the nebula.

I think I will avoid Baader for the 2" as they are very narrow. I do wonder how the Skywatcher one bears up.....

Andrew

[Doc]

Well I have the Baader 2" O111 filter John, you can try it at Salisbury.