Robindavid

Hi All, I think my problem is solved. As Andrew suggested I checked the focal length and it was 3275 instead of 3250. I could increase the distance between the mirrors by screwing the supporting screw of the secondary. I estimate that I moved the secondary some 5 mm. The results was a decrease of the focal length. After that the collimation took 15 minutes following the DSI method. The stars are now round in the center even when little defocused. Thank you everybody.

I will try to check the focal length after a plate solving so I will see if the focal length calculated is in the specification of the supplier. Concerning lateral displacement I can see there is some play for the pulling screws of the primary but only one mm. I will try to move slightly the primary to see any difference. Thanks both for you comments.

Thank you for the answers. These elongated stars are in the center. That's why I don't understand. I think the primary/secondary spacing has an influence on the spherical aberration but not on the astigmatism because the error would be on the axis of the mirrors. But I may be wrong.

I All, I have spent several months in trying to collimate my RC16" TS but there are strange things hapenning. Is it normal that in a Ritchey Chretien telescope the outfocus and infocus image of stars are elongated and perpendicular to each other? I can make good images FWMH between 1.5 and 2 arcsec but as soon as I am slightly out of focus the stars are elongated. Here is a picture of the elongated stars outfocus and a picture of visually aligned spider vanes; Robin