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Re: Fused 12" f/6 conical mirror-Photos

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  • Ric Rokosz
    ... That 2 inch edge thickness must make the tool pretty heavy.For my first 24 incher,used hydrostone that produced -by design- an edge thickness of about 3/4
    Message 1 of 13 , Jan 1, 2005
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      --- In atm_free@yahoogroups.com, "starzkey" <starzkey@c...> wrote:


      > Right now, i am putting the finishing touches on the
      > grinding tool, which is a full-size hydrocal affair about 2" thick
      > at the edge, sealed all around with kleer-cote epoxy; the actual
      > grinding surface is 2" square plate glass "tiles" that I will bond
      > this weekend.

      That 2 inch edge thickness must make the tool pretty heavy.For my
      first 24 incher,used hydrostone that produced -by design- an edge
      thickness of about 3/4 inch for the f/3.3 mirror.Expoied ceramic
      tiles to the surface and it worked very well.
      >
      > Right now, my plan is to work entirely MOT on a full size tool and
      > lap; I am predicting this will work mainly because the final figure
      > needed is spherical. Any control I give up on the absolute radius
      > of curvature can be acccomodated with a slight reoptimization of
      the
      > relay cass design before begining fabrication of the mangin
      > secondary and relay lens elements.

      Didn't realize the primary was to be spherical,does make things
      easier overall.


      > Part of the reason I went with this plan was on the advice of the
      > mirror blank supplier, who thought I would probably run into
      trouble
      > with edge zone flexure if working TOT without edge supports.

      It did occur to me after I sent the last post as to what advise your
      supplier might give you on working his blank.Seems he's saying people
      have done his blanks with some kind of edge support,so he'd be the
      best source for info on that.Of couse you didn't mention if he meant
      small tools or full size,or the sophistication of their polishing
      machines -as in controlling pressure per unit area as the tool goes
      over the surface.Makes me wonder if he tailors his blank design to
      match the level of sophistication of the fabricators processing
      machines...almost doesn't wont atm business..:)

      >Any
      > reasonable edge support scheme just seeemed like a lot of potential
      > trouble because of the desire to periodically index the blank to
      > minimize astigmatism; any details that you can provide of how you
      > handled these issues would be interesting.

      Well,like I said in the previous post when I worked the 18 inch
      double arch blank, had numerous wooden post supports around the edge
      of the rim that were bolted into the turntable base -adjustable to a
      small degree back and forth.I marked the blank relative to one post
      and always returned it to that position after testing.Not random to
      be sure but if any astig showed up I knew where the problem area was
      and could figure out why.
      I cann't imagine the pro's moving 2 meter or larger blanks around
      like atm's do their smaller mirrors.Just my approach,and the 18
      incher showed no astig in the star test.



      > The next interesting problem will be to measure the
      > runnout of the fine ground surface with respect to the center hole
      > and the flat back surface; my tolerance analysis has suggested that
      > minimizing wedge here is necessary to achieve the best possible
      > collimation with fewest higher order chromatic residuals over a
      wide
      > spectral band.

      Yes,trickey problem.Mark Suching wasn't all that pleased about the
      Newport blanks he worked on because of the uneven casting. Conicals
      do have their own problems/solutions.
      >
      > My co-conspirator and I have already fabricated a center
      > mount/polishing pin socket/lifting fixture, so as soon the grinding
      > tool is finsihed, we are ready to begin. Wish us luck (I'm sure
      > will will need some of that before this project is completed).

      Hate to say this but you might have been better prepared if a smaller
      conical blank had been done first-easier on the nerves.When I did the
      18 incher it was a:the only such fused blank ever made,(still is as
      far as I know)b:was f/2.8,c:had the problem of quiltting to contend
      with,d: I had no idea at the start how to support the thing,and e:I
      had no idea if this design would work,not to mention the time and
      money it took to make the blank.Not quite the relaxing atm project...

      >
      > I will update the forum on our progress as it occurs.

      That'd be nice.Perhaps you could upload the photos of the raw blank
      sometime.

