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RE: [William-Optics] Re: I thought the 80mm ED triplet was considered a Apo scop

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  • James Stephens
    We re getting hung up on terminology. An achromat brings two colors to a coincident focus, and his can be achieved with a doublet. A apo brings three colors
    Message 1 of 8 , Dec 10, 2004
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      We're getting hung up on terminology. An achromat brings two colors to a
      coincident focus, and his can be achieved with a doublet. A apo brings
      three colors to a coincident focus, and this is a triplet lens. An ED
      triplet and a fluorite triplet will show different degrees of color
      correction, but if they are both triplets designed to bring three colors to
      a coincident focus, they are both apo's, by definition.
      _______________________________________
      James Stephens, PhD 601-599-4260 (h)
      29 Park Place #301 601-268-2681 ext. 40 (w)
      Hattiesburg MS 39402

      >
      >Hi Again Chris,
      >
      >I have always believed the ED is as much an APO as Flourite. I also
      >have 'read' that a lot has to do with the quality of the glass and
      >optical design, and of course the focal ratio and seeing conditions.
      >There are trade offs. It is also my understanding that generally
      >speaking, Flourite will correct color better than SD; but at an
      >expense. If anyone can speak from experinece in the field for having
      >compared the Megrez triplets in both SD and Flourite versions, it
      >would be most interesting to hear from them. I wouldn't think there
      >would be much difference visually... Perhaps a bit more difference
      >when imaging... A 80 mm objective has only so much chromatic
      >abberation anyway unless you really get to a very fast focal ratio
      >and in the old f/15 ratios, a good quality 80 mm achromat shows
      >little color, at least under the most common observing conditions...
      >
      >Clear Skies I am hoping for,
      >
      >Ed Crandall
      >-- In William-Optics@yahoogroups.com, "Chris Appleton"
      ><denturist89@s...> wrote:
      > >
      > > Hi Group
      > >
      > > I have a Megrez 80mm SD refractor and a WO110mm triplet
      >Fluorostar.
      > > I am considering the purchase of a 80mm Megrez either the Fluorite
      > > triplet or the ED triplet. I do like the results so far with the
      >ED
      > > version, but when I heard some people saying they might buy a true
      > > Apo 85mm TV, I get a little discouraged. I want to sell my old
      >80mm
      > > SD Megrez and buy a Megrez triplet for CCD imaging (widefield
      >scope)
      > > and a great travel scope for visual use. I used to have a 85mm TV
      > > and that was a good scope for viewing (never tried it for
      >imaging),
      > > but the focuser had too much image shift and would not hold it's
      > > position properly. The 85mm TV is only a doublet like my current
      > > Megrez, but I believe one of it's lenses is an SD (NOT LIKE THE
      > > MEGREZ SD) special dispersion glass. I thought ED glass was Apo
      > > material esp. having the scope setup as a trplet?
      > >
      > > Could someone please clarify this issue for us future potential
      > > Megrez buyer's. There is a huge difference in price between the
      > > Megrez 80mm Fluorite and the ED version.
      > >
      > > Thank you!
      > >
      > > Regards,
      > >
      > > Chris A
      > > Toronto
      >
      >
      >
      >
      >
      >
      >
      >Yahoo! Groups Links
      >
      >
      >
      >
      >
      >
      >
    • Chris Appleton
      ... wrote: Hi Ed Thanks for the feedback! There is a difference in SD and ED glass though. ED glass is extra-low dispersion glass and SD is
      Message 2 of 8 , Dec 10, 2004
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        --- In William-Optics@yahoogroups.com, "Edward Crandall"
        <ecrandall@t...> wrote:

        Hi Ed

        Thanks for the feedback! There is a difference in SD and ED glass
        though. ED glass is extra-low dispersion glass and SD is super-low
        (even lower)dispersion type glass. I have read articles stating that
        SD and Fluorite glass produced the same results and the most
        important concern is that the scope is a triplet design in which the
        TV85mm is only a doublet. In the reports though, I've read that
        fluorite does not last as long (shorter lifespan) as SD type glass.
        I know that Televue say's it uses SD as one of the lenses in the
        TV85mm and 102mm refractors and they classify their SD as special
        dispersion glass. I know my older Megrez 80mm SD has one of the two
        lenses as SD, but I am not sure what William Optics means by this SD
        label?

