I just bought a 90mm refractor from an astromarter and have found that it has rather bad astigmatism (it's an Orion f/11). I know it's "caveat emptor" here on the 'Mart and am not complaining - i got a decent mount as part of the deal. My question is this... Is there any way to correct it (I doubt it)? I have verified that the problem lies in the objective and have taken it out and replaced it to see if anything is "pinched" or misaligned (it wasn't). I did see a "K" on the side of the second lens (of the doublet) and two sideways "D"s on the "front" lens, 180 degrees apart. The previous owner had obviously removed the lens set at least once.
Thanks for any help - I suspect that I'm just stuck with an astigmatic 'scope...
I think somene else mentioned rotating the front element relative to the rear element. Sometimes the two glasses are "hand matched" and if they are, you may be able to find a rotation where the astigmatism goes away. This has sometimes worked for me.
Try tilting the entire lens assembly. If there are no adjustment screws, you will need to try more inventive measures. Loosen the entire lens assembly by unscrewing it from the front of the tube until it flops. Then test it horizontally on a distant artificial star. Rotate the scope to control the direction of flop/tilt (gravity will make the lens assembly flop down) and loosen more or less to control the amount of tilt. If you can find some amount and direction of tilt that fixes the astigmatism, you can then go on to find a more permanant fix. One method is to shim the lens in the lens cell so it is tilted -- i.e. put 2 spacers (cork, tape, etc) at 90 or 120 degree where the rear element seats against the metal, and two more spacers where the front element sets but rotated 180 degrees to the rear shims. Another way is to somehow make the flop permanant, but that may look ugly.
If it turns out you can't fix the lens, you can try fixing it at the diagonal. Get one of these cheap mirror-type diagonals (with the thin plate glass mirror), and jam some stuff between the glass and back cover so it deforms the mirror a little bit. This usually works, but the down side is that the correction will be sensitive to how the diagonal is rotated in the focusser -- you'll have to keep it a certain way even if the eyepiece is awkward to look through.
Yet another correction is to put a piece of flat glass (like a UV camera filter) in the focusser draw tube and tilt it. Probably will need big tilt, like 20 degrees, to see much effect. This may introduce some color error depending on how much tilt you need.
HI WE JUST BOUGHT A S-80 WITH THE SAME PROBLEM ,I FLIPPED THE FRONT LENS OVER AND PUT NEW SHIMS BETWEEN ,IT TOOK CARE OF 95% OF THE PROBLEM,BUT FOUND ANOTHER LENS FOR SALE AND REPLACED IT IN THE END TERRY