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  • teveotron
    Hi friends, I am glad to faind a groop dedicated to the vacuum technology. I live in Argentina, and my english is not very good, I hope you scuse my errors. My
    Message 1 of 12 , Jun 5, 2003
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      Hi friends, I am glad to faind a groop dedicated to the vacuum
      technology.
      I live in Argentina, and my english is not very good, I hope you
      scuse my errors.

      My main activity is the rebuilding of catode ray tubes (television
      screens)but my hoby is atronomy.

      I have the idea of using the front of obsolete cathode ray tubes, as
      parabolic mirrow for telescopes.

      This front is made of good cuality glass.

      How you like the idea?

      I am waiting for answers.

      Albert Gasparini
    • RON WHITE
      Hello Albert Glad to have you join us How large is the glass from the cat. tubes and how thick? Because the light dosen t go thru the type of glass is less
      Message 2 of 12 , Jun 5, 2003
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        Hello Albert
        Glad to have you join us
        How large is the glass from the cat. tubes and how thick?
        Because the light dosen't go thru the type of glass is less important than how stable it is. It's been a long time since I've broken a TV screen but I think the glass is too thin to grind into a good mirror. let us know we have some real expert mirror makers here (I'm not one of the experts :-)
        Good luck
        Ron

        teveotron <teveotron@...> wrote:
        I have the idea of using the front of obsolete cathode ray tubes, as
        parabolic mirrow for telescopes.



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      • AGUSTIN GASPARINI
        ... Hello Ron, I am glad of your answer. When you talk of stability of the glass, it is about temperature or other factors? Respect the thickness of the glass,
        Message 3 of 12 , Jun 7, 2003
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          --- RON WHITE <ron_white@...> wrote:
          > Hello Albert
          > Glad to have you join us
          > How large is the glass from the cat. tubes and how
          > thick?
          > Because the light dosen't go thru the type of glass
          > is less important than how stable it is. It's been a
          > long time since I've broken a TV screen but I think
          > the glass is too thin to grind into a good mirror.
          > let us know we have some real expert mirror makers
          > here (I'm not one of the experts :-)
          > Good luck
          > Ron

          Hello Ron,
          I am glad of your answer.

          When you talk of stability of the glass, it is about
          temperature or other factors?
          Respect the thickness of the glass, in the front, it
          is about 12 milimeters (1/2 inch).
          The size of the front of the screen, as you know, the
          usual crt(cathode ray tube) are from 14" to 29"
          diagonal.
          Also is important to consider that already has a
          concave shape in the inside.
          I am glad to know there are real experts mirror makers
          in the group.

          I appreciate your interest in the subjet.



          =====
          Alberto Gasparini
          Perito Moreno 1032
          Godoy Cruz (5501)
          Mendoza
          ARGENTINA
          telefono 0261-422-0923

          __________________________________
          Do you Yahoo!?
          Yahoo! Calendar - Free online calendar with sync to Outlook(TM).
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        • Jeff Anderson-Lee
          ... 12 millimeters (1/2 inch) is pretty thin for any sizable mirror. You might get away with it for a small mirror if you are careful and the glass is not
          Message 4 of 12 , Jun 7, 2003
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            "AGUSTIN GASPARINI" <teveotron@...> wrote:
            > Respect the thickness of the glass, in the front, it
            > is about 12 milimeters (1/2 inch).
            > The size of the front of the screen, as you know, the
            > usual crt(cathode ray tube) are from 14" to 29"
            > diagonal.
            > Also is important to consider that already has a
            > concave shape in the inside.

            12 millimeters (1/2 inch) is pretty thin for any sizable mirror. You might
            get away with it for a small mirror if you are careful and the glass is not
            tempered (which would surprise me from the way that TV screens shatter).

            Any-old concave shape is not necessarily a plus. You want the inside to be
            a spherical section of a relatively large sphere before you are done with
            grinding.

            Radius of curvature is twice the focal length, so an 8in (203mm) f/6 mirror
            would have an 8*6=48in (1.224m) focal length and a 96in (2.448m) radius of
            curvature. The sagitta (depth of curve at center of mirror) is
            approximately S=D/(16*fratio) so for said 8in f/6 mirror, we are looking for
            1/12inch (2.12mm) of depth at the center. A 6in f/6 would have a 1/16in
            saggita as would a 4in f/4).

            One tough part may be trepanning out a small "disk" to work on. Then you
            will need to figure out how to work with a blank that has a convex bottom.
            (Perhaps the lens makers out there can jump in?)

            It sounds like it might be a fun experiment if you have the time and are not
            heavily invested in it working out.