        My WO110mm trplet Apo from what I've read on their web site uses
        crown glass on both sides of a SD super-low dispersion glass lens.
        There are two example pictures on the WO web site showing how the
        colors all merge at the exact same point in both the triplets being
        either SD or Fluorite. I could not find this eg. for the ED triplet
        though.

        BTW Ed- the new 80mm triplet uses ED and NOT SD glass for their
        center lens.

        Cheers,

        Chris Appleton
        >
        > Hi Again Chris,
        >
        > I have always believed the ED is as much an APO as Flourite. I
        also
        > have 'read' that a lot has to do with the quality of the glass and
        > optical design, and of course the focal ratio and seeing
        conditions.
        > There are trade offs. It is also my understanding that generally
        > speaking, Flourite will correct color better than SD; but at an
        > expense. If anyone can speak from experinece in the field for
        having
        > compared the Megrez triplets in both SD and Flourite versions, it
        > would be most interesting to hear from them. I wouldn't think
        there
        > would be much difference visually... Perhaps a bit more difference
        > when imaging... A 80 mm objective has only so much chromatic
        > abberation anyway unless you really get to a very fast focal ratio
        > and in the old f/15 ratios, a good quality 80 mm achromat shows
        > little color, at least under the most common observing
        conditions...
        >
        > Clear Skies I am hoping for,
        >
        > Ed Crandall
        > -- In William-Optics@yahoogroups.com, "Chris Appleton"
        > <denturist89@s...> wrote:
        > >
        > > Hi Group
        > >
        > > I have a Megrez 80mm SD refractor and a WO110mm triplet
        > Fluorostar.
        > > I am considering the purchase of a 80mm Megrez either the
        Fluorite
        > > triplet or the ED triplet. I do like the results so far with the
        > ED
        > > version, but when I heard some people saying they might buy a
        true
        > > Apo 85mm TV, I get a little discouraged. I want to sell my old
        > 80mm
        > > SD Megrez and buy a Megrez triplet for CCD imaging (widefield
        > scope)
        > > and a great travel scope for visual use. I used to have a 85mm
        TV
        > > and that was a good scope for viewing (never tried it for
        > imaging),
        > > but the focuser had too much image shift and would not hold it's
        > > position properly. The 85mm TV is only a doublet like my current
        > > Megrez, but I believe one of it's lenses is an SD (NOT LIKE THE
        > > MEGREZ SD) special dispersion glass. I thought ED glass was Apo
        > > material esp. having the scope setup as a trplet?
        > >
        > > Could someone please clarify this issue for us future potential
        > > Megrez buyer's. There is a huge difference in price between the
        > > Megrez 80mm Fluorite and the ED version.
        > >
        > > Thank you!
        > >
        > > Regards,
        > >
        > > Chris A
        > > Toronto
      • Chris Appleton
        ... wrote: Hi James Thank you very much for the feedback. That s what I have always read or been told that only a Triplet Lens Design can or
        Message 3 of 8 , Dec 10, 2004
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          --- In William-Optics@yahoogroups.com, "James Stephens"
          <j_m_step@h...> wrote:

          Hi James

          Thank you very much for the feedback. That's what I have always read
          or been told that only a "Triplet Lens Design" can or should be
          classified as a true Apo design. This would mean that the TV 85,
          102mm & all other doublet refractors are not true Apo refractors!

