Thread: colour gradient
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Old 26-06-2012, 07:40 PM
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pvelez (Pete)
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Join Date: Aug 2006
Location: Sydney
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colour gradient

I have an issue that is driving me bananas

I've taken a few images of M4 in Scorpio. I have found with each image that there is a noticeable colour gradient. To the left of centre, blue is accentuated while to the right I have a surfeit of green.

No dramas I thought, that must be some of the emission nebulae in that area.

Turning to the region around Eta Carinae - same issue - magenta to the left, green to the right. The bright areas are fine, its in the background that it is noticeable.

Must be dodgy flats - so I reshot them. And reshot them. And reshot them. No change.

Am I missing something? Its not light pollution - the colour gradation looks the same irrespective of which area of the sky I am imaging.

The flats look fine. I'm setting them at 20,000 but have also tried 25,000 and 15,000. I've checked that they are in fact flat by test calibrating a flat - even illumination results. I've checked it with the graph and info tools in Maxim.

I'm shooting RGB and LRGB. I'm median combining 20 - 30 flats taken with a lightbox. I've rotated the box in case its not flat - no change. Darks and bias frames are applied and I'm median combining in Maxim. There's no difference using average or sigma combine (though I wouldn't expect any difference).

So what else am I doing wrong?

Pete
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