            Jeff
          • RON WHITE
            Hi Alberto Stability = I mean lack of stress - Worst case example is tempered glass- the glass is quickly cooled usually a blast of cold air to both sides of
            Message 5 of 12 , Jun 9, 2003
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              Hi Alberto

              Stability = I mean lack of stress - Worst case example is tempered glass- the glass is quickly cooled usually a blast of cold air to both sides of the glass the outside cools quickly and the inside slower as the inside of the glass cools it puts compressive tension (stress) on the outside surface making the glass much stronger. Now if you grind one surface away the glass will warp because of the uneven stress if you keep grinding it will explode into a pile of small pieces. ---If your glass was not cooled slowly enough as you grind the glass could warp. Remember we are talking about 0.02 microns surface error.

              You can test the glass using cross polarized light for stress.

               

              It is possible to grind a mirror up to 250MM from 12 � 15 MM glass even larger sizes have been done successfully but the chance of failure is getting high.

               

              I am going to grind a 600MM diameter flat mirror from blank fabricated from 12.5MM soda glass. This is still experimental but if you are interested E-mail me directly at ron@...

              Ron



              AGUSTIN GASPARINI <teveotron@...> wrote:

              When you talk of stability of the glass, it is about
              temperature

               thickness of the glass, 12 milimeters ).
              The size of the front of the screen, 14" to 29"
              concave shape


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            • AGUSTIN GASPARINI
              ... As far as I know, the glass of crt(catode ray tubes) is not tempered. As I told to Ron, I wish I could make the test with palarized light. Is there a way
              Message 6 of 12 , Jun 10, 2003
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                > 12 millimeters (1/2 inch) is pretty thin for any
                > sizable mirror. You might
                > get away with it for a small mirror if you are
                > careful and the glass is not
                > tempered (which would surprise me from the way that
                > TV screens shatter).
                >
                > Any-old concave shape is not necessarily a plus.
                > You want the inside to be
                > a spherical section of a relatively large sphere
                > before you are done with
                > grinding.
                >
                > Radius of curvature is twice the focal length, so an
                > 8in (203mm) f/6 mirror
                > would have an 8*6=48in (1.224m) focal length and a
                > 96in (2.448m) radius of
                > curvature. The sagitta (depth of curve at center of
                > mirror) is
                > approximately S=D/(16*fratio) so for said 8in f/6
                > mirror, we are looking for
                > 1/12inch (2.12mm) of depth at the center. A 6in f/6
                > would have a 1/16in
                > saggita as would a 4in f/4).
                >
                > One tough part may be trepanning out a small "disk"
                > to work on. Then you
                > will need to figure out how to work with a blank
                > that has a convex bottom.
                > (Perhaps the lens makers out there can jump in?)
                >
                > It sounds like it might be a fun experiment if you
                > have the time and are not
                > heavily invested in it working out.
                >
                > Hi Jeff:

                As far as I know, the glass of crt(catode ray tubes)
                is not tempered.
                As I told to Ron, I wish I could make the test with
                palarized light.
                Is there a way to make a home made polariscope?
                The radius of curvature of a 21" crt is pproximatelly
                120mm wich make a too small radius.

                In wich way is affected the cuality
                of the telescope, when the radius is that small?

                The convex bottom could be filled up wit some epoxy
                or similar compound?
                >
                >


                =====
                Alberto Gasparini
                Perito Moreno 1032
                Godoy Cruz (5501)
                Mendoza
                ARGENTINA
                telefono 0261-422-0923

                __________________________________
                Do you Yahoo!?
                Yahoo! Calendar - Free online calendar with sync to Outlook(TM).
                http://calendar.yahoo.com
              • Jeff Anderson-Lee
                ... Good. That helps. ... I m not an expert but two things come to mind. (1) a pair of polarized sunglasses can be taken apart to make two polarizers, or (2)
                Message 7 of 12 , Jun 10, 2003
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                  "AGUSTIN GASPARINI" <teveotron@...> wrote:
                  > As far as I know, the glass of crt(catode ray tubes)
                  > is not tempered.

                  Good. That helps.

                  > As I told to Ron, I wish I could make the test with
                  > palarized light.
                  > Is there a way to make a home made polariscope?

                  I'm not an expert but two things come to mind. (1) a pair of polarized
                  sunglasses can be taken apart to make two polarizers, or (2) reflected light
                  is plane polarized, so a mirror plus a a super-bright led might provide a
                  polarized light source.

                  > The radius of curvature of a 21" crt is pproximatelly
                  > 120mm wich make a too small radius.
                  >
                  > In wich way is affected the cuality
                  > of the telescope, when the radius is that small?

