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Re: TMB-152 supercharged project

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  • Tube Tim
    ... Hey Randy, ... I m not a designer either, but if I understood Thomas description of the baffeling correctly, you have to take into account how large an
    Message 1 of 7 , Oct 3, 2002
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      --- In tmboptical@y..., "randyskies" <randallrp@a...> wrote:
      > Hello
      >

      Hey Randy,

      > > Also, I moved the focal point back about 10mm so that I canbinoView
      > > with my 2.8 orthos and no path corrector (needed more in-travel). I
      > > hope I did this right, and didn't go the wrong direction.
      > >
      > > One thing I am very concerned with is the extension tube had to end
      > up
      > > being 175mm internal diameter. Will this effect the light cone??
      > The
      > > lens is 175mm clear aperature.
      > >
      > > Wayne's comments to someone else comtemplating an 80mm ATM project
      > > were pretty much on the money and apply to me. The pitfalls on this
      > > simple extension tube project are many. I've read some of the ATM
      > > books, but I have to say a 3 minute talk by Mr. Back at the BFSP
      > saved
      > > my bacon on this project.
      > >
      >
      > I am the guy doing the 80mm achromat, and I am no optical designer by
      > any means. However my thoughts are:
      >
      > The size of the light cone is calculated as:
      >
      > APERTURE * ((FL-RIL)/FL)
      >
      > Where:
      >
      > FL = Focal Length
      > RIL = Racked In Length, ie..The distance from the objective to the
      > end of the focuser draw tube, when the focuser is racked in all the
      > way.
      >
      > Of course various factors come into play. But for my 80mm 500mm FL
      > design it seems like RIL = 309mm to 389mm. with the main influence
      > being whter or not to use an extender on the draw tube. Yet
      > regardless diameter of the light cone seems to be 31mm and 18mm
      > respectively. Assumnng the above formual is correct. As I said I am
      > no optical designer, just using some plain geometery here, so my
      > calculations may well be mistaken. Perhaps someone with more
      > knowledge could confirm this. However, asssuming the above formula is
      > correct, it seems like a none issue unless one plans on using a 1.25
      > inch diagonal.

      I'm not a designer either, but if I understood Thomas' description of
      the baffeling correctly, you have to take into account how large an
      image circle you want at the focal 'point'. If you treat the focal
      point as strictly a small point you will 'vignette'. For example if
      you wish to use a CCD that has a 6mm x 6mm chip size, then at the
      focal point you will need a circle of 6mm in diameter.

      To do my calculations, I used a CAD program.

      I hope I stated this correctly.



      `tube' Tim



      >
      > Any thoughts?
      >
      > Thanks
      > Randy
    • Mike Fowler
      how large an ... at the ... More likely, if the CCD is ROUND and 6mm in diameter you will need a 6mm image circle. If it is square, you need an image circle of
      Message 2 of 7 , Oct 3, 2002
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        how large an
        > image circle you want at the focal 'point'. If you treat the focal
        > point as strictly a small point you will 'vignette'. For example if
        > you wish to use a CCD that has a 6mm x 6mm chip size, then
        at the
        > focal point you will need a circle of 6mm in diameter.
        >
        > `tube' Tim
        >

        More likely, if the CCD is ROUND and 6mm in diameter you will
        need a 6mm image circle.

        If it is square, you need an image circle of at least 8.48mm.
        Hint: A squared + B squared = C squared.