          Cheers,

          Chris Appleton
          >
          > We're getting hung up on terminology. An achromat brings two
          colors to a
          > coincident focus, and his can be achieved with a doublet. A apo
          brings
          > three colors to a coincident focus, and this is a triplet lens.
          An ED
          > triplet and a fluorite triplet will show different degrees of
          color
          > correction, but if they are both triplets designed to bring three
          colors to
          > a coincident focus, they are both apo's, by definition.
          > _______________________________________
          > James Stephens, PhD 601-599-4260 (h)
          > 29 Park Place #301 601-268-2681 ext. 40 (w)
          > Hattiesburg MS 39402
          >
          > >
          > >Hi Again Chris,
          > >
          > >I have always believed the ED is as much an APO as Flourite. I
          also
          > >have 'read' that a lot has to do with the quality of the glass and
          > >optical design, and of course the focal ratio and seeing
          conditions.
          > >There are trade offs. It is also my understanding that generally
          > >speaking, Flourite will correct color better than SD; but at an
          > >expense. If anyone can speak from experinece in the field for
          having
          > >compared the Megrez triplets in both SD and Flourite versions, it
          > >would be most interesting to hear from them. I wouldn't think
          there
          > >would be much difference visually... Perhaps a bit more difference
          > >when imaging... A 80 mm objective has only so much chromatic
          > >abberation anyway unless you really get to a very fast focal ratio
          > >and in the old f/15 ratios, a good quality 80 mm achromat shows
          > >little color, at least under the most common observing
          conditions...
          > >
          > >Clear Skies I am hoping for,
          > >
          > >Ed Crandall
          > >-- In William-Optics@yahoogroups.com, "Chris Appleton"
          > ><denturist89@s...> wrote:
          > > >
          > > > Hi Group
          > > >
          > > > I have a Megrez 80mm SD refractor and a WO110mm triplet
          > >Fluorostar.
          > > > I am considering the purchase of a 80mm Megrez either the
          Fluorite
          > > > triplet or the ED triplet. I do like the results so far with
          the
          > >ED
          > > > version, but when I heard some people saying they might buy a
          true
          > > > Apo 85mm TV, I get a little discouraged. I want to sell my old
          > >80mm
          > > > SD Megrez and buy a Megrez triplet for CCD imaging (widefield
          > >scope)
          > > > and a great travel scope for visual use. I used to have a 85mm
          TV
          > > > and that was a good scope for viewing (never tried it for
          > >imaging),
          > > > but the focuser had too much image shift and would not hold
          it's
          > > > position properly. The 85mm TV is only a doublet like my
          current
          > > > Megrez, but I believe one of it's lenses is an SD (NOT LIKE THE
          > > > MEGREZ SD) special dispersion glass. I thought ED glass was Apo
          > > > material esp. having the scope setup as a trplet?
          > > >
          > > > Could someone please clarify this issue for us future potential
          > > > Megrez buyer's. There is a huge difference in price between the
          > > > Megrez 80mm Fluorite and the ED version.
          > > >
          > > > Thank you!
          > > >
          > > > Regards,
          > > >
          > > > Chris A
          > > > Toronto
          > >
          > >
          > >
          > >
          > >
          > >
          > >
          > >Yahoo! Groups Links
          > >
          > >
          > >
          > >
          > >
          > >
          > >
        • Ed Crandall
          Thanks for a well thought out response. I still think Fluorite is better than SD. I also read stuff that suggest that Fluorite doesn t break down. Don t ask
          Message 4 of 8 , Dec 10, 2004
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            Thanks for a well thought out response. I still think Fluorite is 'better' than SD. I also read stuff that suggest that Fluorite doesn't break down. Don't ask me to cite that authority!  As you may recall, I just recently acquired a WO 110 FLT f/6.5 APO. So far I can confirm (not that it needs confirmation!) that I have a true APO, visually. Thus, I haven't been able to induce color and the images of stars are pin point and distortion hasn't been perceptible although I need some more time under clear skies to do more fun testing. The quality of the glass is another thing that seems to be left out from time to time, as does the definition of an APO as James Stephens points out. A true APO --triplet--is probably more important for imaging than visual applications. Anyway, all this is interesting and fun to talk about. Question: What quantitative parameters differentiates SD for ED? Sounds like a fancy question, but I don't recall seeing a definitive answer although I am sure there is one?
             