                  Sounds like a bit too much curvature for a primary mirror. Even a small
                  mirror would have an extremely small f-ratio which would lead to major coma.
                  However it might be fine to use the convex side for a secondary for a
                  Cassegrain.

                  > The convex bottom could be filled up wit some epoxy
                  > or similar compound?

                  From what I hear, you don't want to glue on something which might have a
                  different stress pattern than the glass, otherwise it makes it hard to
                  create the proper shape as you remove glass.

                  Jeff
                • Charles Mitchard
                  Hi AGUSTIN, I have considered using the front screen of a tv before but it didnt break where I wanted it. However I came to the conclusion that the best way to
                  Message 8 of 12 , Jun 11, 2003
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                    Hi AGUSTIN,
                    I have considered using the front screen of a tv before but it didnt break
                    where I wanted it. However I came to the conclusion that the best way to
                    proceed would be to first
                    1. trepann out the circle size you want.
                    2. Make a mould of the shape you desire, ie the curve, and include sides.
                    3. line the sides of the mould with kiln shelf paper to allow a bit of
                    expansion
                    4. find someone with a temperature controlled kiln who can slump the glass
                    and then anneal it for you.
                    Find a glass artist as they have plenty of practice in these procedures and
                    discuss exactly what you want to do and use their experience in what mould
                    material to use.
                    Further grinding and polishing with a curved back surface will require
                    excellent support.
                    I am experimenting here myself and will be sitting the mirror on a flat
                    surface front face down. Placing a ring around the mirror and flooding with
                    dental stone to form a level flat surface.
                    Rubbing the dental stone down to make sure the mirror sits dead flat and
                    that there is no wedge.
                    Hope that this does not stress the mirror.
                    Mount, grind, polish in the normal manner.
                    Sand of the dental stone when finished. (I hope)
                    I will report back when I have some hard data.
                    I must stress that I haven't done this yet and the screen glass is very thin.
                    Maybe this will generate some other ideas.
                    regards
                    Charles


                    I just got lost in thought. It was unfamiliar territory.
                  • RON WHITE
                    Alberto Polarized light filters are available at some surplus optics company or from theater lighting filters. You can use the light from a LCD monitor and a
                    Message 9 of 12 , Jun 11, 2003
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                      Alberto

                      Polarized light filters are available at some surplus optics company or from theater lighting filters.

                      You can use the light from a LCD monitor and a pair of polarized   sunglasses

                      The light going thru clouds is often polarized, hold the glass up and turn it while wearing polarized   sunglasses.

                      I�ve heard that if you stack up 5 or 6 layers of window glass it will act as a polarized filter.

                      As you know when you cross polarized filters they block much of the light.

                      Stress areas in the glass will rotate the polarized light differently than other areas

                       

                      Set up the glass so the polarized light is going thru it. Look at this thru the other filter (sun glasses) Rotate until it darkens and look for light and dark patterns.

                       

                      Try this with tempered glass first so you can see what really bad stress looks like.

                      If your CRT tubes are not tempered they are probably properly annealed (stress relieved)

                       

                      Please use the same units when giving specs mixing inches and MM confuses this old brain of mine. 21 inch (530MM) diagonal size with a 4 �� (120 MM) Radius of curvature are impossible numbers I think your 120MM number refers to somthing diffrent.

                      Radius of curvature-- is the center point of a sphere, the point that a light point will focus back on itself.

                       

                      Focal length ---is the point that light from something far away comes to focus this equals � the radius of curvature.

                       

                      Sagia is the distance a curve deviates from flat. Hold a straight edge to the concave glass, measure from the center of the straight edge to the glass.

                       

                      Because the of the thickness of the glass you will not be changing the curve much

                      So the first step is finding the focal length. You can use the sagia to calculate it �easiest�, measure the distance from the glass to the focus point of the sun, or � of the distance a light bulb focuses on itself.

                      Divide the focal length by 4 the result is the largest mirror diameter that will work with this curve. (F4) This is a very fast mirror and very difficult to accurately shape.

                      Dividing the focal length by 9 (F9) will give you a slow mirror that is very easy to shape (figure).

                      A quick guess holding a piece of paper to my 18� monitor face looks like 1000MM focal length, small 14� TV = 300MM focal length, 27� TV = 600MM focal length

                      Sounds to me that you can use a CRT front glass to make a mirror 12MM thick glass is a bit thin but because it�s curved it should be no harder to work with than �� flat glass, using epoxy auto body filler to make a holder for the glass should work well just don�t glue the glass to the holder and put somthing soft between the glass and holder. if you only grind with mirror on top you will not need a holder. Glue a block to the back of the glass  for a hand hold to keep the heat from your hand away from the glass. Using pitch for glue works good.

