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Old 03-09-2020, 04:46 PM
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codemonkey (Lee)
Lee "Wormsy" Borsboom

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Join Date: Jul 2013
Location: Kilcoy, QLD
Posts: 2,055
Tilted corrector vs faulty corrector

I have astigmatism + tilt which is manifesting in elongated stars from the top left of the image, to the bottom right.

If I rotate my coma corrector and camera (and the filter wheel, OAG, spacers etc in between them) as one monolithic unit the aberrations stay fixed relative to the sensor. If I then rotate the camera independently of the corrector, the aberrations move relative to the sensor.

I understand that a coma corrector that is tilted in the focuser can/will cause astigmatism in images and it doesn't take much to do so. How can I distinguish between a faulty corrector and a tilted corrector?

Since this is astigmatism and it causes elongation either side of focus, I'm currently mitigating the issue by correcting the tilt between the corrector and the camera. If I remove that tilt adjustment and instead try to tilt the corrector in the focuser's drawtube at the same spot, it makes no difference to the star shape, it just gets more defocused. This has me thinking it's the corrector itself rather than how it's registered in the focuser, but before I go and spend $900 on a new corrector, I'd like to see if anyone has ideas on how I can easily confirm it's the corrector itself.
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