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RE: [taigtools] Digest Number 1170

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  • Elliot Burke
    Re: Making a really thin paraboloid I think this started out by trying to make a paraboloidal reticle. Optical systems with field curvature sometimes
    Message 1 of 2 , Sep 1, 2003
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      Re: Making a really thin paraboloid

      I think this started out by trying to make a paraboloidal reticle.
      Optical systems with field curvature sometimes approximate the field
      curvature as a paraboloid, but very often it is actually spherical (example:
      Schmidt or Maksutov system where field is exactly spherical). For some area
      around the vertex there is little difference.
      A common way to make a field of pinholes is to dust a glass surface with
      small particles of the appropriate size and evaporate aluminum over them. A
      brush is then used to remove the particles. Plastic and glass beads are
      available in the 10's of micron size.
      Alternatively I've used a spark to make small holes in aluminum films. A
      few kV to charge some capacitance through a large resistor will do it. This
      makes a relaxation oscillator, tune it to give a usable repetition rate.

      The glass surface could be a concave surface from a lens.

      regards-
      Elliot B.




      -----Original Message-----
      From: taigtools@yahoogroups.com [mailto:taigtools@yahoogroups.com]
      Sent: Monday, September 01, 2003 1:41 AM
      To: taigtools@yahoogroups.com
      Subject: [taigtools] Digest Number 1170



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      Let the chips fly!

      ------------------------------------------------------------------------

      There are 3 messages in this issue.

      Topics in this digest:

      1. Re: Making a really thin paraboloid
      From: "Robin S." <lasernerd@...>
      2. Re: Making a really thin paraboloid?
      From: Paul Anderson <wackyvorlon@...>
      3. Re: Making a really thin paraboloid?
      From: Tom Benedict <benedict@...>


      ________________________________________________________________________
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      Message: 1
      Date: Sun, 31 Aug 2003 14:54:54 -0000
      From: "Robin S." <lasernerd@...>
      Subject: Re: Making a really thin paraboloid

      --- In taigtools@yahoogroups.com, Don Rogers <Don@C...> wrote:
      > At 09:16 AM 8/30/03 +0000, you wrote:
      > >Any idea how much 0.001" aluminum foil tends to spring back
      > >after being pressed like that?
      >
      > I used to be a machine repairman in a press room where they made
      finders,
      > hoods, etc for cars. The press operation for curved surfaces
      always used a
      > two stage (at least) die. The outer would capture the metal and
      hold it
      > while the second stage would press it into shape.

      That's the pressure pad.

      >
      > For your 0.001" aluminum, start with 0.002" and with one or two
      punching
      > operations stretch it out to the final shape and thickness. The
      holes
      > would have to go in after so you are back to your dilemma.

      That's an important issue. Your previously-created hole features
      will be distorted. Also, these imperfections will cause the material
      to form in a different way than if they were not there. The holes
      could also cause the aluminum to fail during forming. If you must
      punch them first (and I wouldn't recommend it), make sure there are
      no sharp corners. That will _definately_ cause failure.

      Regards,

      Robin



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      Message: 2
      Date: Sun, 31 Aug 2003 18:41:14 -0400
      From: Paul Anderson <wackyvorlon@...>
      Subject: Re: Making a really thin paraboloid?

      Tom Benedict wrote:

      > Apparently some people at ESO pointed a four meter
      > telescope at Venus and the collected light heated up their CCD enough to
      > damage it.
      >
      Piffle! Makes you wonder what would happen if they pointed it at the sun:)



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      Message: 3
      Date: Sun, 31 Aug 2003 15:49:18 -1000 (HST)
      From: Tom Benedict <benedict@...>
      Subject: Re: Making a really thin paraboloid?

      On Sun, 31 Aug 2003, Paul Anderson wrote:

      > Tom Benedict wrote:
      >
      > > Apparently some people at ESO pointed a four meter
      > > telescope at Venus and the collected light heated up their CCD enough to
      > > damage it.
      > >
      > Piffle! Makes you wonder what would happen if they pointed it at the
      sun:)

      Got a funny on that one, too!

      If anyone gets the chance to visit McDonald Observatory in the Davis
      Mountains in West Texas, be sure to go on a tour of the summit, but also
      try to catch the solar observing.

      Frank Cianciolo, who's now director of the Visitor's Center, used to give
      the solar observing tour on a little 5" or 8" telescope. At first he did
      it with a 3" aperture mask, and used the telescope to project the image
      onto a 3x5 card, but without fail someone would ALWAYS try to stick their
      head in front of the card and look through the telescope directly. Frank
      took to starting his speech by using the telescope to burn holes in one of
      the visitor's center pamphlets, and handing it to whoever looked most
      likely to stick their head in the beam. Even with that, people still
      tried it.

      These days I think they've got some dedicated video setup, so the risk is
      a think of the past. Still, one of my fondest memories of the place is of
      Frank burning holes in a pamphlet, handing the still smoking paper to some
      kid, and saying, "Don't."

      Tom


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    • Tom Benedict
      ... In this instance the optics have yet to be designed, so it s really up to him. He figured making a parabolic shape would be tougher, from a machining
      Message 2 of 2 , Sep 1, 2003
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        On Mon, 1 Sep 2003, Elliot Burke wrote:

        > Re: Making a really thin paraboloid
        >
        > I think this started out by trying to make a paraboloidal reticle.
        > Optical systems with field curvature sometimes approximate the field
        > curvature as a paraboloid, but very often it is actually spherical
        > (example: Schmidt or Maksutov system where field is exactly spherical).
        > For some area around the vertex there is little difference.

        In this instance the optics have yet to be designed, so it's really up to
        him. He figured making a parabolic shape would be tougher, from a
        machining standpoint, than making a spherical one, so he opted to toss me
        the parabola. I'm on my third attempt at cutting the shape. Hope to have
        pictures to share by the end of the day. (The contour milling left a very
        slight, but very obvious ridge where the cutter began and ended each
        contour. I looked at approach/depart, but figured since this is an
        axially symmetric shape, I'd skip the whole milling bit and try a lathe
        instead.)

        > A common way to make a field of pinholes is to dust a glass surface with
        > small particles of the appropriate size and evaporate aluminum over
        > them. A brush is then used to remove the particles. Plastic and glass
        > beads are available in the 10's of micron size. Alternatively I've used
        > a spark to make small holes in aluminum films. A few kV to charge some
        > capacitance through a large resistor will do it. This makes a
        > relaxation oscillator, tune it to give a usable repetition rate.
        >
        > The glass surface could be a concave surface from a lens.

        Neat trick! I'll have to remember that. I can think of a couple of
        places where that sort of spark setup would be handy.

        We're either going to use a laser machining center to cut the holes, or
        we're going to use 20 micron single-mode fiber. The jury's still out on
        that. I think the guy doing the optics is leaning toward the fiber
        because it'll make illuminating the "pinholes" easier, optically. No beam
        expander and collimator coming off a laser. We'd be able to stick the
        fibers into the source end of our spectrophotometer and get whatever
        wavelength we need.

        Tom
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