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ZS80FD - use at close range

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  • Acme Zenith
    I understand these telescopes are optimised for astronomical use, but I enjoy birding and if I can get something that does both well, so much the better. I m
    Message 1 of 17 , Feb 20, 2007
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      I understand these telescopes are optimised for astronomical use, but
      I enjoy birding and if I can get something that does both well, so
      much the better.

      I'm concerned however about how the telescope will perform when
      extended to focus on something the size of a pop can. I've been unable
      to get a handle on at what photographic magnification aberrations will
      begin to degrade an image I take. (With an APS sized sensor.)

      My main worry is spherical aberration, but I'm also curious what close
      focusing does to field curvature.

      I'd be happy if anyone could point me toward enough optical theory to
      answer the question from first principles, but pixel level detail
      crops from the centre and corners of, for example, an image of a page
      of magazine text, would be great too.
    • Timm Bottoni
      Hi, I can t provide any optical theory but I have taken some pictures that are posted on the William Optics website and here in the photo section. I have the
      Message 2 of 17 , Feb 20, 2007
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        Hi,

        I can't provide any optical theory but I have taken some pictures that
        are posted on the William Optics website and here in the photo section.

        I have the WO Megrez 80FD (after I APOgraded it) and have taken some
        bird pictures with it attached to a Canon 20D. Look for them by my
        name, Timm Bottoni, but be sure to look for a variety of others as well.

        It works extremely well for this, but keep in mind that you will need a
        good tripod, along with a keen eye for manual focusing.

        Timm

        --- In William-Optics@yahoogroups.com, "Acme Zenith" <acme_y@...> wrote:
        >
        > I understand these telescopes are optimised for astronomical use, but
        > I enjoy birding and if I can get something that does both well, so
        > much the better.
        >
        > I'm concerned however about how the telescope will perform when
        > extended to focus on something the size of a pop can. I've been unable
        > to get a handle on at what photographic magnification aberrations will
        > begin to degrade an image I take. (With an APS sized sensor.)
        >
        > My main worry is spherical aberration, but I'm also curious what close
        > focusing does to field curvature.
        >
        > I'd be happy if anyone could point me toward enough optical theory to
        > answer the question from first principles, but pixel level detail
        > crops from the centre and corners of, for example, an image of a page
        > of magazine text, would be great too.
        >
      • Acme Zenith
        ... Thanks for the reassurance, and I m sure the scope would serve well for most applications. But what I m trying to establish is where an uncorrected
        Message 3 of 17 , Feb 22, 2007
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          --- In William-Optics@yahoogroups.com, "Timm Bottoni" <t.bottoni@...>
          wrote:
          >
          > Hi,
          >
          > I can't provide any optical theory but I have taken some pictures that
          > are posted on the William Optics website and here in the photo section.
          >
          > I have the WO Megrez 80FD (after I APOgraded it) and have taken some
          > bird pictures with it attached to a Canon 20D. Look for them by my
          > name, Timm Bottoni, but be sure to look for a variety of others as well.
          >
          > It works extremely well for this, but keep in mind that you will need a
          > good tripod, along with a keen eye for manual focusing.

          Thanks for the reassurance, and I'm sure the scope would serve well
          for most applications. But what I'm trying to establish is where an
          uncorrected telescope will begin to not do well, either through
          testing or through well described theory.

          I'm not the kind of person who buys something, tests it for a few
          days, decides it's unacceptable and sends it back. I tend to commit to
          my purchases.

          But if I did have the scope I would begin to gauge the validity of my
          concerns by photographing test charts at 100m, 10m, and as close as I
          could get it to focus with the extensions I have available, to see how
          the MTF changes. I know that at some point the results will become
          poor, but just where I have been unable to discover from the
          information available.

          There are a lot of people using telescopes for birding, etc, but it
          seems very little published data to serve them. Everyone seems to be
          obsessed with chromatic aberration these days, without regard to other
          issues of interest!

