Fixing refractor astigmatism
Jeff Quinn
How do I fix 1/4 wave of astigmatism in my 152ED? I love the scope,
it has smooth lenses, can be collimated unlike the stock Meade ED's,
shows a clean airy disc and first ring at high power, etc. but has 1/4
wave of astig. I don't know how to grind lenses nor do I have the
tools. Is there a way to miscollimate the lens with the astig. (once
I pinpoint which of the two lenses it is) to add in some astigmatism
at 90 degrees to the amount present, to null it out? Valery? Roland?
Tom? Anyone done this before? (The scope gives wonderful planetary
views, BTW, so it is worth fixing IMO.)
Thanks in advance for any ideas - Jeff -
Bob May
to do is to start moving one glass relative to the other and see if you can
"fix" it. You will, at the least, find out which one is producing the
astigmitism. If you have to, you can polish out the problem so you don't
have to go back to a grinding stage to fix the problem.
--
Bob May
Imagine the terrorist's fun when they realize that a "nubile virgin" is a 3
month old girl!
Wayne Hoffman
(Jeff Quinn) wrote:
>How do I fix 1/4 wave of astigmatism in my 152ED?
Have you eliminated the diagonal as the source of your
problem?
Wayne Hoffman
http://home.pacbell.net/w6wlr/
33-49-17 N
117-56-41 W
"Don't Look Down"
Jeffrey R. Hapeman
have:
1) The diagonal. Many diagonals are the source of slight astigmatism, or
horrible astigmatism. Even expensive ones.
2) Your eyes :-). Sounds funny, but I first found out I had astigmatism
when star testing...after ruling out the oculars, diagonal, and then the
scope, I realized I needed an eye exam.
3) The eyepiece(s) you are using.
Then, take a look at the lens retaining ring on the objective. If it is too
tight, it can be the source of astigmatism. I have heard this is common,
and my 5" refractor had that problem, and I had it fixed. Very easy to fix
if you can get a way to adjust that retaining ring.
Jeff Hapeman
"Jeff Quinn" <jeff.g...@worldnet.att.net> wrote in message
news:62144f02.01102...@posting.google.com...
ValeryD
Jeff,
Several different reasons can cause an astigmatism.
At first of all - which kind of astigmatism?
1. Potato-like ?
2. Elliptical (when defocus, you see an ellips)
3. Triangle astigmatism.
1. Mostly caused by poor glass homogeneity
Objective must be replaced.
2. Can be caused by
a. lens defect
If only one of objective lens has this defect, then you can't
help and an objective must be replaced.
If both lenses has comparable astigmatism, then relative rotation
can help much or even totally eliminate this defect.
b. too big objective miscollimation
(so called field astigmatism).
Try to use a Cheshire eyepiece to see if your objective is properly
collimated to the tube axis. I met once such case in my friend's 150mm
F/10 refractor. When his objective was properly collimated, images was
perfect for achromat.
c. If a star-diagonal mirror is not flat enough and has concave
or convex surface.
3. Can be caused by:
a. Pinched by three screws optics.
Untight centering screws to eliminate optics stressing.
b. lens defect if during last stages of polishing a lens
was slightly pinched by its cell (cell for polishing).
This can't be fixed and an objective should be replaced.
Hope this help.
Valery Deryuzhin.
Jeff Quinn
Thanks a lot for the tips. I do know that the astig. is present while
viewing straight-through and also with the diag. My eyepieces all
show the astig. in the 152ED, but work fine in my C102 which has no
astig. One thing I need to do is remove a layer of the tape I put
around the back element, as it may be causing some slight stress on
the lens (it takes a slight push to get it in the cell). This tape is
to keep it centered, BTW while I adjust the front lens' centering via
the star test. Bob, your idea of polishing the lens a little sounds
like a probable last resort for me, as the ED element isn't even
coated to begin with and I have pitch and rouge laying around from my
latest reflector project.
The scope is the 152ED reviewed on cloudynights.com. It showed no
astig. to the previous owner, though it was not even close to being
collimated in the normal sense when it got here (verified with my
cheshire) so I think he may have been experimenting with some airspace
wedge mixed in with a miscollimation of the push-pull screws, set
opposite of each other as to tilt the back element while leaving the
front element square to the light path.
The scope gave a very crisp view of Saturn tonight at 275x when the
seeing settled so I am really just wanting my star test clean up a
bit. I know that the scope could be crisper in the crappy seeing,
like a Takahashi so this is why I am trying to get the star test more
perfect. To all who are wondering, the color correction is fair to
good, but there is still a purple ring around Vega (not a halo - just
a ring). There is the slightest purple fringe around Jupiter, and
most of the false color is seen when out of focus, similar to the
Televue 102 but more prominent.
Chris1011
like a probable last resort for me, as the ED element isn't even
coated to begin with and I have pitch and rouge laying around from my
latest reflector project.>>
AHHH! Don't do this!!!! You will not polish ED with rouge or anything you can
scare up. Leave it alone. Chances are it's in the front element anyway. The way
to find out is to rotate that element, and if the astigmatism rotates with it,
you have found the source.
Trying to polish an ED with rouge will result in tons of pits and sleeks that
you will never get out. It takes a special pad and polishing materials to even
get it to polish, and then the final step must be done on soft pitch with
diamond powder or equivalent. Finally, I bet even if you can do all this, you
will put in more astigmatism than you are trying to take out unless you really
know how to hold this very fragile material.
Roland Christen
Jeff Quinn
Well, Roland, I don't want to touch the lens (it is the back on this
scope which is ED) as I already found out the hard way how fragile it
is compared to crown/flint by putting a small chip in it with a
collimating setscrew experiment gone bad. The stuff is really soft to
the point where I may not even consider getting it coated down the
road with MgFl2. I may be putting stress on it with my tape around
its edge to seat it snugly in the cell, but either way, the scope
focuses the airy discs right down to hard points. I just know that in
bad seeing, there would be less flare and extra diffraction rings
visible if I could get rid of the astig. which causes kind of a cross
pattern of flares like mini reflector spider spikes, going outward a
few rings away from the disc (only visible at higher powers).
Thanks for the tips!
rande...@aol.com
>>>Bob, your idea of polishing the lens a little sounds
>like a probable last resort for me, as the ED element isn't even
>coated to begin with and I have pitch and rouge laying around from my
>latest reflector project.>>
>
>AHHH! Don't do this!!!! You will not polish ED with rouge or anything you can
>scare up. Leave it alone. Chances are it's in the front element anyway. The way
>to find out is to rotate that element, and if the astigmatism rotates with it,
>you have found the source.
>
>Trying to polish an ED with rouge will result in tons of pits and sleeks that
>you will never get out
Hence the danger of people who hang onto ancient mirror making texts
as if they were some kind of holy scripture.
"I remember when...cough, cough."
-Rich
ValeryD
Jeff,
Try to rotate an outer element as Roland suggested. If this will not
help, try to give to lenses a bit (very very little) space for play
and see if an astigmatism is gone.
If all this will not help, you can fix astigmatism by:
1. making your star-diagonal slightly concave of convex
2. Install a small optical window with flat sides and
inclinate it to the optical axis. This will cause an
astigmatism. As thicker glass and as larger inclination
angle, as larger astigmatism. So you can fix it completely.
Valery Deryuzhin.