      Ric
    • Ken Hunter
      The 1 meter glass blank I was offered had a cellular cast shape and was to have been supported on the machine face up after having a aluminum flashing dam
      Message 2 of 13 , Jan 1, 2005
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        The 1 meter glass blank I was offered had a cellular cast shape and
        was to have been supported on the machine face up after having a
        aluminum flashing dam wrapped around it and filling the voids with a
        2 part mix of closed-cell foam... not exactly styrofoam but something
        very similar. I do not remember the details of the foam but after
        finishing the mirror, the foam was to be removed by chemical action
        with Acetone.

        Lynn Hepburn had all the details worked out (from his similar work at
        Kitt Peak) on how he was going to do the 1 meter. He knew that he was
        not going to live long enough to work it and offered it and all the
        materials to me... now I sometimes wish I had taken on the project as
        a rememberance of him.

        Live and learn...

        Ken Hunter
      • starzkey
        ... About 90 pounds or thereabouts. Not too bad if you have a lift, or a friend. But it is interesting that you found that a thinner tool worked well for
        Message 3 of 13 , Jan 1, 2005
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          > That 2 inch edge thickness must make the tool pretty heavy.
          About 90 pounds or thereabouts. Not too bad if you have a lift, or
          a friend. But it is interesting that you found that a thinner tool
          worked well for your projecct.



          Makes me wonder if he tailors his blank design to
          > match the level of sophistication of the fabricators processing
          > machines...almost doesn't wont atm business..:)

          I don't think that he considers the ATM business to be his primary
          customer, but he is small enough that an individual can still work
          with him and not get steamrollered.
          >
          >
          > Well,like I said in the previous post when I worked the 18 inch
          > double arch blank, had numerous wooden post supports around the
          edge
          > of the rim that were bolted into the turntable base -adjustable to
          a
          > small degree back and forth.I marked the blank relative to one
          post
          > and always returned it to that position after testing.

          Him, that sounds like a fairly simple approach that worked well for
          you. I will file that away for future reference.
          >
          >>
          >Mark Suching wasn't all that pleased about the
          > Newport blanks he worked on because of the uneven casting.

          My own problems with newport started way before that. It was five
          months and multiple phone calls to just get a quotation from these
          guys; needless to say, this did not leave me with a warm, fuzzy
          feeling. I also taked to Stabilite, who has since gone out of
          business amid a cloud of legal problems. So I would definitely
          recommend a little due diligence to consider the reputation,
          technical expertise as evidenced in past projects, and financial
          health (if you can get it) of any supplier before making a decision
          to buy.

          >
          > Hate to say this but you might have been better prepared if a
          smaller
          > conical blank had been done first-easier on the nerves.

          Yeah, I know, but my partner in this venture had already completed a
          32" newt some years ago, and was quite firm on his desire to go with
          the 32". If this project had been my own alone, I would definitely
          have fo0llowed your advice here. I don't ever see myself doing
          anything larger than this one.


          Scott
        • starzkey
          ... and ... a ... something ... action ... That is a very interesting and creative approach to the problem. Of course, there is all that acetone + disolved
          Message 4 of 13 , Jan 1, 2005
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            --- In atm_free@yahoogroups.com, "Ken Hunter" <kb7h@c...> wrote:
            >
            > The 1 meter glass blank I was offered had a cellular cast shape
            and
            > was to have been supported on the machine face up after having a
            > aluminum flashing dam wrapped around it and filling the voids with
            a
            > 2 part mix of closed-cell foam... not exactly styrofoam but
            something
            > very similar. I do not remember the details of the foam but after
            > finishing the mirror, the foam was to be removed by chemical
            action
            > with Acetone.

            That is a very interesting and creative approach to the problem. Of
            course, there is all that acetone + disolved residue to deal with at
            the end, which might pose a few problems in a non-industrial setting.
            >
            > Lynn Hepburn had all the details worked out (from his similar work
            at
            > Kitt Peak) on how he was going to do the 1 meter. He knew that he
            was
            > not going to live long enough to work it and offered it and all
            the
            > materials to me... now I sometimes wish I had taken on the project
            as
            > a rememberance of him.
            >
            > Live and learn...

            I'm saddened to hear that this aborted project is connected to the
            loss of a friend. There is no future in this business of growing
            old.