          >
          > Timm
          >
        • Timm Bottoni
          Hi Acme Zenith (is that your name?), I have a variety of test shots I did when I created my review for Astromart and Cloudy Nights of the APOgrade. They are
          Message 4 of 17 , Feb 22, 2007
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            Hi Acme Zenith (is that your name?),

            I have a variety of test shots I did when I created my review for
            Astromart and Cloudy Nights of the APOgrade. They are quite large,
            so they can't be emailed, but I included crops of them in the reviews.

            http://www.astromart.com/articles/article.asp?article_id=398
            http://www.cloudynights.com/item.php?item_id=1420

            If you want originals, I can re-setup a 4shared.com location for you
            to download them. The one I had went dormant from lack of use, and
            the pics all got deleted but it's a free site for sharing large files.

            Let me know if that would help you.

            Timm



            --- In William-Optics@yahoogroups.com, "Acme Zenith" <acme_y@...>
            wrote:
            >
            >
            > Thanks for the reassurance, and I'm sure the scope would serve well
            > for most applications. But what I'm trying to establish is where an
            > uncorrected telescope will begin to not do well, either through
            > testing or through well described theory.
            >
            > I'm not the kind of person who buys something, tests it for a few
            > days, decides it's unacceptable and sends it back. I tend to commit
            to
            > my purchases.
            >
            > But if I did have the scope I would begin to gauge the validity of
            my
            > concerns by photographing test charts at 100m, 10m, and as close as
            I
            > could get it to focus with the extensions I have available, to see
            how
            > the MTF changes. I know that at some point the results will become
            > poor, but just where I have been unable to discover from the
            > information available.
            >
            > There are a lot of people using telescopes for birding, etc, but it
            > seems very little published data to serve them. Everyone seems to be
            > obsessed with chromatic aberration these days, without regard to
            other
            > issues of interest!
            >
            >
          • Acme Zenith
            ... My parents would have had to have been more than a little vertically preoccupied if so.. ... These crops are great, and are close to what I m looking for,
            Message 5 of 17 , Feb 22, 2007
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              --- In William-Optics@yahoogroups.com, "Timm Bottoni" <t.bottoni@...>
              wrote:
              >
              > Hi Acme Zenith (is that your name?),

              My parents would have had to have been more than a little vertically
              preoccupied if so..

              >
              > I have a variety of test shots I did when I created my review for
              > Astromart and Cloudy Nights of the APOgrade. They are quite large,
              > so they can't be emailed, but I included crops of them in the reviews.
              >
              > http://www.astromart.com/articles/article.asp?article_id=398
              > http://www.cloudynights.com/item.php?item_id=1420

              These crops are great, and are close to what I'm looking for, but I
              need you to crank up the magnification ratio. The rain cap on that
              chimney spans approximately one fifth of the field. If we assume it's
              15cm in diameter then the field is 75cm. Your 20D's sensor is about
              2.2cm across, so the magnification here is about 1:30.

              I'm concerned about aberrations that will occur at a magnification
              more like 1:10. Don't think of this as a hard number though. I'm
              trying to get a handle on where they start to creep in and how quickly
              they become unacceptable. I don't expect to be put two feet of extra
              tube on my telescope and photograph aphids with it.

              In the original image the screen around that vent is just about the
              right kind of detail to show any lurking problems, but it'd be too big
              at 1:10. Plus you'd be hanging in space to take the picture.

              It'd probably be more practical to print out one of the test charts
              from this page:

              http://www.normankoren.com/Tutorials/MTF5.html

              And photograph it at several magnifications, to determine how the MTF
              changes.

              Thanks very much if you or anyone else wants to tackle this challenge!

