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 Hi all, I have been sitting on some data I shot back in September of NGC 6726 around the Corona Australis complex, but I have never been really happy with the end results. I used my Radian 61 as that was the only telescope I had at the time for my first light with my ZWO ASI183MC Pro. Before I shot this target, I had previously only used dual narrowband filters with my Radian 61 with minimal halos, which was fine until I used only a UV/IR cut filter for this target which then revealed that I had indeed received one of the earlier variants of the Radian 61 that is plagued with the excessive chromatic aberrations and halos. I am now on the fence of just chucking this data out completely and calling it a loss, but a part of me still wants to salvage this dataset as I really love the Corona Australis complex. I am hoping that I will be still able to salvage this data in some capacity, so I have attached a screenshot of the stack that was generated with GraXpert AI. I look forward to hearing everyone's thoughts about this. Zak  |
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| I wouldn’t throw the data away. Adam Block demonstrated a way to deal with blue CA by splitting the channels and using BlurX on the blue then recombining. That could be an option. It will slightly change the color balance I think. |
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Hi Zak, Just my opinion - I would not chuck away the data. See if you can seperate out the star and process them seperately. Perhaps, don't stretch them aggressively. The image is very good Second, try Astronomik L3 UV/IR cut filter. See this link https://www.astronomik.com/en/uv-und-ir-block-filter/luminance-filter-l-1-l-2-l-3.html?___store=en. The L3 filter is claimed to minimize the problem of blue halos. I have never used it but have read good things about the filter. Clear skies! |
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Joseph Biscoe IV: I definitely agree with not throwing the data away as I really want to try and get something out of this. I did see Adam's BXY technique and have tried it, but it still results in more chromatic aberrations than in his demonstration. I am hoping that I can somehow reduce it to a point where it doesn't really impact the overall feeling of the image whilst maintaining some star colour but that seems a bit unlikely as it seems the star colours have been washed out by the chromatic aberration and halos. Zak |
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Rajat Kumar: Hi Rajat, I agree, I am hoping that I will be able to salvage something out of this data. I am currently in the process of re-stacking my data using 2x drizzle so hopefully that might help with the taming of the colour fringing and processing the stars. I don't normally stretch my stars too aggressively anyway, but I will definitely keep that in mind when I am re-processing the data. Thanks for the suggestion about the Astronomik L-3 filter, I will definitely look into it and might even end up getting one to see if that takes care of most (but hopefully all) of the excessive chromatic aberrations. Zak |
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If you can, try and get the scope fixed. I am not sure what kind of customer support you can find in that endeavor but fixing the actual source of the problem would be the best thing. If that is not feasible, then the previously mentioned filters are your next best bet. Instead of running BlurX on the blue channel, you can just use your favourite program to reduce the star sizes in that channel. I know a lot of people who shoot with lenses that do this. |
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If you can, try and get the scope fixed. I am not sure what kind of customer support you can find in that endeavor but fixing the actual source of the problem would be the best thing. If I definitely picked this up before now I could have done something, but now unfortunately OPT shut their doors earlier this year so I’ve missed the boat there. I did see that OPT sent out replacement objective lenses a while ago but now that Agena Astro owns the Radian brand, they said on their website that they are not releasing anything Radian related for the foreseeable future although that could change later down the line. I am not sure if Agena Astro can assist me here but I’m definitely way too late in picking it up. I’ll probably just end up getting the Astronomik L-3 filter and see if that helps. If it doesn’t, I’ll just end up replacing the Radian 61 with something like the Askar FRA300 Pro. Thanks for the suggestion, will definitely try that once the data finishes re-stacking. Zak 🙂 |
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| Try the Baader semi-APO filter. It works wonders with this sort of issues and basically removes the violet/purple halo and restore some semblance of normality within the colour balance. Plus, it acts as a LP supression filter too. |
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| I've recently suffered with the same level of CA. To resolve it I took the image into PhotoShop & used the CameraRaw /Defringe action. I've not tried the option of splitting the channels within PI and then using BlurX, but I may try that next time. |
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I have been there Zak. I bought a Sharpstar 61 EDPHII several years ago (as far as I can tell, the same scope as the Radian 61) and used it exclusively with an Optolong L-Enhance filter and the results were wonderful. Until, I imaged M31 and M33 with a uv/ir cut filter. The CA was horrible! I bought the scope from Agena and contacted them first. They were absolutely no help. They told me the CA was expected at the scope's price point. Then I saw a video by Cuiv (his Sharpstar 61 had the same issue) and he helped me contact Sharpstar and they sent me a new main objective. That was years ago and I believe that the time replacement window has expired. The blue halo's were horrible in my images (luckily, only M31 and M33), but I ended up tossing the data. I didn't try Adam's method, but Defringe in Photoshop Camera Raw worked well at the expense of other blue hues in the image. If you find a good method dealing with the CA, including trying the filters mentioned by others in this thread, please post it here. Otherwise, I would just use it for narrowband only. Jim |