                      I�m probably giving you way more info than you need or want. I hope some of this is usable.

                      If you have any other questions I�ll be glad to help please regard all information as my opinion and not as hard fact.

                      Good luck

                      Ron

                       



                      AGUSTIN GASPARINI <teveotron@...> wrote:
                      >
                      =====
                      Alberto Gasparini
                      Perito Moreno 1032
                      Godoy Cruz (5501)
                      Mendoza
                      ARGENTINA
                      telefono 0261-422-0923

                      __________________________________
                      Do you Yahoo!?
                      Yahoo! Calendar - Free online calendar with sync to Outlook(TM).
                      http://calendar.yahoo.com


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                    • AGUSTIN GASPARINI
                      ... I was wrong when I said 120 mm, it is 1200mm!! And your brain is OK, because you found the error! I will measure the Sagia of the 21 glass as you said,
                      Message 10 of 12 , Jun 12, 2003
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                        --- RON WHITE:
                        >
                        >As you said mixing inches and MM is confusining.
                        I was wrong when I said 120 mm, it is 1200mm!!
                        And your brain is OK, because you found the error!

                        I will measure the Sagia of the 21" glass as you
                        said, and will report it to you.
                        The slow mirrow is easy to shape because the curvature
                        is almost esferical?
                        Your quick guess holding a piece of paper to your 18�
                        monitor face, is veriy realistic.
                        I am thinking of working the parabolic shape in a
                        different way, is there eny alternative procedure.

                        I can see you have a large experience in this subjet,
                        and I thank you for your attention.

                        =====
                        Alberto Gasparini
                        Perito Moreno 1032
                        Godoy Cruz (5501)
                        Mendoza
                        ARGENTINA
                        telefono 0261-422-0923

                        __________________________________
                        Do you Yahoo!?
                        Yahoo! Calendar - Free online calendar with sync to Outlook(TM).
                        http://calendar.yahoo.com
                      • AGUSTIN GASPARINI
                        ... About annealing the glass i have some experience, I have an oven for crt(cathode ray tubes) procesisng which can reaach more than 430 centigrades. The
                        Message 11 of 12 , Jun 12, 2003
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                          --- Hi Charles Mitchard:
                          About annealing the glass i have some experience, I
                          have an oven for crt(cathode ray tubes) procesisng
                          which can reaach more than 430 centigrades.

                          The temperature of 430 centigrades is the annealing
                          point for this tipe of glass.
                          About the further grinding and polishing I am
                          thinking of holding the glass from the center an make
                          it to rotate.
                          I am not worry about not stressing the mirror, I
                          think the important cuestion is to have an even
                          stress.
                          Mount, grind, polish in the normal manner could be
                          changed for faster and more accurrate results.
                          Did eny one in the group trayed other alternatives?
                          Maybe this will also generate some other ideas.
                          I thanks your attention, regards.


                          =====
                          Alberto Gasparini
                          Perito Moreno 1032
                          Godoy Cruz (5501)
                          Mendoza
                          ARGENTINA
                          telefono 0261-422-0923

                          __________________________________
                          Do you Yahoo!?
                          Yahoo! Calendar - Free online calendar with sync to Outlook(TM).
                          http://calendar.yahoo.com
                        • Ken Hunter
                          ... Alberto, Welcome... I am the owner and moderator of this YAHOO Group, I am also the owner/moderator of an Amateur Telescope Making Group on YAHOO. Check us
                          Message 12 of 12 , Jun 15, 2003
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                            --- In VacuumX@yahoogroups.com, AGUSTIN GASPARINI <teveotron@y...>
                            wrote:

                            > Maybe this will also generate some other ideas.
                            > I thanks your attention, regards.
                            >
                            >
                            > =====
                            > Alberto Gasparini

                            Alberto,

                            Welcome...

                            I am the owner and moderator of this YAHOO Group, I am also the
                            owner/moderator of an Amateur Telescope Making Group on YAHOO.

                            Check us out:
                            http://groups.yahoo.com/group/atm_free

                            You will find MANY MORE members there that are more familiar with the
                            questions you are asking here... But please remain a member here
                            also... you will eventually need to have an aluminum coating on your
                            glass and that is where you will need the resources of THIS GROUP!

                            From past experience, getting a useful mirror out of a CRT is not
                            likely at least in sizes over 150mm diameter. The glass (if larger)
                            is too easily stressed while working and difficult to finish into an
                            acceptable figure but... in smaller sizes it is a cheap source of
                            glass and easily worked.

                            Good luck!

                            Ken Hunter
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