              >
              > If you want originals, I can re-setup a 4shared.com location for you
              > to download them. The one I had went dormant from lack of use, and
              > the pics all got deleted but it's a free site for sharing large files.
              >
              > Let me know if that would help you.
              >
              > Timm
              >
              >
            • Acme Zenith
              ... Ok, next time read until the browser bottoms out. Yes, your test chart is on the right track but I d like to know what happens when your field spans only
              Message 6 of 17 , Feb 22, 2007
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                --- In William-Optics@yahoogroups.com, "Acme Zenith" <acme_y@...> wrote:

                > It'd probably be more practical to print out one of the test charts
                > from this page:
                >

                Ok, next time read until the browser bottoms out. Yes, your test chart
                is on the right track but I'd like to know what happens when your
                field spans only about a third of it!
              • Timm Bottoni
                Hi, I only had time to set this up and upload one file here. It s one of the original full shots, JPEG format, from the basement charts,shot about 35 feet
                Message 7 of 17 , Feb 22, 2007
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                  Hi,

                  I only had time to set this up and upload one file here. It's one of
                  the original full shots, JPEG format, from the basement charts,shot
                  about 35 feet away.

                  Let me know if this is useful and if you want me to try to upload more.

                  http://www.4shared.com/dir/2079858/391585ff/80FD_Charts.html

                  Timm

                  --- In William-Optics@yahoogroups.com, "Acme Zenith" <acme_y@...> wrote:
                  >
                  > --- In William-Optics@yahoogroups.com, "Acme Zenith" <acme_y@> wrote:
                  >
                  > > It'd probably be more practical to print out one of the test charts
                  > > from this page:
                  > >
                  >
                  > Ok, next time read until the browser bottoms out. Yes, your test chart
                  > is on the right track but I'd like to know what happens when your
                  > field spans only about a third of it!
                  >
                • Acme Zenith
                  ... I really appreciate your entertaining my questions on this. I m just going to ramble with some possibly fuzzy math for a second. Based on a f/l of 555mm I
                  Message 8 of 17 , Feb 23, 2007
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                    --- In William-Optics@yahoogroups.com, "Timm Bottoni" <t.bottoni@...>
                    wrote:
                    > the original full shots, JPEG format, from the basement charts,shot
                    > about 35 feet away.
                    >
                    I really appreciate your entertaining my questions on this. I'm just
                    going to ramble with some possibly fuzzy math for a second.

                    Based on a f/l of 555mm I make your horizontal angle of view with the
                    22mm wide sensor as 2.3 degrees. As you say the test chart is 35 feet
                    away. tan(2.3/2) * 2 * 35 * 12 * 2.54 = 42.8. So I figure your
                    horizontal field there is 42.8cm.

                    428mm / 22mm equals an image magnification of about 20:1, the maximum
                    for this telescope without extension. And it looks good, so this is
                    very useful info.

                    I'd like to know how things looks if you drop in enough extension to
                    make a magnification of, say, 10:1. You'll probably need a new test
                    chart, to keep some pixel level detail (as in the chart that went all
                    the way to extinction at the bottom of your image).
                  • Timm Bottoni
                    Hi, Good job on the math. Can you be more specific on what I should try? The back side of the chart you see has a 1x1cm grid that I made, and I have another
                    Message 9 of 17 , Feb 23, 2007
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                      Hi,

                      Good job on the math.

                      Can you be more specific on what I should try? The back side of the
                      chart you see has a 1x1cm grid that I made, and I have another chart
                      that Scott Walker made for me, but I haven't tried any shots of it
                      yet. I don't have any other extensions though, so I can't really get
                      any closer.

                      I have other pics I can post if you like. How about a nice mourning
                      dove sitting in the tree? I have a nice Cardinal shot, and a nice
                      American Goldfinch shot too.