            Best, and again thanks for your helpful and interesting messages,
             
            Ed
            ----- Original Message -----
            Sent: Friday, December 10, 2004 7:50 PM
            Subject: [William-Optics] Re: I thought the 80mm ED triplet was considered a Apo scope?


            --- In William-Optics@yahoogroups.com, "Edward Crandall"
            <ecrandall@t...> wrote:

            Hi Ed

            Thanks for the feedback! There is a difference in SD and ED glass
            though. ED glass is extra-low dispersion glass and SD is super-low
            (even lower)dispersion type glass. I have read articles stating that
            SD and Fluorite glass produced the same results and the most
            important concern is that the scope is a triplet design in which the
            TV85mm is only a doublet. In the reports though, I've read that
            fluorite does not last as long (shorter lifespan) as SD type glass.
            I know that Televue say's it uses SD as one of the lenses in the
            TV85mm and 102mm refractors and they classify their SD as special
            dispersion glass. I know my older Megrez 80mm SD has one of the two
            lenses as SD, but I am not sure what William Optics means by this SD
            label?

            My WO110mm trplet Apo from what I've read on their web site uses
            crown glass on both sides of a SD super-low dispersion glass lens.
            There are two example pictures on the WO web site showing how the
            colors all merge at the exact same point in both the triplets being
            either SD or Fluorite. I could not find this eg. for the ED triplet
            though.

            BTW Ed- the new 80mm triplet uses ED and NOT SD glass for their
            center lens.

            Cheers,

            Chris Appleton
            >
            > Hi Again Chris,
            >
            > I have always believed the ED is as much an APO as Flourite. I
            also
            > have 'read' that a lot has to do with the quality of the glass and
            > optical design, and of course the focal ratio and seeing
            conditions.
            > There are trade offs. It is also my understanding that generally
            > speaking, Flourite will correct color better than SD; but at an
            > expense. If anyone can speak from experinece in the field for
            having
            > compared the Megrez triplets in both SD and Flourite versions, it
            > would be most interesting to hear from them. I wouldn't think
            there
            > would be much difference visually... Perhaps a bit more difference
            > when imaging... A 80 mm objective has only so much chromatic
            > abberation anyway unless you really get to a very fast focal ratio
            > and in the old f/15 ratios, a good quality 80 mm achromat shows
            > little color, at least under the most common observing
            conditions...
            >
            > Clear Skies I am hoping for,
            >
            > Ed Crandall
            > -- In William-Optics@yahoogroups.com, "Chris Appleton"
            > <denturist89@s...> wrote:
            > >
            > > Hi Group
            > >
            > > I have a Megrez 80mm SD refractor and a WO110mm triplet
            > Fluorostar.
            > > I am considering the purchase of a 80mm Megrez either the
            Fluorite
            > > triplet or the ED triplet. I do like the results so far with the
            > ED
            > > version, but when I heard some people saying they might buy a
            true
            > > Apo 85mm TV, I get a little discouraged. I want to sell my old
            > 80mm
            > > SD Megrez and buy a Megrez triplet for CCD imaging (widefield
            > scope)
            > > and a great travel scope for visual use. I used to have a 85mm
            TV
            > > and that was a good scope for viewing (never tried it for
            > imaging),
            > > but the focuser had too much image shift and would not hold it's
            > > position properly. The 85mm TV is only a doublet like my current
            > > Megrez, but I believe one of it's lenses is an SD (NOT LIKE THE
            > > MEGREZ SD) special dispersion glass. I thought ED glass was Apo
            > > material esp. having the scope setup as a trplet?
            > >
            > > Could someone please clarify this issue for us future potential
            > > Megrez buyer's. There is a huge difference in price between the
            > > Megrez 80mm Fluorite and the ED version.
            > >
            > > Thank you!
            > >
            > > Regards,
            > >
            > > Chris A
            > > Toronto




          • Chris Appleton
            ... wrote: Hi Ed I think this question is best left for Mr. William Yang to answer accurately for us all. I would then like to know if the
            Message 5 of 8 , Dec 10, 2004
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              --- In William-Optics@yahoogroups.com, "Ed Crandall"
              <ecrandall@t...> wrote:


