RE: ATM 8"f/7 astigmatism

["CSC" <optic@gwi.net>]

The test is really based on the eye's ability to compare shapes.  When
rotating the mirror does not change the shape of the "star" image, You're
there.

Now you are saying that your tan/sag focal difference is 1/8"?  That should
polish out easily.

I hope you have been polishing this disk by hand while deciding what to do.
Hours of good technique might be enough to cure it.

colin



-----Original Message-----
From: owner-atm@shore.net [mailto:owner-atm@shore.net]On Behalf Of
david.a.stevens@gm.com
Sent: Saturday, October 04, 2003 5:10 PM
To: Atm
Subject: RE: ATM 8"f/7 astigmatism




Hi Colin,

I actually did the test this morning after spending about 45 min. getting
the alignment right and making sure I had the eyepiece and the pinhole
about 2.5 inches apart ( just enough to get my eye to the eyepiece). I made
a two hole mask and sure enough I had two circular images that I merged on
the horizontal axis. I marked a line underneath the tester then proceeded
to do the vertical axis, marked another line. I then did the intermediate
axis at 45 deg.. After I was done I had 4 markes on the sheet of paper and
the distance between the longest and shortest axis was about 1/8".

I am wondering what the variables to this test are?? The pinhole and the
eyepiece were not on the same horizontal plane, the pin hole was about 3"
closer to the mirror. The pinhole probably could have been smaller and the
mirror was not spherical in one axis.

Will these things make a big difference??

Dave



                      "CSC"
                      <optic@gwi.net>          To:       "Atm"
<atm@shore.net>
                      Sent by:                 cc:
                      owner-atm@shore.n        Subject:  RE: ATM 8"f/7
astigmatism
                      et


                      10/04/2003 08:35
                      AM
                      Please respond to
                      "CSC"







Great test! 1" focal difference is bad for the image, of course, but can be
polished out, I think.

The first thing you need to do is reduce the difference.  As I mentioned,
it
is possible to make toroidal mirrors, which yours is, approximately.  In
the
old telescope making magazines, regarding making the Yolo secondary, there
is a bracket rig that allows polishing strokes, and does not allow the
mirror to rotate. This creates astigmatism, precisely.

I'd say you need to create enough astigmatism to cancel out what's there.
Just enough to get it about right.

Then spend many hours smoothing the surface with standard techniques that
will remove the roughness, etc from your special strokes.

If it was me, I'd make a regular lap and mark it permanently on the edge.
Same with the mirror.  Mark one axis, let's say across the more weakly
curved diameter (longer radius). Using mirror on top, use 1/3 to 1/2
strokes
along the direction of the mark on the mirror.  This will tend to shorten
that radius. 30 minutes will probably make a difference.

Keep the relationship between the tool and mirror throughout the session.

Now Tool on Top, and do a session stroking the tool 90 degrees from the
mark
on the mirror.

Without some mechanical restraint to hold the mirror and tool perfectly
aligned, it will probably be good for the mirror, in that it will slightly
blend the work. Remember we aren't going for perfection here.  That will be
later.


Keep testing.

The suggestion to use an elliptical lap is good.  When pressing the lap,
put
down two aluminum foil strips under the edges of the mirror at the ends of
the mark you made.  This makes the lap a sub diameter lap in one direction
and it will work to reduce the radius across the mark on the mirror.

Once the values from your test get close to zero, re press the lap and go
for a sphere.

You know what you did to get astigmatism?  Don't do that.


Colin
-----Original Message-----
From: david.a.stevens@gm.com [mailto:david.a.stevens@gm.com]
Sent: Friday, October 03, 2003 9:26 AM
To: CSC
Subject: RE: ATM 8"f/7 astigmatism



Hi Colin,

I was going to use the method described in book #2 of ATM on page 70. The
method is to use a pin hole light source and an eyepiece to catch the
reflection. By using a mask with two holes at the edges, I focus by moving
the eyepiece forward or backward to merge the two images together. I mark
where thefocus is and then rotate the mask to a different position on the
mirror and refocus for that mask position. I continue doing this to find
the long and short axis of the astig.

This is the way the book describes it, I have not tried it yet but I have
found the two focuses for one axis and it seemes like it is about 1 inch
for that particular diameter.

My question is ,once I figure out which diameter is long and short, how do
I correct this error, i.e. what stroke to use??

How bad does the astig. have to be before I need to regrind??