                      Timm


                      In William-Optics@yahoogroups.com, "Acme Zenith" <acme_y@...> wrote:
                      >
                      > --- In William-Optics@yahoogroups.com, "Timm Bottoni" <t.bottoni@>
                      > wrote:
                      > > the original full shots, JPEG format, from the basement
                      charts,shot
                      > > about 35 feet away.
                      > >
                      > I really appreciate your entertaining my questions on this. I'm just
                      > going to ramble with some possibly fuzzy math for a second.
                      >
                      > Based on a f/l of 555mm I make your horizontal angle of view with
                      the
                      > 22mm wide sensor as 2.3 degrees. As you say the test chart is 35
                      feet
                      > away. tan(2.3/2) * 2 * 35 * 12 * 2.54 = 42.8. So I figure your
                      > horizontal field there is 42.8cm.
                      >
                      > 428mm / 22mm equals an image magnification of about 20:1, the
                      maximum
                      > for this telescope without extension. And it looks good, so this is
                      > very useful info.
                      >
                      > I'd like to know how things looks if you drop in enough extension to
                      > make a magnification of, say, 10:1. You'll probably need a new test
                      > chart, to keep some pixel level detail (as in the chart that went
                      all
                      > the way to extinction at the bottom of your image).
                      >
                    • Acme Zenith
                      ... The centre portions where the lines converge until extinction look good. I d move the vertical bar chart so it runs along an edge of the newly reduced
                      Message 10 of 17 , Feb 23, 2007
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                        --- In William-Optics@yahoogroups.com, "Timm Bottoni" <t.bottoni@...>
                        wrote:
                        >
                        > Hi,
                        >
                        > Good job on the math.
                        >
                        > Can you be more specific on what I should try? The back side of the
                        > chart you see has a 1x1cm grid that I made, and I have another chart
                        > that Scott Walker made for me, but I haven't tried any shots of it
                        > yet. I don't have any other extensions though, so I can't really get
                        > any closer.

                        The centre portions where the lines converge until extinction look
                        good. I'd move the vertical bar chart so it runs along an edge of the
                        newly reduced field. Focus as best as possible along that edge and you
                        may be limited by spherical aberration, while the centre will probably
                        be mostly affected by field curvature. At least, this is what I'm
                        trying to get to the bottom of. If it's bad, you'll find it difficult
                        to figure out where the best point of focus is.

                        Are you using a field flattener? If not, you can use an extension tube
                        on your camera. If so, you need a 2" extension tube in the focuser.
                        Since you don't have any or any more currently I guess we can consider
                        that a future test.

                        >
                        > I have other pics I can post if you like. How about a nice mourning
                        > dove sitting in the tree? I have a nice Cardinal shot, and a nice
                        > American Goldfinch shot too.

                        How about a hummingbird? Only kidding, but if I do buy one I pledge to
                        contribute some images too!

                        >
                        > Timm
                        >
                        >
                        > In William-Optics@yahoogroups.com, "Acme Zenith" <acme_y@> wrote:
                        > >
                        > > --- In William-Optics@yahoogroups.com, "Timm Bottoni" <t.bottoni@>
                        > > wrote:
                        > > > the original full shots, JPEG format, from the basement
                        > charts,shot
                        > > > about 35 feet away.
                        > > >
                        > > I really appreciate your entertaining my questions on this. I'm just
                        > > going to ramble with some possibly fuzzy math for a second.
                        > >
                        > > Based on a f/l of 555mm I make your horizontal angle of view with
                        > the
                        > > 22mm wide sensor as 2.3 degrees. As you say the test chart is 35
                        > feet
                        > > away. tan(2.3/2) * 2 * 35 * 12 * 2.54 = 42.8. So I figure your
                        > > horizontal field there is 42.8cm.
                        > >
                        > > 428mm / 22mm equals an image magnification of about 20:1, the
                        > maximum
                        > > for this telescope without extension. And it looks good, so this is
                        > > very useful info.
                        > >
                        > > I'd like to know how things looks if you drop in enough extension to
                        > > make a magnification of, say, 10:1. You'll probably need a new test
                        > > chart, to keep some pixel level detail (as in the chart that went
                        > all
                        > > the way to extinction at the bottom of your image).
                        > >
                        >
                      • Scott Walker
                        Per the ATMOS model a 80mm aperture 555mm focal length scope with perfect spherical aberration in green at infinite focus degrades to .1580 wave peak to valley
                        Message 11 of 17 , Feb 25, 2007
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                          Per the ATMOS model a 80mm aperture 555mm focal length scope with perfect spherical aberration in green at infinite focus degrades to .1580 wave peak to valley when focused at 5550mm and to .06 waves peak to valley at 11100mm. The close focusing causes under correction. These numbers are all at the center of the FOV. I think the degrading is much worse for faster systems.
                           
