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Re: Colour coding on the aberration graph

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  • Craig Stark
    OK, so here is what I believe it means. I am no expert, but I did look into this a bit some time ago. FWIW, this page may help explain the issues:
    Message 1 of 7 , Jan 23, 2006
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      OK, so here is what I believe it means. I am no expert, but I did
      look into this a bit some time ago. FWIW, this page may help explain
      the issues:
      http://www.vanwalree.com/optics/chromatic.html

      The "Longitudinal Aberration" is showing the distance between focal
      planes for light of varying wavelengths. Red light focuses in a
      different place that blue light. As we visually focus on green (where
      we are most sensitive), the shift in focal planes for different freqs
      of light "blur" those freqs and lead to things like color halos.

      So, what this graph is showing is for different frequencies (I think
      we can safely ignore the 435 and 707nm) how far off focus are we? If
      we find the spot where they cluster the most (about the third tick
      from the top) and say that we have our focuser at this position
      (y-axis here is focal plane position), we can see that we have a nice
      clustering. Ignore the dark blue and the dark red (getting pretty UV
      and IR) and the focal plane shift for a nice cluster in there save the
      ~656 (Ha-like line) is on the order of .05mm.

      Craig
    • Scott Walker
      Someone has already answered the color question. I thought I would answer your question on what the graph means. The graph shows how different wavelengths
      Message 2 of 7 , Jan 23, 2006
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        Someone has already answered the color question. I thought I would answer your question on what the graph means. The graph shows how different wavelengths focus as a function of lens radial position. The bottom of the y axis is the center of the lens. The top is the outer diameter. Restated this graph shows the spherical aberration of the lens at different colors and chromatic aberration. The eye is most sensitive to light around 550nmeter, so the graphs should be about a straight line at this wavelength in a well designed lens. The graph for the Fluorite Doublet shows the lens to be very well corrected at all but the wavelength extremes. At 435nmeters the focus point is about .125mm from the 550nmeter point. (typically the focus position is calculated at 70% of full aperture). This shift in focus will cause a blur diameter of about .02mm (.125/f-stop). So at 435nm the blur is much larger than due to diffraction(around .008mm). For visual use this is of little concern because the eye has very little sensitivity at this wavelength. One of the questions floating around has been what is the difference between an APO doublet and an APO triplet. WO address this question with the longitudinal aberration chart shown in Megrez 80 fluorite triplet. It shows the curves for a doublet and a triplet. In general the triplet can bring the focus points a the 70% of max radius, much closer together over the full visual range. Also look at the curves for the Megrez 80 II ED triplet APO. Even though it uses a lesser glass than the fluorite doublet, the color correction at the extremes is much better. The comparison between a triplet and a doublet is also shown on the WO website in the 66 ED APO section. Here spot diameters are shown. As one can see the spot diameters are much small at the wavelength extremes for the triplet. The photographic process tends to be more sensitive to the color extremes. This is why triplet are often preferred for photographic work. I hope this helps.
         
        Scott
         
         
        ----- Original Message -----
        Sent: Monday, January 23, 2006 7:19 AM
        Subject: [William-Optics] Re: Colour coding on the aberration graph

        Good question Dmitri!

        I want to know more than that, I also want someone to explain how to
        read it and what it really means.

        I want to be able to add this to the FAQs I am working on for the
        group, as well as for my own understanding.

        I actually went out and downloaded the Zemax demo, but even after
        reading and understanding the basics, and searching Google, I still
        don't understand what the charts mean, to be honest, and how to use
        them to compare one scope to another.

        The real software costs $2000-$4000 and then you have to pay for
        training, so maybe this is one of those things that you can't learn
        for free, but lets see if anyone has an explanation in our group.

        Anyone?  Tom?  Ron?  Anyone feel free to offer to explain these.

        Also - Tom Trusak has posted his opinions and thoughts on doublets
        vs. triplets, and I will try to includes some of that in the FAQs
        since it comes up often as well.  I have a few sites that are very
        technical, but if anyone has an easy web site that helps explain it
        feel free to post it.

        The majjority of companies do NOT post these types of charts, or the
        glass specs, or the color correction levels, or quality control
        specifications.  Perhaps WO is just more honest, or perhaps they feel
        that the charts help people, or perhaps other companies just don't
        want to open the kimono that far because it lets competitors see what
        they are doing, and gives them a chance to try to one-up them with a
        better looking chart.

        Either way - if anyone has any information, let us all know.

