Ways of Dealing with Excessive Chromatic Aberrations

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!

Rajat Kumar:
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!

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

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.

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 🙂

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.

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!

Guiem Kimi:
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.

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

andrea tasselli:
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.

Thanks for the suggestion, will definitely have a look at it!

Would it also remove the halos that are around the white stars?

Zak

Jim Raskett:
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

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

Zak Jones:
Thanks for the suggestion, will definitely have a look at it!

Would it also remove the halos that are around the white stars?

Zak

Not sure I had any of those but I'll check.

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.

Jim Raskett:
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!

You're welcome Jim!