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Combating Chromatic Aberration?

Scott Iver
in PixInsight
My triplet still get's some CA.  If anyone is interested in the extreme details I'm willing to post more. The short version is that I've tried to do some spectroscopy with a grating, which revealed a "fishtail" in the violet/blue end of the spectrum that's produced by the stars.

What this translates to in imaging is violet/purple/ strong blue halo's around brighter stars.   As an example I took RGB at 15sec exposures and put them together in PI.  I only did an auto stretch (screen transform, histogram), star alignment, and finally channel combo to create the color image.  Sure enough violet/blue halo's around the brighter stars.

I've contact the telescope manufacturer for assistance, and I'm waiting for a reply from their optical engineer.

However my fear at the moment is that replacement of the optics is what's going to end up being required, either by replacement of the lens cell, or by replacement of the whole OTA.  Either way I'm looking at that much more "downtime".  If I end up having to replace the OTA, I've been thinking about going to a newtonian, meaning a complete setup change.

I recently saw Adam's video on removal of scattered light, I wonder if something similar could be done for my blue frames?

Comments

  • James Goodall
    Or I'm hoping the new PixInsight RGB channel alignment will help reduce CA. I'm still trying to figure our at what stage it is applied in the calibration and star alignment process in WBPP 2... I hope Adam does a video for this new feature.
  • Scott Iver
    I'm really struggling with it,  I've found some "tricks" online but none of them worked for me.

    Right now my Triplet Apochromat with it's ED glass and all the other marketing hoopla speak,  is producing images that look like they were taken using a achromat Doublet.  The manufacture so far has only kept asking me for more test images (feels like they are searching for any excuse to not repair it) ...

    Guess I'm SOL
  • Gordon Aspin
    Are you using OSC or mono with RGB filters? If the latter I found that changing to better filters helped with blue bloat on my original doublet setup. But it does really sound like you're not getting the performance you should from a decent triplet - once I upgraded to better optics the problem with blue halos went away completely for me.
  • Scott Iver
    It's a mono, I have LRGB and narrow band.  The saving grace I guess has been that I can still get Ha, Oiii, and Sii out of the rig.

    They are Baader Planetarium brand filters.  The original set of filters from ZWO was from the batch that had severe problems (guess I just get lucky all the time), so I replaced them all with these, which at the time were the #2 choice in filters other than the Astrodon filters which were way more $ ,  not to say these didn't set me back a good amount either.

    What's really aggravating is that the scope went back to the manufacture already this year when I reported this problem.  They cleaned it, columnated it and sent it back, and the CA issue seemed to get worse instead of better.

    Hopefully I will hear back from my latest round of test images for them this week.  I was just hoping for some way to salvage the data
  • Gordon Aspin
    I'm also using Baader and they're fine on my setup. Do the blue stars look like they're properly focused?

    To salvage something from your data you could try using a blue colour mask to select the halos and then replace with local background from a blurred copy, or perhaps using MT to shrink the halos on the blue channel. If most of the halos are on a dark sky background it's not too difficult to deal with, a bit more care is needed if they're in front of a galaxy or nebula.

    Think of it as a challenge to get the best out of a data set with problems - that's what Adam's videos are all about!
  • Scott Iver
    Well the official position from the manufacturer is this: “It looks like what to expect out of the scope and the equipment used. You're not going to totally remove all blue fringe. Even a very expensive Takahashi with 4 elements you'll have about the same on bright stars. Also, processing can help out too to cut back on the blue.” 

    So I'm going to have to find some trick in processing.

    However I've had some very knowledgeable and respected people in the hobby tell me that my scope is "not right' for a triplet.  So I'm more than likely going to replace it with a different manufacturer and go with maybe a very large imaging newt for more aperture. 
  • Scott Iver
    further update, the manufacturer gave me a couple of options,  I have to think them over.
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