Help

I have a WO 66SD Petzval w/2 speed rotatable focuser....

I am wondering if what I am seeing is abnormal, or is there something that needs to be fixed here.

When I am slightly inside focus, the star is oblong. When outside focus the star is again oblong, but 90* perpendicular to the image inside focus. I am using an inexpensive 2" diagonal, this happens no matter what eyepiece I use. Any input would be appreciated

Clear Skies.

Have you looked "straight through" minus the diagonal? That might indicate if the diagonal is the culprit, and start process of elimination.

Sounds like astigmatism. If the diagonal is not the culprit, looks like optics in the scope.

When looking into the eyepiece at the enlongated image, be sure to turn (rotate) your head to see if your eyeball lens is adding to the problem.

Also, loosen the setscrew and turn the eyepiece in the diagonal to see if it adds to the problem.

Anything in the optics path, from objective to diagonal to eyepiece to eyeball can contribute to astigmatism.

Rotate the diagonal in the focuser by 90 degrees without
changing anything else. If the astigmatism keeps the
same direction relative to the diagonal body, the problem is
either the diagonal or the eyepiece. Then try rotating the
eyepiece (as someone else suggested) -- if it does not
rotate with the eyepiece, then the problem is the diagonal.

If, when you rotate the diagonal in the focuser, the
astigmatism pattern rotates by 90 degrees (e.g. changes from
along the telescope tube to perpendicular), then the problem
is the OTA.