                          Scott Walker
                          ----- Original Message -----
                          Sent: Friday, February 23, 2007 9:59 AM
                          Subject: [William-Optics] Re: ZS80FD - use at close range

                          --- In William-Optics@ yahoogroups. com, "Timm Bottoni" <t.bottoni@. ..>
                          wrote:
                          > the original full shots, JPEG format, from the basement charts,shot
                          > about 35 feet away.
                          >
                          I really appreciate your entertaining my questions on this. I'm just
                          going to ramble with some possibly fuzzy math for a second.

                          Based on a f/l of 555mm I make your horizontal angle of view with the
                          22mm wide sensor as 2.3 degrees. As you say the test chart is 35 feet
                          away. tan(2.3/2) * 2 * 35 * 12 * 2.54 = 42.8. So I figure your
                          horizontal field there is 42.8cm.

                          428mm / 22mm equals an image magnification of about 20:1, the maximum
                          for this telescope without extension. And it looks good, so this is
                          very useful info.

                          I'd like to know how things looks if you drop in enough extension to
                          make a magnification of, say, 10:1. You'll probably need a new test
                          chart, to keep some pixel level detail (as in the chart that went all
                          the way to extinction at the bottom of your image).

                        • Acme Zenith
                          ... perfect spherical aberration in green at infinite focus degrades to .1580 wave peak to valley when focused at 5550mm and to .06 waves peak to valley at
                          Message 12 of 17 , Feb 26, 2007
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                            --- In William-Optics@yahoogroups.com, "Scott Walker" <sdwalker@...>
                            wrote:
                            >
                            > Per the ATMOS model a 80mm aperture 555mm focal length scope with
                            perfect spherical aberration in green at infinite focus degrades to
                            .1580 wave peak to valley when focused at 5550mm and to .06 waves peak
                            to valley at 11100mm. The close focusing causes under correction.
                            These numbers are all at the center of the FOV. I think the degrading
                            is much worse for faster systems.

                            Thanks for this - I can stop torturing Timm.

                            Any idea how to get ATMOS to run the numbers for what will be the edge
                            of the field?

                            >
                            > Scott Walker
                            > ----- Original Message -----
                            > From: Acme Zenith
                            > To: William-Optics@yahoogroups.com
                            > Sent: Friday, February 23, 2007 9:59 AM
                            > Subject: [William-Optics] Re: ZS80FD - use at close range
                            >
                            >
                            > --- In William-Optics@yahoogroups.com, "Timm Bottoni" <t.bottoni@>
                            > wrote:
                            > > the original full shots, JPEG format, from the basement charts,shot
                            > > about 35 feet away.
                            > >
                            > I really appreciate your entertaining my questions on this. I'm just
                            > going to ramble with some possibly fuzzy math for a second.
                            >
                            > Based on a f/l of 555mm I make your horizontal angle of view with the
                            > 22mm wide sensor as 2.3 degrees. As you say the test chart is 35 feet
                            > away. tan(2.3/2) * 2 * 35 * 12 * 2.54 = 42.8. So I figure your
                            > horizontal field there is 42.8cm.
                            >
                            > 428mm / 22mm equals an image magnification of about 20:1, the maximum
                            > for this telescope without extension. And it looks good, so this is
                            > very useful info.
                            >
                            > I'd like to know how things looks if you drop in enough extension to
                            > make a magnification of, say, 10:1. You'll probably need a new test
                            > chart, to keep some pixel level detail (as in the chart that went all
                            > the way to extinction at the bottom of your image).
                            >
                          • Scott Walker
                            The image at the edge of the FOV should be limited by the field curvature. Scott Walker ... From: Acme Zenith To: William-Optics@yahoogroups.com Sent: Monday,
                            Message 13 of 17 , Feb 26, 2007
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                              The image at the edge of the FOV should be limited by the field curvature.
                               
