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Hi folks. Just putting something out there about the Antlia 2.5nm OIII filter in case anyone else has noticed. Recently I purchased 2" Antlia 2.5nm SHO Ultra filters. I've had a chance to try all filters and compare them to the Antlia 3nm version (I've had for a couple years now) on the same nebula (NGC3572). I have no issue with the 2.5nm SII and Ha filters. However, when directly comparing the 2.5nm OIII and 3nm OIII, the 2.5nm OIII had a number very small artifacts (small fuzz patches) and a couple of stars were noticeably brighter than they were in the 3nm image. I've attached some images to show what I mean (whole image, a fuzz patch, and brightened star in each 2.5nm and 3nm filter). If you jump between them, the artifacts and brighter stars become obvious. One of my fellow astrophotographers, who uses 50mm Antlia 3nm filters, recently imaged the same nebula (NGC3572). His 3nm OIII subs were largely the same as my 3nm OIII subs in that they both showed no artifacts or brightened stars. At this point, I began to suspect that the 2.5nm OIII filter is passing light out of band; likely in the near-infrared region (~700-1100nm). To test the theory, I took an OIII image of the same nebula (NGC3572) but with the Luminosity filter screwed to the back of my coma corrector (blocks the UV and near infrared). The resulting image gave no fuzz patches and the few stars that were brighter than normal, had returned to the level of luminosity seen in the 3nm OIII filter. For some additional evidence, I did an experiment whereby I pointed by imaging train at a light source (just a light bulb) and took frames through each 2.5nm SHO filter (~30kADU) with and without a lum filter (UV-IR blocker) in front of the imaging train. The results show that the OIII frames were duller by 1.36% with the lum filter (mean ADU reduced from ~31000 to ~30600). The SII frames were duller by 0.24%. The Ha frames showed virtually no change (<0.1%). So I can only conclude that the OIII filter is transmitting light out of the visible band. This would explain why some stars brightened appreciably in the OIII subs (those stars may give off a lot of NIR) and perhaps those "fuzz patches" are legitimate (i.e not artifacts) but not composed of OIII. I don't know if this is a problem for every 2.5nm OIII filter or just mine. I have engaged with Antlia and they're apparently going to test the transmission up to 1150nm (their spec chart stops at 850) but they are very slow and I don't know the results yet (been a few weeks). I'm hoping I just got a defective filter. Otherwise it could be an issue with the whole line up.       |
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Jacob, thanks for the excellent, detailed information. Have you any update? Did Antlia test the OIII filter? Thanks for any update. Steve |
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| I'm sorry that you are having this trouble. I was worried about these filters too and it definitely looks strange. It is a good idea to do these experiments, I'd suspect the same thing. In addition, maybe you can try to see if it passes the IR light from the remote's LED. They should be around 950nm if I'm not wrong. |
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it looks like the 2.5nm gives halos and bloating, so I'm not getting those in the future thx |
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Hi, I had similar effects with the Antlia OIII 2.8 nm ultra some months ago. It produced blotches near IR stars that looked like tiny Planetary Nebulae after the stars were extracted. It became apparent when I compared the filter with an Astronomik OIII 6 nm. By blinking the two results it was easy to identify these blotches at the positions of more than 30 Mira stars and other long period variables inside the test image. I sent the filter back and bought a 3 nm pro instead. Antlia replied to my complaint with a very nebulous answer that said nothing. CS Pete |
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So, What happens if you stack your Antila 2.5 nm OIII filter with your UV/IR Cut filter? Does that make the 2.5 nm noticeably better than the 3 nm? |
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Steve Viltoft: Hey mate. The filter is on it's way back to China for analysis. Once it arrives, they'll send a replacement, which Antlia said was tested out to 1150nm. I'll post here the results when I get the filter but it's at least a few weeks away. If the new filter still has the same problems, I'll get the 2" OIII 3nm instead. I had no such issue with my 36mm OIII 3nm filter. |
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Kay Ogetay: Good suggestions with the remote. If the filter wasn't already on it's way to China, I would try it. Cheers. |
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Hi, I had similar effects with the Antlia OIII 2.8 nm ultra some months ago. It produced blotches near IR stars that looked like tiny Planetary Nebulae after the stars were extracted. It became apparent when I compared the filter with an Astronomik OIII 6 nm. By blinking the two results it was easy to identify these blotches at the positions of more than 30 Mira stars and other long period variables inside the test image. I sent the filter back and bought a 3 nm pro instead. Antlia replied to my complaint with a very nebulous answer that said nothing. Yep that's exactly my problem!! Beginning to think the problem lies with the whole production of 2.5nm OIII filters and not just mine. Communication with Antlia has been ok-ish. They were generally responsive to my emails but they're replies were very short and the English was very bad (Although I don't hold that against them as I don't speak a word of any Chinese language!). |
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John Stone: I did take a single OIII sub with the Lum filter screwed to the back of my coma corrector, which eliminated the problem as I saw no blotches and the star brightness was normal. I didn't take enough to get a stack though. A possible solution is to permanently screw a Lum filter to the back of the CC but I'd prefer not to if I can help it. Would rather just get the 3nm, which I know doesn't have the problem. When directly comparing the 3nm and 2.5nm subs from the Ha and SII, I did see a noticeable improvement; probably because less light pollution was transmitted. |
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it looks like the 2.5nm gives halos and bloating, so I'm not getting those in the future I actually found the 2.5nm gives less haloing than the 3nm. If you compare the bright star in the top right you'll see what I mean. As for the bloating, the 3nm sub was taken with much better seeing than the 2.5nm sub hence the extra bloating in the 2.5nm image. I've since taken more subs with the 2.5nm image under much better seeing. |
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Here you see a comparison of some of my filters under IR Light from my Wyze Cam: That should be enough proof ;-) The reducer does not flatten that wavelenght. Here is how the effect looks with a Mira star in an HOO:  These are the positions of LPV stars that showed the effect in my test image:  Astronomik 6nm and Antlia 2.8nkm in a gif. Watch out for the blinking stars at 6:30 and 8:30:  And finally a tight gif comparing Astronomik 6nm and Antlia 2.8nm:  Cheers Pete |
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Jacob Heppell:it looks like the 2.5nm gives halos and bloating, so I'm not getting those in the future I had made a direct comparisin with a 3nm and a 2.8nm side by side in the same filterwheel at that time. They were on par, maybe the 2.8 showed a tiny tiny bit more structure in extremely dark OIII emissions. Concerning halos they had performed identical. |
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Here you see a comparison of some of my filters under IR Light from my Wyze Cam: Wow! No room for doubt now! Thanks for sharing. Kind of surprised I've never heard of this issue yet (or I just don't spend enough time on the forums). Do you think it was just your 2.8nm filter or do you suspect it's a problem with the whole production of 2.8nm OIII filters? |
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Jacob Heppell:it looks like the 2.5nm gives halos and bloating, so I'm not getting those in the future Yeah cool. For me, the 3nm vs 2.5nm difference was most obvious in the [SII] structure. Ha looked marginally improved but otherwise on par. When comparing the advertised bandpass of the 3nm and 2.5nm [SII], I noticed a slight red shift, which shifts the maximum transmission of the SII doublet from 671.6nm to 673.1nm. with only a 0.4nm blue shift of the bandpass (likely with my F4 system), both doublets will be at maximum transmission. This may explain why the SII stands out a bit more in the 2.5nm.  |
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| This is awesome information. Could you please update this in a month or so when Antlia gets back to you? I was about to pull the trigger on a set of the 2.5's, but will hold off for now hoping for news on this. Cheers! |
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Steve Viltoft: Will do mate. |
