Image of the day

Captured by
Herb Bubert

Sun on June 20th

My Account

New to Astromart?

Register an account...

Need Help?

Refractor Astigmatism

Started by plunkett, 10/11/2003 05:57PM
Posted 10/11/2003 05:57PM Opening Post
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...

Pat
Posted 10/12/2003 03:00AM #1
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.

Astigmatism is no fun... Good luck!
Posted 10/12/2003 07:31AM #2
I am curious if you discussed the astigmatism with the seller. While astromart is "buyer beware" in some regards, I think selling a scope with astigmatism and not mentioning it is definitely bad form.

I buy and sell some on Astromart and try to conduct my business like a gentleman, I am up front with any problems and want to be sure that the buyer is happy with the item.

Unfortunately you have disassembled the objective, this makes addressing the issue with seller a bit trickier. But if the guy is reputable and up front, you should be able to work something out.

jon isaacs
Posted 10/12/2003 09:21AM #3
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
Posted 10/12/2003 09:34AM #4
Apogee has a 90mm F 11.1 Objective listed for $85. You could replace it.

Posted 10/12/2003 05:26PM #5
Got some good ideas from the group - thanks tons!!!

Pat