A question for all lens experts!

I just picked up a used 8" f6 Achro lens on Amart knowing not to expect too much, based on the sellers description. I star-tested it tonight, it has some SA probably 1/3 to 1/2 wave but also astigmatism that really bothers me more then anything else. Retaining ring is loose so no problem there. It looks like someone already messed with it by taking it apart. Spacers seems to be there, but my question is: can astigmatism be reduced if I rotate the lenses in relative to eachother? I do not have an optical bench but will dive into the project if there is hope. Attached is a picture of the lens in the temporary tube from a different perspective...

Thank you!

oren.

Attached Image:

It could be the astigmatism is in your eye, not the scope. Your eye will cause more trouble at low power views with a wider exit pupil from the scope. To eliminate your eye as a problem, make sure the lens and focuser are well aligned and then look at the star image at high power. There will be the usual color problems but any astigmatism in the image will show up as out of round images that change orientation as you go from inside focus to outside focus.

good luck,

dan

What Dan said, but I'd first collimate everything (focuser and lens..focuser first). Astigmatism can creep in due to collimation in exactly the way Dan describes.

Use a quality laser that itself is well collimated with an aperture mask that's got a small hole in the middle. Using the mask, adjust the focuser until the beam exits the hole in the mask. Then, leave the mask on but put something black over the lens and use a good Cheshire to adjust the lens so that the dots you see all line up. That will get you close but star testing (using a green filter) at 400X and above is really needed to dial it in. When star testing first go to about 4-5 rings on each side of focus. If there is still astigmatism, adjust the lens collimation screws to make the images round.

Also, I suggest an aperture mask of maybe 6". If that really cleans up the image, then you know the problem is towards the edge, where they usually are.

Good Luck:

Jeff