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Red Halos??


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Hi all,

This image of Cygnus was taken with an old F4-5.6, 17mm-85mm Canon EFS lens, set at 17mm, on my modded Canon and Astrotrac. I didn't use a LPF as the lens has a rubber ring that protrudes inwards so it wouldn't fit. Melsky suggested Custom White Balance, which was good. But I get sooooo fed up with the red halos around the stars. I thought it was the Astronomic LPF but it's still happening here. Must be that the red end of the spectrum in the modded camera isn't focusing the same as the other colours. I've softened the red halos in processing but they're still a blasted nuisance. How to I get rid of them?

Alexxx

post-1704-0-46199600-1407776911_thumb.jp

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You can isolate just the stars in photoshop and invert selection then refine the edge and desaturate a selected color with the eyedropper or replace color at will.

Thanks Leveye. I tried that with Colour Balance but didn't want to lose any natural colour. I didn't include Deneb and the other bright stars. Maybe I should force it a bit more.

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Hi

I'm not experienced enough to help but the problem looks quite similar to the effect I've seen when I've overstretched/overexposed stars and tried to replenish the colours by increasing saturation and colour bias. I've only really used StarTools plus tweaking with cs2. I'm sure I've read another thread about reducing the prominence of stars in order to increase nebulosity - some actions in PS?

Also this one: http://stargazerslounge.com/topic/221052-red-fringing-on-bright-stars-after-dslr-mod/page-2?hl=%2Breducing+%2Bstars#entry2378924

Sorry I'm not much help!

Louise

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That's interesting. Thank Louise. The red is already in the RAW images and stretching just brings them out. I did use custom white balance but I didn't look at the channels. I'll have a closer read of that thread to see if I can understand it all!

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The additional red allowed through by the modified camera will not focus to the same plane.

The design specification on the lens would have been for something like 650-400nm to be at the same plane.

You have now allowed 700nm to get through and that was never in the original design of the lens.

The filter took out these wavelengths so the lens was never designed to handle them.

Think about it, why design a lens to handle a wavelength that is not (should not) be present?

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I tried the tutorial to reduce halos but one part wouldn't work, so I used the Magic Wand to select the stars and made the stars smaller as on the tutorial. Still a lot to learn with processing!

What do you think?

post-1704-0-34699300-1408302137_thumb.jp

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have you tried a normal ir filter for the lens i know its a modded camera but it will be worth a try at least you could take some with the filter on the lens and use them as a color layer mask.

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