        Timm

        --- In William-Optics@yahoogroups.com, "kulginoff"
        <dmitri.kulginov@e...> wrote:
        >
        > I would like to know the colour legend on the chromatic aberration
        > graph for Zenith Star Fluorite Doublet on the WO web site,
        >
        > http://www.william-optics.com/wowebs/prod_tel/zsfd80/chrom.htm
        >
        > Thankfully,
        > Dmitri
        >





      • Timm Bottoni
        Thanks Scott, It does help - I will have to read it a few times over while looking at the charts when i am not at work, and being interupted by other less
        Message 3 of 7 , Jan 23, 2006
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          Thanks Scott,

          It does help - I will have to read it a few times over while looking
          at the charts when i am not at work, and being interupted by other
          less important topics to fully grasp it ;-)

          Do you know of any web sites that explain it?

          I have Googled a number of times on things like "longitudinal
          chromatic aberration", but never really found anything that would
          explain those charts, so perhaps there aren't any, or perhaps I am
          Googling the wrong words.

          Thanks,

          Timm

          --- In William-Optics@yahoogroups.com, "Scott Walker" <sdwalker@c...>
          wrote:
          >
          > Someone has already answered the color question. I thought I would
          answer your question on what the graph means. The graph shows how
          different wavelengths focus as a function of lens radial position.
          The bottom of the y axis is the center of the lens. The top is the
          outer diameter. Restated this graph shows the spherical aberration of
          the lens at different colors and chromatic aberration. The eye is
          most sensitive to light around 550nmeter, so the graphs should be
          about a straight line at this wavelength in a well designed lens. The
          graph for the Fluorite Doublet shows the lens to be very well
          corrected at all but the wavelength extremes. At 435nmeters the focus
          point is about .125mm from the 550nmeter point. (typically the focus
          position is calculated at 70% of full aperture). This shift in focus
          will cause a blur diameter of about .02mm (.125/f-stop). So at 435nm
          the blur is much larger than due to diffraction(around .008mm). For
          visual use this is of little concern because the eye has very little
          sensitivity at this wavelength. One of the questions floating around
          has been what is the difference between an APO doublet and an APO
          triplet. WO address this question with the longitudinal aberration
          chart shown in Megrez 80 fluorite triplet. It shows the curves for a
          doublet and a triplet. In general the triplet can bring the focus
          points a the 70% of max radius, much closer together over the full
          visual range. Also look at the curves for the Megrez 80 II ED triplet
          APO. Even though it uses a lesser glass than the fluorite doublet,
          the color correction at the extremes is much better. The comparison
          between a triplet and a doublet is also shown on the WO website in
          the 66 ED APO section. Here spot diameters are shown. As one can see
          the spot diameters are much small at the wavelength extremes for the
          triplet. The photographic process tends to be more sensitive to the
          color extremes. This is why triplet are often preferred for
          photographic work. I hope this helps.
          >
          > Scott
          >
          >
          > ----- Original Message -----
          > From: Timm Bottoni
          > To: William-Optics@yahoogroups.com
          > Sent: Monday, January 23, 2006 7:19 AM
          > Subject: [William-Optics] Re: Colour coding on the aberration
          graph
          >
          >
          > Good question Dmitri!
          >
          > I want to know more than that, I also want someone to explain how
          to
          > read it and what it really means.
          >
          > I want to be able to add this to the FAQs I am working on for the
          > group, as well as for my own understanding.
          >
          > I actually went out and downloaded the Zemax demo, but even after
          > reading and understanding the basics, and searching Google, I
          still
          > don't understand what the charts mean, to be honest, and how to
          use
          > them to compare one scope to another.
          >
          > The real software costs $2000-$4000 and then you have to pay for
          > training, so maybe this is one of those things that you can't
          learn
          > for free, but lets see if anyone has an explanation in our group.
          >
          > Anyone? Tom? Ron? Anyone feel free to offer to explain these.
          >
          > Also - Tom Trusak has posted his opinions and thoughts on
          doublets
          > vs. triplets, and I will try to includes some of that in the FAQs
          > since it comes up often as well. I have a few sites that are
          very
          > technical, but if anyone has an easy web site that helps explain
          it
          > feel free to post it.
          >
          > The majjority of companies do NOT post these types of charts, or
          the
          > glass specs, or the color correction levels, or quality control
          > specifications. Perhaps WO is just more honest, or perhaps they
          feel
          > that the charts help people, or perhaps other companies just
          don't
          > want to open the kimono that far because it lets competitors see
          what
          > they are doing, and gives them a chance to try to one-up them
          with a
          > better looking chart.
          >
          > Either way - if anyone has any information, let us all know.
          >
          > Timm
          >
          > --- In William-Optics@yahoogroups.com, "kulginoff"
          > <dmitri.kulginov@e...> wrote:
          > >
          > > I would like to know the colour legend on the chromatic
          aberration
          > > graph for Zenith Star Fluorite Doublet on the WO web site,
          > >
          > > http://www.william-optics.com/wowebs/prod_tel/zsfd80/chrom.htm
          > >
          > > Thankfully,
          > > Dmitri
          > >
          >
          >
          >
          >
          >
          >
          >
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