the focusing/defocusing seems a convincing explanation for the observed effect. That it shows up particularly in the fish tail is due to the astigmatism present there. Since the lens system was designed to give resolution along the spectral direction, some image curvature in the perpendicular direction (parallel to the slits cannot be avoided at the outer edges of the CCD chip. A move of the focus at the slit translates into a move of focus at the CCD. If exactly in focus, a slight defocusing has little effect (moving within the waist), while out of focus the width of the spot increases much faster (see sketch below).
- effect of moving the focus with astigmatism
- astigmatism ISIS.PNG (19.12 KiB) Viewed 2031 times
In principle one could shift the focus so that the tails become narrower, but this would reduce the resolution and/or the intensity of the spectrum in other parts.
Fortunately the optics is designed in such a way that despite these fishtails the resolution does not seem to suffer, since the spectral lines do not broaden even if the fishtails get larger.
One could confirm these arguments by measuring the fishtail shape at different positions of the telescpe focuser.
I hope we have clarified this issue to some degree.
Regards,
Martin