              Hi Ed

              I think this question is best left for Mr. William Yang to answer
              accurately for us all. I would then like to know if the middle lens
              on the new 80mm ED triplet is the SAME type of glass that is used in
              the middle lens of the WO110mm triplet in which clearly states SD
              glass and not ED glass. I am sure the quality of the ED & SD glass
              is not the same. Look on the WO web site and you will see the graph
              showing how the colors merge with the triplet SD and fluorite are
              identicle in pattern. I would love to have the WO110mm and 80mm
              fluorite side by side for a color comparison test.

              Cheers,

              Chris A


              > Thanks for a well thought out response. I still think Fluorite
              is 'better' than SD. I also read stuff that suggest that Fluorite
              doesn't break down. Don't ask me to cite that authority! As you may
              recall, I just recently acquired a WO 110 FLT f/6.5 APO. So far I
              can confirm (not that it needs confirmation!) that I have a true
              APO, visually. Thus, I haven't been able to induce color and the
              images of stars are pin point and distortion hasn't been perceptible
              although I need some more time under clear skies to do more fun
              testing. The quality of the glass is another thing that seems to be
              left out from time to time, as does the definition of an APO as
              James Stephens points out. A true APO --triplet--is probably more
              important for imaging than visual applications. Anyway, all this is
              interesting and fun to talk about. Question: What quantitative
              parameters differentiates SD for ED? Sounds like a fancy question,
              but I don't recall seeing a definitive answer although I am sure
              there is one?
              >
              > Best, and again thanks for your helpful and interesting messages,
              >
              > Ed
              > ----- Original Message -----
              > From: Chris Appleton
              > To: William-Optics@yahoogroups.com
              > Sent: Friday, December 10, 2004 7:50 PM
              > Subject: [William-Optics] Re: I thought the 80mm ED triplet was
              considered a Apo scope?
              >
              >
              >
              > --- In William-Optics@yahoogroups.com, "Edward Crandall"
              > <ecrandall@t...> wrote:
              >
              > Hi Ed
              >
              > Thanks for the feedback! There is a difference in SD and ED
              glass
              > though. ED glass is extra-low dispersion glass and SD is super-
              low
              > (even lower)dispersion type glass. I have read articles stating
              that
              > SD and Fluorite glass produced the same results and the most
              > important concern is that the scope is a triplet design in which
              the
              > TV85mm is only a doublet. In the reports though, I've read that
              > fluorite does not last as long (shorter lifespan) as SD type
              glass.
              > I know that Televue say's it uses SD as one of the lenses in the
              > TV85mm and 102mm refractors and they classify their SD as
              special
              > dispersion glass. I know my older Megrez 80mm SD has one of the
              two
              > lenses as SD, but I am not sure what William Optics means by
              this SD
              > label?
              >
              > My WO110mm trplet Apo from what I've read on their web site uses
              > crown glass on both sides of a SD super-low dispersion glass
              lens.
              > There are two example pictures on the WO web site showing how
              the
              > colors all merge at the exact same point in both the triplets
              being
              > either SD or Fluorite. I could not find this eg. for the ED
              triplet
              > though.
              >
              > BTW Ed- the new 80mm triplet uses ED and NOT SD glass for their
              > center lens.
              >
              > Cheers,
              >
              > Chris Appleton
              > >
              > > Hi Again Chris,
              > >
              > > I have always believed the ED is as much an APO as Flourite. I
              > also
              > > have 'read' that a lot has to do with the quality of the glass
              and
              > > optical design, and of course the focal ratio and seeing
              > conditions.
              > > There are trade offs. It is also my understanding that
              generally
              > > speaking, Flourite will correct color better than SD; but at
              an
              > > expense. If anyone can speak from experinece in the field for
              > having
              > > compared the Megrez triplets in both SD and Flourite versions,
              it
              > > would be most interesting to hear from them. I wouldn't think
              > there
              > > would be much difference visually... Perhaps a bit more
              difference
              > > when imaging... A 80 mm objective has only so much chromatic
