The Astronomik 2 inch OIII Filter in a 4 inch scope

[John]

I wrote these notes last year but forgot about them. Having now re-discovered them on a memory stick I thought I’d post them in case they are of interest to anyone.

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 that were heavily dimmed by the Baader and Celeston OIII filters I’d owned briefly in the past. Personally I prefer to see a deep sky object (DSO) in the context of the starfield it’s in so I have used 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 that they are clones 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 can be effective even in small aperture scopes.

Realising that I should probably give OIII filters another chance and having now graduated to larger aperture scopes, I’d 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 late 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, and to me 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 (NGC6692 / 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 very pleasantly surprised at the performance of the Astronomik OIII in the 4 inch scope. The views of all 4 of the DSO’s listed were significantly enhanced.

The contrast of M57 (the famous Ring Nebula) against the background sky was noticeably 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 visible when using the OIII and they were more sharply defined with the Astronomik filter than they were with the Ultrablock and, from what I recall, 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. It was quite different 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 enhance 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 visible or not visible 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 components of the Veil – the fainter of which, known also as the Witches Broom, 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 Pickering’s 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 faint object.

Note: My hopes were later realised – when used with the 12” scope in due course with the Astronomik OIII I was able to pick out Pickering’s Wisp quite clearly on dark nights with good transparency.

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 bandwidth that the stars disappear 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 quality and performance seem excellent to me.

[Wizz]

John, your bang on regarding the Astronomik OIII. I tested the Astronomik 2" OIII a few years ago against Lumicon and Televue 2" OIII filters and the Astronomik beat the other two. It showed more in terms of contrast and nebulosity in the diffuse nebula I targeted, the nebula also took on a more 3d effect. I was testing with a C14.

Mark.

[bish]

I've toyed with the idea of an OIII filter but wasn't sure which one. I had heard good things about the Astronomik from other posts so will probably go for that one.

[John]

I've toyed with the idea of an OIII filter but wasn't sure which one. I had heard good things about the Astronomik from other posts so will probably go for that one.

I'm dead chuffed with mine as you can tell from the review. The only thing against them is the cost which is higher than the Baader or Orion OIII's - worth it though IMHO.

[bish]

Hi John. Nice report by the way. It's always a tricky one when it comes to whether to go for cheaper options. I do a bit of photography and face the same dilema with lenses . I think in this case it sounds worth spending that bit extra. At least it's not the difference between spending £500 or spending £1000 to get that bit extra as with camera lenses (not that I'm in position to spend £1000 on anything at the moment!).

[adam88]

Good to know of the good quality filters astronomik produce. That report may have aquiesced me into buying a UHC from them!