Dark CA halos/rings around stars

[stroggvorbis]

Hi All,

Looking for help on this as it doesnt seem like any examples of CA that ive seen before which can easily be removed in photoshop etc.
As in the attached images, the CA on the brightest stars has created dark halos at the edges which when the frames are stacked also stacks to create a horrendous output. The frames were taken on a Bresser 102/460 achro with an aps-c sensor.

Thanks

[Elp]

You can try running an action on every frame within a folder to remove the fringing before stacking. If you're unfamiliar with actions, you record every step you take processing an image until you choose to stop. You can then run that action on a folder of files to repeat it automatically.

For future maybe try a baader fringe killer filter. I haven't used one myself as I bought an apo instead after the acro.

 

Edited December 22, 2022 by Elp

[ollypenrice]

I don't think that you can really expect good results from an F4.5 achromat. At around 100mm aperture, F4.5 is a big ask for an apochromat. Even Takahashi, with their legendary FSQ106, settled for F5. The larger the aperture, the harder it is to achieve good colour correction in a refractor.

The light which is creating the magenta halos should be focused into the star itself. Nothing you can do in post-processing will put it back there.  For broadband astrophotography I'd look for a more suitable set of optics.

Olly

[happy-kat]

To help suggest a trick or two to help minimise CA what software for stacking and processing are you using?

[stroggvorbis]

33 minutes ago, happy-kat said:

To help suggest a trick or two to help minimise CA what software for stacking and processing are you using?

Thanks, deepskystacker and Adobe photoshop. 

I know the optics aren't the best and fringing is to be expected but I wasn't expecting such a harsh defined halo. 

[happy-kat]

With deep sky stacker on the cosmetic tab don't use remove hot pixels (can give strange star centres). Could use your highest scoring light as the reference frame.

Could consider making a mask to reduce the aperture of the telescope increasing the f ratio reducing the CA.

Could play with the exact point of focus being slightly past or behind where you had focused, can help with camera lens but YMMV.

Post processing perhaps there are some CA tools already in Photoshop. I use StarTools and there's a fringe killer in the though I first choose to leave offending stars in luminance (no colour).

[Ian McCallum]

The Baader Contrast Booster seems to be better for reducing CA, than other Baader filters. Your mileage may vary. 

[alacant]

4 hours ago, stroggvorbis said:

dark halos at the edges

Hi

Stick a yellow filter, available cheaply, before the sensor.

A #8 will control the blue without removing it completely. A #12 or 495 long-pass will remove most of it.  Former preferred.

If you're looking for a software method, split to rgb, use deconvolution with stars+halo selected on b only, then recombine.

Cheers and HTH

Edited December 22, 2022 by alacant

[Ian McCallum]

46 minutes ago, alacant said:

Hi

Stick a yellow filter, available cheaply, before the sensor.

A #8 will control the blue without removing it completely. A #12 or 495 long-pass will remove most of it.  Former preferred.

If you're looking for a software method, split to rgb, use deconvolution with stars+halo selected on b only, then recombine.

Cheers and HTH

I've got the Wratten #8 and it does help, it's the poor man's Baader Contrast Booster. 👍

[stroggvorbis]

Thanks for all the advice.