                              Scott Walker 
                              ----- Original Message -----
                              Sent: Monday, February 26, 2007 7:19 PM
                              Subject: [William-Optics] Re: ZS80FD - use at close range

                              --- In William-Optics@ yahoogroups. com, "Scott Walker" <sdwalker@.. .>
                              wrote:
                              >
                              > Per the ATMOS model a 80mm aperture 555mm focal length scope with
                              perfect spherical aberration in green at infinite focus degrades to
                              .1580 wave peak to valley when focused at 5550mm and to .06 waves peak
                              to valley at 11100mm. The close focusing causes under correction.
                              These numbers are all at the center of the FOV. I think the degrading
                              is much worse for faster systems.

                              Thanks for this - I can stop torturing Timm.

                              Any idea how to get ATMOS to run the numbers for what will be the edge
                              of the field?

                              >
                              > Scott Walker
                              > ----- Original Message -----
                              > From: Acme Zenith
                              > To: William-Optics@ yahoogroups. com
                              > Sent: Friday, February 23, 2007 9:59 AM
                              > Subject: [William-Optics] Re: ZS80FD - use at close range
                              >
                              >
                              > --- In William-Optics@ yahoogroups. com, "Timm Bottoni" <t.bottoni@>
                              > wrote:
                              > > the original full shots, JPEG format, from the basement charts,shot
                              > > about 35 feet away.
                              > >
                              > I really appreciate your entertaining my questions on this. I'm just
                              > going to ramble with some possibly fuzzy math for a second.
                              >
                              > Based on a f/l of 555mm I make your horizontal angle of view with the
                              > 22mm wide sensor as 2.3 degrees. As you say the test chart is 35 feet
                              > away. tan(2.3/2) * 2 * 35 * 12 * 2.54 = 42.8. So I figure your
                              > horizontal field there is 42.8cm.
                              >
                              > 428mm / 22mm equals an image magnification of about 20:1, the maximum
                              > for this telescope without extension. And it looks good, so this is
                              > very useful info.
                              >
                              > I'd like to know how things looks if you drop in enough extension to
                              > make a magnification of, say, 10:1. You'll probably need a new test
                              > chart, to keep some pixel level detail (as in the chart that went all
                              > the way to extinction at the bottom of your image).
                              >

                            • Acme Zenith
                              ... curvature. Yep, I gathered this, but it should still be possible to bring the edges to focus at the expense of the centre. As I understand field curvature
                              Message 14 of 17 , Feb 27, 2007
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                                --- In William-Optics@yahoogroups.com, "Scott Walker" <sdwalker@...>
                                wrote:
                                >
                                > The image at the edge of the FOV should be limited by the field
                                curvature.

                                Yep, I gathered this, but it should still be possible to bring the
                                edges to focus at the expense of the centre. As I understand field
                                curvature if one took a photo of the inside of a sufficiently large
                                sphere(oid) all of it could be focus, but this is not the case with
                                spherical aberration. Hence my suggestion in testing to focus on an
                                edge.

                                But now you also have me wondering what, for example, the .8x field
                                flattener does to spherical aberration!