              > > abberation anyway unless you really get to a very fast focal
              ratio
              > > and in the old f/15 ratios, a good quality 80 mm achromat
              shows
              > > little color, at least under the most common observing
              > conditions...
              > >
              > > Clear Skies I am hoping for,
              > >
              > > Ed Crandall
              > > -- In William-Optics@yahoogroups.com, "Chris Appleton"
              > > <denturist89@s...> wrote:
              > > >
              > > > Hi Group
              > > >
              > > > I have a Megrez 80mm SD refractor and a WO110mm triplet
              > > Fluorostar.
              > > > I am considering the purchase of a 80mm Megrez either the
              > Fluorite
              > > > triplet or the ED triplet. I do like the results so far with
              the
              > > ED
              > > > version, but when I heard some people saying they might buy
              a
              > true
              > > > Apo 85mm TV, I get a little discouraged. I want to sell my
              old
              > > 80mm
              > > > SD Megrez and buy a Megrez triplet for CCD imaging
              (widefield
              > > scope)
              > > > and a great travel scope for visual use. I used to have a
              85mm
              > TV
              > > > and that was a good scope for viewing (never tried it for
              > > imaging),
              > > > but the focuser had too much image shift and would not hold
              it's
              > > > position properly. The 85mm TV is only a doublet like my
              current
              > > > Megrez, but I believe one of it's lenses is an SD (NOT LIKE
              THE
              > > > MEGREZ SD) special dispersion glass. I thought ED glass was
              Apo
              > > > material esp. having the scope setup as a trplet?
              > > >
              > > > Could someone please clarify this issue for us future
              potential
              > > > Megrez buyer's. There is a huge difference in price between
              the
              > > > Megrez 80mm Fluorite and the ED version.
              > > >
              > > > Thank you!
              > > >
              > > > Regards,
              > > >
              > > > Chris A
              > > > Toronto
              >
              >
              >
              >
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            • Jim Johnson
              ... is better than SD. I also read stuff that suggest that Fluorite doesn t break down. Don t ask me to cite that authority! As you may recall, I just
              Message 6 of 8 , Dec 13, 2004
              • 0 Attachment
                --- In William-Optics@yahoogroups.com, "Ed Crandall"
                <ecrandall@t...> wrote:
                > Thanks for a well thought out response. I still think Fluorite
                is 'better' than SD. I also read stuff that suggest that Fluorite
                doesn't break down. Don't ask me to cite that authority! As you may
                recall, I just recently acquired a WO 110 FLT f/6.5 APO. So far I
                can confirm (not that it needs confirmation!) that I have a true
                APO, visually. Thus, I haven't been able to induce color and the
                images of stars are pin point and distortion hasn't been perceptible
                although I need some more time under clear skies to do more fun
                testing. The quality of the glass is another thing that seems to be
                left out from time to time, as does the definition of an APO as
                James Stephens points out. A true APO --triplet--is probably more
                important for imaging than visual applications. Anyway, all this is
                interesting and fun to talk about. Question: What quantitative
                parameters differentiates SD for ED? Sounds like a fancy question,
                but I don't recall seeing a definitive answer although I am sure
                there is one?
                >
                > Best, and again thanks for your helpful and interesting messages,
                >
                > Ed
                > ----- Original Message -----
                > From: Chris Appleton
                > To: William-Optics@yahoogroups.com
                > Sent: Friday, December 10, 2004 7:50 PM
                > Subject: [William-Optics] Re: I thought the 80mm ED triplet was
                considered a Apo scope?
                >
                >
                >
                > --- In William-Optics@yahoogroups.com, "Edward Crandall"
                > <ecrandall@t...> wrote:
                >
                > Hi Ed
                >
                > Thanks for the feedback! There is a difference in SD and ED
                glass
                > though. ED glass is extra-low dispersion glass and SD is super-
                low
                > (even lower)dispersion type glass. I have read articles stating
                that
                > SD and Fluorite glass produced the same results and the most
                > important concern is that the scope is a triplet design in which
                the
                > TV85mm is only a doublet. In the reports though, I've read that
                > fluorite does not last as long (shorter lifespan) as SD type
