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Green haloes around stars

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#1 Michal1

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Posted 26 December 2012 - 09:49 AM

I have a problem with some frames on E200. The stars are sometimes surrounded by green haloes, that I'm unable to effectively reduce by postprocessing. In some frames, the haloes don't appear everywhere, but only in some parts of the image.
Here you can see an example.
http://www.astro.cz/...grhalo.jpg.html
It is a cut from a 70 min photo of the Lacerta contellation with Takumar 105mm at f/5.6. The green haloes are strongest in this image. In this illustration, I increased the green black point and overall brightness to make the haloes apparent. The original cut (2.4MB) can be downloaded here:
http://netload.in/da...Zau/lac.tif.htm
Please can you advise me, what are they caused by?

#2 Eric P

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Posted 26 December 2012 - 10:45 AM

Is this home developed or did you send the film out for development?

#3 Michal1

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Posted 26 December 2012 - 10:54 AM

It was developed by a professional lab. Bad developement is the only thing that comes to my mind, but I wanted to hear the opinion of the others.

#4 Eric P

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Posted 26 December 2012 - 11:09 AM

Well if it was a lab then I guess it could also be the film being improperly stored at some point. Though if your lab is anything like my local photo-store then maybe it is bad QC during the development process. My local doesn't even have a working film fridge... the remaining stock just sits there like it's in a closet.

#5 Hikari

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Posted 26 December 2012 - 02:03 PM

Or classic CA if you are seeing it in a few frames on the roll, which would be green for a lens corrected for two wavelengths. Atmospheric conditions could be exasperating it. That would cause fringing.

If it is one a whole roll, it could be a processing problem. You have a cross curve where the magenta layer, in this case, is developed to a higher (or lower contrast) to the other two layers. What results is highlights and shadows of complementary colors--if you have green highlights, you should see a magenta cast in the shadows.

BTW, both issue might not be significant under normal shooting condition, but the extreme contrast needed in processing astrophotography will accentuate them.

#6 Michal1

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Posted 26 December 2012 - 05:37 PM

Hikari, you are right, the shadows are magenta. The haloes aren't green on the unprocessed images. They become green after that I increase the green black point to neutralize the background. Some frames of the roll are OK, but others are not. Is there any way to adjust the contrast of the magenta layer by software? I tried to convert the image to the CMYK color space in Photoshop, adjust and convert to RGB again, but the background get completely grey in the result.

#7 Hikari

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Posted 26 December 2012 - 05:55 PM

You should be able to simply go into color balance in photoshop and adjust the shadows and highlights (there are radio buttons to select those) in the green channel. You could also do the same thing in curves by also adjusting the green channel. There is no need to convert to CMYK.

#8 Michal1

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Posted 26 December 2012 - 07:58 PM

Thanks for the avice. Unfortunatelly, I always end up with something like this using curves. The dense MW region comes out magenta and the isolated stars with those green haloes. I'll have to make do with green reduction in Pixinsight for now and try a different lab next time. Several other frames from the roll are perfect, though.

#9 Michal1

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Posted 30 December 2012 - 02:27 PM

BTW, what is the colour of the background sky on your shots on E200? Is it neutral or it has a color tint? It is always blue or purple on my images. Does it have something to do with light pollution?






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