Crescent Halos around Red Stars

[NorthernLight]

Hi There,

I keep getting crescent shaped Halos around stars of reddish colour. I get them with my ED80 and with the 6"Newton+2.5Powermate.

Does somebody know why? Example attached.

Cheers

[billdan]

Hi Max,

I get that as well and it was explained to me (on the Startools forum) that it is a stacking artifact.

Startools has a module (Lens) that has a fix for this, you shift the Red (or Blue) pixels left/right or up/down a small amount. Basically moving the crescent so it wraps around the whole star.

Cheers
Bill

[iborg]

Hi

Registax has a similar tool, but, I would never have guessed this problem was a stacking artifact.

Philip

[cometcatcher]

Not sure that's a stacking artifact. You seem to have some edge distortion, are you using a field flattener for the refractor and a coma corrector for the reflector?

[billdan]

It didn't sound logical to me either (stacking artifact), I suppose a simple test would be to stretch a single sub and see if the crescent remains.

[samgibbs]

Can you tell us more about the image capture process please.
What device were you using to make this image? What was the exposure length, iso settings, filters?, and did you refocus during the capture process.
Maybe we can sleuth this one with more info.

[NorthernLight]

Hi Guys, the image has been taken with a CAnon450D Baader Mod.; on a 6" Newton without Coma Corrector + a 2.5x Powermate. Post processing was done in DSS and the FITS output was processed in Startools. The final TIFF was then massaged in Lightroom and exported as JPEG.
I will try the Startools function. The issue keeps persisting regardless of whether I use a ED80 with field flattener or the Newton without correction. So it may be a stacking artifice after all. Maybe it's got to do with dithering of sun exposures???

[billdan]

Hi Max,

To be honest I have only ever seen this issue when processing with Startools, if I stretch and process within Astroart I don't get this problem.

But I can't get good star colour or remove gradients with Astroart so I stick with Startools.

Cheers
Bill

[samgibbs]

Hello Max

What i am seeing (within the limitations of a 200kb image) is a lot of coma, possible pixel alignment issues and certainly a considerable amount of blooming due to the sensitivity of the ccd at the red end of the spectrum.
I am also thinking there may be mechanical sagging or some alignments errors in the optical train of your setup. What i find intriguing is that the problem is confined to the lower 1/4 of the image only. Bright stars above dont seem to be affected. Its also interesting how the halo is oriented toward the left corner of the image on the left side and the right corner on the right and almost symmetrically near the middle.

Maybe during post processing, you can try to make adjustments to the red channel and curves to try to reduce the effect.

[NorthernLight]

Thanks Sam, I will definitely try that.