                                >
                                > Scott Walker
                                > ----- Original Message -----
                                > From: Acme Zenith
                                > To: William-Optics@yahoogroups.com
                                > Sent: Monday, February 26, 2007 7:19 PM
                                > Subject: [William-Optics] Re: ZS80FD - use at close range
                                >
                                >
                                > --- In William-Optics@yahoogroups.com, "Scott Walker" <sdwalker@>
                                > wrote:
                                > >
                                > > Per the ATMOS model a 80mm aperture 555mm focal length scope with
                                > perfect spherical aberration in green at infinite focus degrades to
                                > .1580 wave peak to valley when focused at 5550mm and to .06 waves peak
                                > to valley at 11100mm. The close focusing causes under correction.
                                > These numbers are all at the center of the FOV. I think the degrading
                                > is much worse for faster systems.
                                >
                                > Thanks for this - I can stop torturing Timm.
                                >
                                > Any idea how to get ATMOS to run the numbers for what will be the edge
                                > of the field?
                                >
                                > >
                                > > Scott Walker
                                > > ----- Original Message -----
                                > > From: Acme Zenith
                                > > To: William-Optics@yahoogroups.com
                                > > Sent: Friday, February 23, 2007 9:59 AM
                                > > Subject: [William-Optics] Re: ZS80FD - use at close range
                                > >
                                > >
                                > > --- In William-Optics@yahoogroups.com, "Timm Bottoni" <t.bottoni@>
                                > > wrote:
                                > > > the original full shots, JPEG format, from the basement
                                charts,shot
                                > > > about 35 feet away.
                                > > >
                                > > I really appreciate your entertaining my questions on this. I'm just
                                > > going to ramble with some possibly fuzzy math for a second.
                                > >
                                > > Based on a f/l of 555mm I make your horizontal angle of view
                                with the
                                > > 22mm wide sensor as 2.3 degrees. As you say the test chart is 35
                                feet
                                > > away. tan(2.3/2) * 2 * 35 * 12 * 2.54 = 42.8. So I figure your
                                > > horizontal field there is 42.8cm.
                                > >
                                > > 428mm / 22mm equals an image magnification of about 20:1, the
                                maximum
                                > > for this telescope without extension. And it looks good, so this is
                                > > very useful info.
                                > >
                                > > I'd like to know how things looks if you drop in enough extension to
                                > > make a magnification of, say, 10:1. You'll probably need a new test
                                > > chart, to keep some pixel level detail (as in the chart that
                                went all
                                > > the way to extinction at the bottom of your image).
                                > >
                                >
                              • Scott Walker
                                One can improve the focus on the edge but it is limited by the astigmatism that most telescope have off the center of the FOV. At the edge the two planes of
                                Message 15 of 17 , Feb 28, 2007
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                                  One can improve the focus on the edge but it is limited by the astigmatism that most telescope have off the center of the FOV. At the edge the two planes of focus are not in focus at the same point.
                                   
                                  Scott Walker
                                  ----- Original Message -----
                                  Sent: Tuesday, February 27, 2007 5:15 PM
                                  Subject: [William-Optics] Re: ZS80FD - use at close range

                                  --- In William-Optics@ yahoogroups. com, "Scott Walker" <sdwalker@.. .>
                                  wrote:
                                  >
                                  > The image at the edge of the FOV should be limited by the field
                                  curvature.

                                  Yep, I gathered this, but it should still be possible to bring the
                                  edges to focus at the expense of the centre. As I understand field
                                  curvature if one took a photo of the inside of a sufficiently large
                                  sphere(oid) all of it could be focus, but this is not the case with
                                  spherical aberration. Hence my suggestion in testing to focus on an
                                  edge.

                                  But now you also have me wondering what, for example, the .8x field
                                  flattener does to spherical aberration!

                                  >
                                  > Scott Walker
                                  > ----- Original Message -----
                                  > From: Acme Zenith
                                  > To: William-Optics@ yahoogroups. com
                                  > Sent: Monday, February 26, 2007 7:19 PM
                                  > Subject: [William-Optics] Re: ZS80FD - use at close range
                                  >
                                  >
                                  > --- In William-Optics@ yahoogroups. com, "Scott Walker" <sdwalker@>
                                  > wrote:
                                  > >
                                  > > Per the ATMOS model a 80mm aperture 555mm focal length scope with
                                  > perfect spherical aberration in green at infinite focus degrades to
                                  > .1580 wave peak to valley when focused at 5550mm and to .06 waves peak
                                  > to valley at 11100mm. The close focusing causes under correction.
                                  > These numbers are all at the center of the FOV. I think the degrading
                                  > is much worse for faster systems.
                                  >
                                  > Thanks for this - I can stop torturing Timm.
                                  >
                                  > Any idea how to get ATMOS to run the numbers for what will be the edge
                                  > of the field?
                                  >
                                  > >
                                  > > Scott Walker
                                  > > ----- Original Message -----
                                  > > From: Acme Zenith
                                  > > To: William-Optics@ yahoogroups. com
                                  > > Sent: Friday, February 23, 2007 9:59 AM
                                  > > Subject: [William-Optics] Re: ZS80FD - use at close range
                                  > >
                                  > >
                                  > > --- In William-Optics@ yahoogroups. com, "Timm Bottoni" <t.bottoni@>
                                  > > wrote:
                                  > > > the original full shots, JPEG format, from the basement
                                  charts,shot
                                  > > > about 35 feet away.
                                  > > >
                                  > > I really appreciate your entertaining my questions on this. I'm just
                                  > > going to ramble with some possibly fuzzy math for a second.
                                  > >
                                  > > Based on a f/l of 555mm I make your horizontal angle of view
                                  with the
                                  > > 22mm wide sensor as 2.3 degrees. As you say the test chart is 35
                                  feet
                                  > > away. tan(2.3/2) * 2 * 35 * 12 * 2.54 = 42.8. So I figure your
                                  > > horizontal field there is 42.8cm.
                                  > >
                                  > > 428mm / 22mm equals an image magnification of about 20:1, the
                                  maximum
                                  > > for this telescope without extension. And it looks good, so this is
                                  > > very useful info.
                                  > >
                                  > > I'd like to know how things looks if you drop in enough extension to
                                  > > make a magnification of, say, 10:1. You'll probably need a new test
                                  > > chart, to keep some pixel level detail (as in the chart that
                                  went all
                                  > > the way to extinction at the bottom of your image).
                                  > >
                                  >