        Mike Fowler
        Chicago
      • Tube Tim
        ... Busted! Your correct of course, I did the math in my head and messed up, but what s a few millimeters among friends. :-) Tim
        Message 3 of 7 , Oct 3, 2002
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          --- In tmboptical@y..., "Mike Fowler" <popocatept@a...> wrote:
          > how large an
          > > image circle you want at the focal 'point'. If you treat the focal
          > > point as strictly a small point you will 'vignette'. For example if
          > > you wish to use a CCD that has a 6mm x 6mm chip size, then
          > at the
          > > focal point you will need a circle of 6mm in diameter.
          > >
          > > `tube' Tim
          > >
          >
          > More likely, if the CCD is ROUND and 6mm in diameter you will
          > need a 6mm image circle.
          >
          > If it is square, you need an image circle of at least 8.48mm.
          > Hint: A squared + B squared = C squared.
          >
          > Mike Fowler
          > Chicago

          Busted! Your correct of course, I did the math in my head and messed
          up, but what's a few millimeters among friends. :-)


          Tim
        • Tube Tim
          ... Put the lens on the dew shield??? ... Markus gave me these (Thomas warned me of this also). It made a difference in the calculations for sure. ... Don t
          Message 4 of 7 , Oct 3, 2002
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            --- In tmboptical@y..., "buchanan8" <jim.buchanan@v...> wrote:
            > Tim,
            >

            >> Hey Jim,

            > Your supercharged project reminds me of a similair thought process
            > as I was building my 130/780. Only a larger APO lens was too
            > expensive, so I was thinking of a D&G 6 or 8 inch f10. In the end,
            > the achro would add too much length to be practical.
            >
            > I actually have one option still open in my case, and Mike C's case
            > for that matter. The Tak tube, lens ring, and dew shield, we used,
            > is of reasonable thickness to purchase a 2nd lens ring and dew
            > shield, and mount a 6" TMB on the 7" dew shield!
            >

            Put the lens on the dew shield???

            > It sounds like you have it all thought out correctly, so my comments
            > are really worth only 02 cents...
            >
            >
            >
            > > Is it correct to assume that if one sets the larger lens to be
            > exactly
            > > the delta in focal length, then it will pretty much focus the same
            > way
            > > as the original 152 lens?
            >
            > Pretty much, but as a triplet gets bigger in aperature, the
            > thickness grows, and the exact lens optical center will move. Thomas
            > can give you the EXACT figures. Having said that, any variation in

            Markus gave me these (Thomas warned me of this also). It made a
            difference in the calculations for sure.

            > actual focal length may cancel or augment the exact lens optical
            > center prediction. My own 130/780 was amazingly close to 780mm
            > actual focal length.
            >
            >
            > > Also, I moved the focal point back about 10mm so that I can
            > binoView
            > > with my 2.8 orthos and no path corrector (needed more in-travel).
            > I
            > > hope I did this right, and didn't go the wrong direction.
            >
            > Good idea, orthos are known for needing extra in-focus. Especially,
            > the short ones have the tiny objectives up close to the top of the
            > eyepiece, requiring in-focus to bring the light cone up to
            > the 'above the base' eyepiece field stop. The 2 Televue eyepieces
            > with 'above the base' field stops are the 31Nag and 22Nag.
            >
            > One thing to think about. If you use no glaspath with the 2.8s,
            > won't you get a little chromic aberation at that higher power?

            Don't know, but I suspect you are correct.

            > I was
            > concerned about a pair of O-10s not focusing w/o a glaspath, but was
            > a non-issue because I only used them with the 1.7x and Barcon.
            >
            > > One thing I am very concerned with is the extension tube had to
            > end up
            > > being 175mm internal diameter. Will this effect the light cone??
            > The
            > > lens is 175mm clear aperature.
            >
            > I would say not. I would use protostar flocking paper up to the
            > first baffle. When there is little space between the tube wall and
            > the light cone, more baffles are needed and the most important ones
            > are at the objective end. Reference Telescope Optics, Rutten. I
            > guess I would put at least 2 and possibly 3 short baffles in the
            > extension part and not worry about the main tube.
            >

            This is a relief to hear the 175 ID will work out. Thanks for the tip
            about protostar also. As to baffels, I'm pretty much stuck to one.
            At this point the flanges are made and the extension tube is next once
            the raw material (tube) arrives. Then I just have to wait for the
            lens. :-(

            > Exciting stuff. Regards,
            >

            Fer Sure.

            Thanks for your input.


            Tim

            > Jim Buchanan
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