Thanks,
Dave



                      "CSC"
                      <optic@gwi.net>          To:       "CSC"
<optic@gwi.net>, "Atm" <atm@shore.net>
                      Sent by:                 cc:
                      owner-atm@shore.n        Subject:  RE: ATM 8"f/7
astigmatism
                      et


                      10/02/2003 06:34
                      PM
                      Please respond to
                      "CSC"







I make several pinholes on the same foil, one obviously huge.  This makes
it
easy to locate the return beam.  the other pinholes are smaller, and on or
two is usually optimal in size.

Sort of like looking at an open cluster.

Remember that lateral separation will introduce some astigmatism, so make
the mirror holder such that you can rotate the mirror to isolate what's on
the glass, and what's in the setup. I null test spheres using a laser
pointer without it's focusing lens as a source.  I have astig in the setup
but I know what it is.

Reflecting light off a ball bearing right at the eyepiece will eliminate
the
astig from the setup, as will other clever ideas using a beam splitter,
microscope slide, etc.

Colin

-----Original Message-----
From: owner-atm@shore.net [mailto:owner-atm@shore.net]On Behalf Of
david.a.stevens@gm.com
Sent: Tuesday, September 30, 2003 12:03 PM
To: Atm
Subject: RE: ATM 8"f/7 astigmatism




Thanks for all the responses,

I have determined  the mirror is astigmatic and not stressed. I still have
to determine how astigmatic it is. After looking in ATM #2 page 70 on
astigmatism I have opted to go with test setup #2. I have made a fixture
for the eyepiece and a separate fixture for the pinhole.
The only thing I have to do is to attach the eyepiece and the maglight w/
pinhole made from aluminum foil.
 Has anybody else tried this??
How hard is the setup i.e. finding the return reflection in the eyepiece??

I am doing this both as an excercise, and to find out how much astig I have
to determine if I have to go back to grinding.


Thanks,

Dave



                      "CSC"
                      <optic@gwi.net>          To:       "Atm"
<atm@shore.net>
                      Sent by:                 cc:
                      owner-atm@shore.n        Subject:  RE: ATM 8"f/7
astigmatism
                      et


                      09/24/2003 05:40
                      PM
                      Please respond to
                      "CSC"







Elegant!  Yeah, try that.

Lenscrafters start with blanks that already have the cylinder on the back
side.  They generate the front to whatever diopter you need.

Drop in some day and some young tech will be excited to give you a tour.

Starts the mind working.

Colin

-----Original Message-----
From: Richard Schwartz [mailto:richas@earthlink.net]
Sent: Wednesday, September 24, 2003 3:24 AM
To: CSC
Subject: Re: ATM 8"f/7 astigmatism


I don't know if this would work, but what happens if you polish on an
eliptical pitch lap without rotating the mirror?

Somehow I can go to lenscrapters and have astimatism-correcting eyeglasses
made while I wait.   How does THAT work?   And while we're at it, how the
blazes do they make blended bifocals and trifocals?

. . . Richard

----- Original Message -----
From: "CSC" <optic@gwi.net>
To: "Atm" <atm@shore.net>
Sent: Tuesday, September 23, 2003 12:59 PM
Subject: RE: ATM 8"f/7 astigmatism


>
> I think it will take a long time to gradually polish it out.
> Toroidal mirrors are made by NOT rotating the tool/mirror in relation to
> each other, but you need to understand which way the astigmatism is
> oriented.  I would think that polishing with tool on top across the short
> radius axis to lengthen it, and mot across the long axis to shorten it
would
> get it in fairly roughly.
>
> Probably the best way to test would be to use an artificial star
(pinhole)
> with the foucault and an eyepiece.
>
> After getting the image reasonably round, it would be a matter of some
long
> polishing spells to smooth it all out.
>
> I did try once to polish out some astig, takes work and gumption, and I
> wasn't that successful...rough results.
>
> colin
>
> -----Original Message-----
> From: owner-atm@shore.net [mailto:owner-atm@shore.net]On Behalf Of
> david.a.stevens@gm.com
> Sent: Tuesday, September 23, 2003 7:48 AM
> To: atm@shore.net
> Subject: ATM 8"f/7 astigmatism
>
>
>
> Hello everyone,
>
>
> OK, I have made a new lap last night and it is pressing as I type. I
> beleive now that the mirror is astigmatic, my question is how to get rid
of
> it??
>
> I am going to assume the astigmatism was polished in and not ground in.
>
>
> Any suggestions would be appreciated,
>
> Thanks,
>
> Dave
>
>
>
>
>
>