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Re: ATM 8"f/7 astigmatism
If astigmatism is the only problem with your mirror, you can compensate for
it by tilting the mirror. You can use this undocumented feature ("defect")
to reduce the distance from the focal plane to the diagonal, and thereby use
a smaller ("cheaper") diagonal. In fact, with enough tilt, you can have the
diagonal mirror outside the cylinder of incoming light. Of course there
are drawbacks to this. I guess that is why I have never done it.
. . . Richrad
----- Original Message -----
From: "CSC" <optic@gwi.net>
To: "Atm" <atm@shore.net>
Sent: Saturday, October 04, 2003 6:35 AM
Subject: RE: ATM 8"f/7 astigmatism
>
> 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
> >
> >
> >
> >
> >
> >
>
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