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Jim, I know you're shooting with an OSC camera. I have some OSC data of the Iris nebula (NGC 7023). It is imaged with a Sky-Watcher 72ED doublet, 0.6x reducer and Astronomik's L3 UV/IR filter. Back-focus was far from accurate and the stars in the corners are deformed. The ones in the center are nice and round, yet with relatively annoying chromatic abberation. I used PixInsight's WBPP to pre-process and integrate the data. I configured it to produce demosaiced/interpolated images for each R, G and B channels along with a combined RGB image. With StarAlignment, I chose the G image as reference, put the R and B images as targets and ran the process. Then used ChannelCombination to generate an RGB image. As a result I ended with two RGB masters - one where the R/B channels are StarAlighed to the G channel (ca_corrected), and another where they're left pristine. As you can see below the chromatic aberration is significantly reduced.  In addition, the SpectrophotometricColorCalibration chart of the CA-corrected image is more... aligned?  Anyway, this is what I used for some of my latest reprocessings (not yet published). In addition, I run BlurXTerminator twice - first with "Correct only" option enabled, and a second time as usual but with the stellar adjustments toned-down. The results look good to me but I may be missing something. I haven't tried Adam's technique yet and I'll give it a try. Thanks! |
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I suffered from a lot of CA with my TS Optics Photoline 130, which is supposed to be well corrected but it is not. After going to factory for collimation I was told it couldn't be corrected (it improved, but still showed the same CA as your subs). I had an Astronomik L2 so I decided to buy a L3. It works like a charm. At least with my equipment. I can provide some subs if you need them. The purple halos disappeared. Guiem. |
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Guiem Kimi: Hi Guiem, Great information! By any chance, do you have subs with and without the filter that you wouldn’t mind sharing? Either way, thanks for the L3 information! Jim |
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Yo tengo un refractor acromático... lo que mejor me resultó hasta ahora es: - Usar una máscara en el objetivo del telescopio para aumentar la /F - Capturas separadas para el canal Blue "en foco" (esto depende del telescopio supongo) y procesar los canales por separado (SIRIL) |
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andrea tasselli: Thanks for the suggestion, will definitely have a look at it! Would it also remove the halos that are around the white stars? Zak |
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I've recently suffered with the same level of CA. To resolve it I took the image into PhotoShop & used the CameraRaw /Defringe action. I've not tried the option of splitting the channels within PI and then using BlurX, but I may try that next time. It is definitely a pain to deal with that's for sure... I have tried that to moderate success, but Lightroom Classic has a new tool called Point Colour, so I might have a play with that and see if it can reduce the chromatic aberrations. I also tried the BXT method of using it on the blue channel, but I haven't got good results as the stars in the blue channel are really bad and bloated. Zak |
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Jim Raskett: Yes, you are right there. The Radian 61 was basically a rebranded Sharpstar 61EDPH II with a higher price tag. It is such a shame that my Radian 61 was one of the earlier units that is affected, but I can't do much about it now as OPT went into liquidation earlier this year. I did see Cuiv's video and the new objective lens definitely helped in reducing the chromatic aberrations. I am not sure if Sharpstar still have the replacement objective lenses that were made for the Radian 61, but I am certain that they will not be able to help there unfortunately... I will try defringe in Adobe Camera Raw again, but I am still thinking about just chucking away the data. I've tried re-processing this data again and again but I am still not happy with it. It must be my OCD or something else, but I am very disappointed that I didn't detect this when I got the Radian 61. I will definitely keep everyone posted in regards to finding another solution. Zak |
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Guiem Kimi: Good to hear that you are having great success with your Astronomik L3! I might have to pick one up and see if it will help significantly reduce the chromatic aberrations. I'm still skeptical that even if I get the filter that it will not reduce the halos that are around the white stars. Zak |
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Zak Jones: Not sure I had any of those but I'll check. |
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Jim Raskett:Guiem Kimi: Hi Jim, Yes, for sure. Left L2, Right L3:  It is not perfect, but for me, the improvement is quite a lot. Those are two uncalibrated subs. Just debayered and unlinked STF. After that, with some of the techniques explained above, CA is gone. I can send you the subs if you need them. Guiem. |
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Just UV/IR filter, 100mm f/6 ED doublet: With Baader Semi-APO filter:  So, yes, it reduces the white-ish bloating as well as the violet halo. The big bright one is Alpha UMi, Polaris, btw. |
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Guiem Kimi:Jim Raskett:Guiem Kimi: Thank you for that! Very significant improvement for sure. Even with the new objective in my Sharpstar 61, I still see some blue fringing in the brighter stars. Lower exposure time will probably help some, but the filter really does make a difference. On my list! Much appreciated Guiem! Jim |
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Jim Raskett: You're welcome Jim! |
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Hey zak Had the same issue with a sharpstar edphii. I spoke to sharpstar, sent some photos. They agreed and sent me a new objective lens and was a great improvement over the old lens |
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Hey Zak definitely do not toss the data out. Dealing with CA is annoying, but there are some ways to get around this as explained above by many others. Unfortunately I do not have pictures to post as examples, I'm no expert but here is what I use to do to get around this issue. ,I would separate the channels, run BlurX on them separately. Then use the FWHM eccentricity script to get the mean values for each channel and keep the G as that as my reference. I would then stretch the R and B channels, and measure the mean FWHM and accordingly change my stretch for these channels to produce identical / very close mean values to that of the G channel then i would combine them together, and if necessary do a star alignment. Once done, I'd do an SPCC, this would significantly quench the CA. Cs |
