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Old 12-08-2013, 05:23 AM
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Placement of Clip-In filters: color edge issues

Yesterday night I made an image of the North America + Pelican Nebula and the whole area in Cygnus around Deneb and Sadr.
Canon EOS 7 + 85mm f/1.8 squeezed to f/2.5, 5 minutes exposed at 800 ASA.
It seems that 800ASA + longer exposure is better than 3200 ASA as the latter loses fine detail. I used a Hutech LPS V2 filter.
And the red colors of the H-alpha nebulas are prominent despite it is an unmodded camera.

But the issue is that on one side there is a greenish hue and the other side a bluish one. Is this because the filter is not placed exactly perpendicular to the light path ? Picture 1 shows how the filter is put into the body behind the lens. It is not fixed with a screw, but when the lens is mounted it seems to fit tightly.

Picture 2 is a stack of 8 frames (north left) and picture 3 is a single frame (north up) in which the colored top and bottom edge is more prominent.

Is filter placement really a strict requirement to prevent these issues ?

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Click for full-size image (deneb-single.jpg)
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