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Results of last nights testing:
Following up on the theory that I needed to make sure that I had the focus right, I changed the SGP pro setting to crop the auto focus image by 50%. I ran auto focus routine and then rechecked the focus by stepping the focus motor 13 steps at a time (half of the CFZ step count) whilst watching the HFR of a star in the centre of the image. This gave me the same result as the auto focus so I was happy with this.
I then:
• Took an image (guided) 180s Lum with OTA on west side of pier
• Found that I needed to do a meridian flip so did so before doing any more test shots
• Took another image 180s Lum with OTA on east side of pier
• Rotated the camera 90d clockwise
• Took another 180s Lum image
When the camera was rotated I refocused and then recalibrated the guiding. All test images were guided. Guiding was performing well at around .5”.
The following images taken with the PI aberration inspector have been attached to this post:
• Test exposure with OTA on west side of pier
• Test exposure with OTA on east side of pier
• Test exposure with OTA rotated 90 d
Also attached are screen shots of a 3:1 zoom of the images taken before and after rotating the camera 90 degrees.
Observations from these exposures, particularly when compared to the previous test images taken a few days ago, are that the radial pattern is no longer evenly distributed around the frame. It is much more pronounced in the upper corners. In the rotated image the elongation in the centre does move with the camera rotation but the radial pattern in the upper corners appears to stay fixed. This supports the conclusion that the issue is tilt.
I also note that the pre and post meridian flip images changes noticeably in that the radial pattern shifted in a way that implies some movement in the focuser or image train. Given that the image train is all secured by screw connections it points to the focuser being the culprit. The only thing that I cannot quite see that fits into my theory is the elongation of the stars in the centre of the image.
My conclusion from this is that I must agree with the view that the aberrations that I am seeing are due to tilt and it also appears that there is some variable movement in the focuser as demonstrated by the changes taken in the image before and after the meridian. That plus the fact that the play in the CAA is really unacceptable for precise work, it would seem that the best option is to replace the focuser, preferably with a Feather Touch.
Comments on my analysis welcome.
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