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Thanks for the responses! I've done a series of little experiments which clearly indicate that the OAG, camera, and CC are not causing the artefact, but rather light coming into the tube (not a light leak though as the artefact disappears when the tube is covered even in broad daylight).
Dennis, I've determined that it is not a light leak, but perhaps an internal reflection? Any suggestions for identifying the origin?
Nik, I think you are right. When uncalibrated, it's a centred dark disk (rather than bright ring). Any suggestions where & how to look? The tube is different with a new focuser, secondary mirror, tube, spider, primary mirror mask and primary cell.
John, thanks. I have indeed test without the OAG and CC. The artefact is still present. So those pieces of equipment are innocent. It must be originating somewhere in the tube.
Dave, great suggestion. The artefact however remains even when not cooling.
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