                glass.
                > I know that Televue say's it uses SD as one of the lenses in the
                > TV85mm and 102mm refractors and they classify their SD as
                special
                > dispersion glass. I know my older Megrez 80mm SD has one of the
                two
                > lenses as SD, but I am not sure what William Optics means by
                this SD
                > label?
                >
                > My WO110mm trplet Apo from what I've read on their web site uses
                > crown glass on both sides of a SD super-low dispersion glass
                lens.
                > There are two example pictures on the WO web site showing how
                the
                > colors all merge at the exact same point in both the triplets
                being
                > either SD or Fluorite. I could not find this eg. for the ED
                triplet
                > though.
                >
                > BTW Ed- the new 80mm triplet uses ED and NOT SD glass for their
                > center lens.
                >
                > Cheers,
                >
                > Chris Appleton
                > >
                > > Hi Again Chris,
                > >
                > > I have always believed the ED is as much an APO as Flourite. I
                > also
                > > have 'read' that a lot has to do with the quality of the glass
                and
                > > optical design, and of course the focal ratio and seeing
                > conditions.
                > > There are trade offs. It is also my understanding that
                generally
                > > speaking, Flourite will correct color better than SD; but at
                an
                > > expense. If anyone can speak from experinece in the field for
                > having
                > > compared the Megrez triplets in both SD and Flourite versions,
                it
                > > would be most interesting to hear from them. I wouldn't think
                > there
                > > would be much difference visually... Perhaps a bit more
                difference
                > > when imaging... A 80 mm objective has only so much chromatic
                > > abberation anyway unless you really get to a very fast focal
                ratio
                > > and in the old f/15 ratios, a good quality 80 mm achromat
                shows
                > > little color, at least under the most common observing
                > conditions...
                > >
                > > Clear Skies I am hoping for,
                > >
                > > Ed Crandall
                > > -- In William-Optics@yahoogroups.com, "Chris Appleton"
                > > <denturist89@s...> wrote:
                > > >
                > > > Hi Group
                > > >
                > > > I have a Megrez 80mm SD refractor and a WO110mm triplet
                > > Fluorostar.
                > > > I am considering the purchase of a 80mm Megrez either the
                > Fluorite
                > > > triplet or the ED triplet. I do like the results so far with
                the
                > > ED
                > > > version, but when I heard some people saying they might buy
                a
                > true
                > > > Apo 85mm TV, I get a little discouraged. I want to sell my
                old
                > > 80mm
                > > > SD Megrez and buy a Megrez triplet for CCD imaging
                (widefield
                > > scope)
                > > > and a great travel scope for visual use. I used to have a
                85mm
                > TV
                > > > and that was a good scope for viewing (never tried it for
                > > imaging),
                > > > but the focuser had too much image shift and would not hold
                it's
                > > > position properly. The 85mm TV is only a doublet like my
                current
                > > > Megrez, but I believe one of it's lenses is an SD (NOT LIKE
                THE
                > > > MEGREZ SD) special dispersion glass. I thought ED glass was
                Apo
                > > > material esp. having the scope setup as a trplet?
                > > >
                > > > Could someone please clarify this issue for us future
                potential
                > > > Megrez buyer's. There is a huge difference in price between
                the
                > > > Megrez 80mm Fluorite and the ED version.
                > > >
                > > > Thank you!
                > > >
                > > > Regards,
                > > >
                > > > Chris A
                > > > Toronto
                >
                >
                >
                >
                Here is a link to a great essay on what constitutes an Apochromatic
                scope. Also, in other posts by Roland Christen, many claims for ED
                glass don't always mean better correction. Also, some doublet lenses
                may qualify as apo given the comonly accepted definition, while some
                triplets are barely semi-apo. I guess the proof is in the in and out-
                of-focus star tests.

                Jim Johnson

                http://www.tmboptical.com/itemsGrid.asp?cat_id=32
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