Antlia 2.5nm OIII Ultra Filter - NIR Transmission [Deep Sky] Acquisition techniques · Jacob Heppell · ... · 86 · 3676 · 63

Ollie Aplin:
This is exactly what I’ve found Andrew I’m afraid, and the squid one probably worst of all considering how critically placed it is… a lot of them don’t spoil the image and can be dealt with but it’s not the point… Other main perpetrators will be Propus and Tejat Posterior next to the Jellyfish Nebula, it’s annoying. Honestly, swap them for the 3nm if you can and put any concerns to bed I wish I had done.

Thanks, that's what I've decided to do.  I've already put in the return request.  I already own an OIII that gives me halos, so there's no way I am paying $590 for another OIII to give me halos!  LOL

I have a feeling with more subs, the Antlia would show more detail and contrast, but not $590 worth, especially with the halos.

Hopefully I will get a good 3nm filter.  It's surprising, but Agena won't let you return any Antlia OIII filter for halos, which surprises me as it seems most people don't have that issue with the 3nm version.

Also interesting, the Antlia had the exact same 30 unit focus position offset from my SII and Ha Optolong Filters as the Optolong OIII did.  Makes me wonder if it's a blue thing as the RG opto filters were parfocal with the SH ones and the blue and OIII have the same 30 unit offset.

Jake Slater:
I am very glad I found this thread. I've been shooting with my unmodified Canon EOS R for a few years and have decided to pull the trigger on the ASI2600MM. I was looking at the 2.5nm filters in question, and to pair them with the Dark Series LRGB particularly because Antlia mentions it will be the closest match to be as parfocal as possible. I kind of have mixed feelings now. I live in a bortle 5 area, but can drive 1-2 hours to reach bortle 2-3. And maybe once or twice a year I will  to go camping in a bortle 1 area.

So should I just get the 3nm? Or take a chance on the 2.5nm? The thing Im nervous about is being able to actually distinguish halos, bloating, and artifacts between "normal/acceptable" and "clearly this filter has an issue" since this will be my first time ever using filters. Heck, it'll even be my first time using a dedicated astrocam as my main imaging camera. So how could I even go about testing them to see if they need to be swapped?

I would definitely just go with the tried and true 3nm.  The 2.5nm I got wasn't much better than the Optolong 3nm OIII that has serious issues and I wasn't about to pay $560 for barely better than what I already had.  I would still make sure to get it from a place that will accept a return on antlia filters.  AgenaAstro won't.  They won't do it, because Antlia is th eonly manufacturer that will admit there is a star issue with their OIII filters, so Agena won't accept returns on a known issue.  So although it's possible you might get one with the IR star issue, most people I've seen on the forums don't report an issue with them.

As far as knowing if you have the problem, you will be able to see it easily when you use a tool like StarExterminator.  Originally, I thought the OIII filters would just manifest as a bloat or halo issue, which I didn't think would be a big deal.  However, when you the stars the issue will cause StarEx to leave behind a serious blotch where the star used to be.  I've actually noticed both, it either leaves the complete start there or removes it but leaves the massive blotch there.  If the star is not close to the edge of some nebula feature, it's not a big deal, but when it's right next to your feature, it's a real pain to try to remove it.



Here it is with the Channels merged.



It's possible to remove it, but it's tedious and not perfect



Here's an example of star artifacts that are easily removed with the healing brush tool in Photoshop.  But note, this is an image with the stars "removed" and yet, some stars look like they weren't touched at all, and lots of them still left a lot behind.  But these are easy to remove with the healing brush tool as shown in the second image.



This took a whopping 10 seconds to remove the stars with that healing brush tool

Jacob Heppell:
Even though the replacement 2.5nm OIII filter I got from Antlia was an improvement over the faulty one, it's still not perfect as IR stars still tend to get a bit bloated. My first SHO target since getting the replacement OIII filter made this obvious as every star that leaves behind a fuzzy blob in the OIII after starXterminator (pics below) is an orange/red in the RGB. My solution is to just put a lum filter on the back of my 2" coma corrector.


A crop of the above images with the RGB for comparison.




The blobs can be removed in PS with the clone tool but is annoying.
No issues with the 2.5nm Ha, 2.5nm SII, and LRGB Dark Series. Seems like if there's ever a problem, it's always the OIII filter!!

So it still sounds like the replacement 2.5nm Oiii still isn’t as good as the 3nm Oiii? Plus I don’t really want to have to rely on a secondary lum filter in the train. So I think I’ll just go with the safe 3nm SHO. As far as LRGB, the only noticeable difference between the Pro and the Dark seems to be that the Dark doesn’t have the sodium light cutoff. Are there any other big differences I should be aware of?

John Stone:
The 2.5nm and Dark filters are made of quartz so have better transmission.

The 2.5 nm are “flat topped” so they have better transmission as the focal ratio decreases

So basically it just needs a slightly longer exposure time?

Jake Slater:

John Stone:
The 2.5nm and Dark filters are made of quartz so have better transmission.

The 2.5 nm are “flat topped” so they have better transmission as the focal ratio decreases

So basically it just needs a slightly longer exposure time?

The 2.5nm have slightly higher transmission than the 3nm (more light per unit time) but nothing that you need to split hairs over.