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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. |
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Well folks I've received the replacement 2.5nm OIII Ultra filter. I was told it was tested to 1150nm but was never given the results. So I decided to do some more NIR pass testing. I decided to take Kay Ogetay's suggestion and use a remote control (in my case the air con remote as the NIR source). Apparently they use a 940nm diode. The test was rather simple. I took my optical train (CC, OAG, FW, Camera), placed an NIR pass filter on the end of the CC (I have a Hoya R72 IR pass filter)(transmits above 720nm) , took a "blank" image (5sec) (I was in a dimly lit room), then took another image (5sec) but while pointing the remote at the NIR pass filter and pressing it 10 times. To my surprise, all seven LRGBSHO filters in the Antlia Ultra Series (2.5nm for SHO) showed incredible NIR transmission (940nm). For good measure, I repeated the experiment but I placed my 36mm Pro Series filter (3nm for the SHO)(haven't sold them yet!) on top of the Hoya R72. As I suspected, I observed very minimal NIR transmission with the remote control (940nm) I repeated the above experiments without the coma corrector and that was even worse!! I saturated the camera sensor when I pressed the remote 10 times so I dropped down to pressing it five times within the 5sec exposure. I've screenshotted my results (see below), which shows the mean ADU for each combination (I saved all the fits files as well). For the camera settings I use, a completely blank exposure of short duration gives a mean ADU in the low 800's. Surprisingly the 2.5nm OIII filter transmits less NIR than the SII and Ha. My previous results whereby I used a light bulb and a lum filter showed that the OIII dulled the most, which I attributed to higher NIR transmission but could easily have been higher UV transmission (plus I don't know the emission curve of the light bulb I used), or the OIII filter has lower transmission at 940nm, but across the whole NIR spectrum, transmits more NIR than SII/Ha. If only I had an instrument that could measure the transmission across the whole spectrum! that would settle the argument! What I now find a little confusing is that, when imaging the target NGC3572, I didn't see any difference in the star luminosity between the SII/Ha Pro/Ultra series whereas the OIII has very obvious differences.  This certainly complicates things a bit and makes me seriously question if Antlia did test the 2.5nm OIII filter to 1150nm. Or if they did, they didn't bother to give me the results. So now I've got a couple of options. I can either try and return all my 2" LRGBSHO Ultra Series filters ( bought them from Agena Astro) and replace them with 2" LRGBSHO Pro Series. Or I can just buy a Pro Series Lum filter and screw it to the back of my CC as I know that filter has significantly less NIR transmission. I can only assume that this is an issue with the entire production of the Ultra Series so I'm not sure what Antlia will do about that. Not sure if they knew the whole time but might also explain why the spec sheets for these filters stop at 850nm instead of 950nm or greater. If you do want Antlia filters, please I consider buying the Pro Series over the Ultra Series. |
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| Wow! You just saved me a fortune on these filters... Glad to know the Pro series works well though. |
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This is very odd, an extra large star appeared from knowhere….🤔🤔 |
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AstroShed: Most of those fuzz patches seem to be associated with a faint star but some do appear to come out of nowhere. I still have no idea if it's legitimate (but not OIII) or an artefact. |
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John Stone: I used the 36mm LRGBSHO pro series for ages no problem. I decided to get 2" filters and thought I would go for the Ultra series. Partly because the RGB encompass the whole visible spectrum (the Pro series has a gap in the orange) and the 2.5nm SHO transmission curve was flatter for longer than the 3nm pro series. Never considered the NIR transmission would be an issue. |
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Jacob Heppell:AstroShed: The faint star that is in the middle of that bright spot, is still there and shows as a darker spot near the middle, you can just make it out, if you zoom right in the the bright spot on the top image I posted.. 🤔🤔 |
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| Are you sure it’s not just gone supernova….😂😂😂 |
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AstroShed: Oh I wish that was the case mate!!! I'll admit, when I first compared the 2.5nm and 3nm OIII filters, I though some stars were going nova. But checking against the Ha and SII burst that bubble. |