            Scott Milligan
          • Randy Smith
            ... I don t think the cost goes up exponentially for an amateur. I built my equatorial mounted 22 for about $4K. In todays dollars I suppose that would be
            Message 5 of 13 , Jan 1, 2005
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              kb7h@... wrote:

              --- In atm_free@yahoogroups.com, "Mitchell" <Funnybone101@c...> wrote:

              " But hey, you can get the blanks, so why not got for it? "

              Because the blank is only a small part of the story. As you go larger
              in size the costs go EXPONENTIALLY... This means if you double the
              size the price does not go up by 2 times. It goes up by 2 X 2 or 4
              times. The biggest concern is not getting the blank, it is how and
              where you are going to use it. Have you ever wondered why all
              Professional or College Observatories are located where they are?
              I don't think the cost goes up exponentially for an amateur. I built my equatorial mounted 22" for about $4K. In todays dollars I suppose that would be about $10K or $15K. On the other hand $4K for a college student who barely made $5K per year was a real stretch. The cost depends a lot on how much you can make, how good a scrounger you are, and how patient you are. Certainly some things are much more time consuming and can be more challenging to handle due to size. My fork is not heavy just big. I found a crane handy for lifting and placing it on the polar axle. I do think they could have been mated without the crane however.

              Several reasons.

              1. If you go beyond about 16 inches in size, the scope is
              tremendously limited by having to look through the lower levels of
              our atmosphere. The "SEEING" conditions are such that the
              average "AIR CELL" is a fair proportion of the optical size and the
              air currents dominate the image seen on most but the best nights
              observing.
              I hear this all the time and I used to believe it, until I started to use my 22". On nights when the seeing is bad it is bad for any telescope. However, when the seeing is reasonable... nothing beats aperture, and when the seeing is good... WOW! I have done side by side comparisons, with my 3" refractor, 6" cassegrain, and the 22", or should I say I did a no comparison between them since there is no comparison between small and big scopes.

              2. Light polution is a major concern with these scopes. If you are
              going to the trouble and expense of making a large scope, pretty soon
              you have to justify building it and spending all that money. This
              puts the scope into the "SCIENTIFIC" class of making measurements
              that are meaningful to others. Light polution effects the images so
              they look for extremely dark skies. Even with a large ATM scope these
              days, observing (Scientifically) in your backyard is not very easily
              done unless you are out in the sticks and can control the lighting
              for a mile in any direction. Sure there are things that you can do
              but the options are much less in the city.
              There needs to be no justification for building a large telescope other than you want to. The only justification I had was I like big apertures for observing and it was fun to build. With the 22" I can see the spiral structure in M51 form my drive. Can't even see M51 with the 3" and I can only see the nucleus in the 6". Certainly light pollution is a problem but aperture still produces better results. Collecting more light for the eye is essential even in light polluted skies. By the way I have a street light 3 feet from my driveway. As for doing scientific work. CCD imagers have made aperture somewhat less important. Much of the raw data for major observatories is available on the internet for researchers. There are many projects that are perfectly suitable for small scopes and medium size telescopes. Doing meaningful research is not trivial.  Just planing a project can be very difficult.

              3. Logistics... Making a 26.5 inch mirror is not hard, I've been
              (mainly not) working on one for 10 years or so... Why? Because I am
              going to retire in a couple of years and I do not relish packing up
              and moving the scope, mounting, building, etc, etc, from Texas to
              Kentucky. So, the mirror is in the crate waiting for the move. In the
              meantime, I am working on smaller fun projects and gathering parts
              for the retirement projects.

              Realistically a proper mounted scope over 24 inches in size is a
              MAJOR undertaking. I passed up on a FREE, 1 METER cast blank a few
              years ago as I realized that I could not afford to do it properly.
              Yes, the glass part would have been easy, aluminizing it would have
              cost a fortune, mounting it another fortune, putting it on top of a
              mountain, another fortune, add a building, air conditioning, a home
              nearby, etc. etc... Just wasn't going to work for me but... I still
              buy lottery tickets ;0)
              The next time you have of offer or a large mirror blank let me know. I'd love to build a 1 meter scope. Building a proper mounting is difficult but there are Dobs of that size. Looking through such a scope regularly would be such a thrill. A one meter scope would be easily trailerable to dark skies.

              You're still young and can dedicate your WHOLE LIFE to a project like
              this if you want to but, I think in 2-3 years you'll discover what
              fun girls and cars are and then look at all the time and money you
              will have wasted.

              Ken Hunter






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