                                • Timm Bottoni
                                  Hi Scott, Can you explain further what you mean by at the edge, the two loanes of focus are not in focus at the same point ? I can easily try to take a few
                                  Message 16 of 17 , Feb 28, 2007
                                  • 0 Attachment
                                    Hi Scott,

                                    Can you explain further what you mean by "at the edge, the two loanes
                                    of focus are not in focus at the same point"?

                                    I can easily try to take a few photos of my 1x1 cm grid in the
                                    basement, at different focus points. This might show what is being
                                    sought.

                                    Thanks,

                                    Timm

                                    --- In William-Optics@yahoogroups.com, "Scott Walker" <sdwalker@...>
                                    wrote:
                                    >
                                    > One can improve the focus on the edge but it is limited by the
                                    astigmatism that most telescope have off the center of the FOV. At
                                    the edge the two planes of focus are not in focus at the same point.
                                    >
                                    > Scott Walker
                                    > ----- Original Message -----
                                    > From: Acme Zenith
                                    > To: William-Optics@yahoogroups.com
                                    > Sent: Tuesday, February 27, 2007 5:15 PM
                                    > Subject: [William-Optics] Re: ZS80FD - use at close range
                                    >
                                    >
                                    > --- In William-Optics@yahoogroups.com, "Scott Walker" <sdwalker@>
                                    > wrote:
                                    > >
                                    > > The image at the edge of the FOV should be limited by the field
                                    > curvature.
                                    >
                                    > Yep, I gathered this, but it should still be possible to bring the
                                    > edges to focus at the expense of the centre. As I understand field
                                    > curvature if one took a photo of the inside of a sufficiently
                                    large
                                    > sphere(oid) all of it could be focus, but this is not the case
                                    with
                                    > spherical aberration. Hence my suggestion in testing to focus on
                                    an
                                    > edge.
                                    >
                                    > But now you also have me wondering what, for example, the .8x
                                    field
                                    > flattener does to spherical aberration!
                                    >
                                    > >
                                    > > Scott Walker
                                    > > ----- Original Message -----
                                    > > From: Acme Zenith
                                    > > To: William-Optics@yahoogroups.com
                                    > > Sent: Monday, February 26, 2007 7:19 PM
                                    > > Subject: [William-Optics] Re: ZS80FD - use at close range
                                    > >
                                    > >
                                    > > --- In William-Optics@yahoogroups.com, "Scott Walker"
                                    <sdwalker@>
                                    > > wrote:
                                    > > >
                                    > > > Per the ATMOS model a 80mm aperture 555mm focal length scope
                                    with
                                    > > perfect spherical aberration in green at infinite focus
                                    degrades to
                                    > > .1580 wave peak to valley when focused at 5550mm and to .06
                                    waves peak
                                    > > to valley at 11100mm. The close focusing causes under
                                    correction.
                                    > > These numbers are all at the center of the FOV. I think the
                                    degrading
                                    > > is much worse for faster systems.
                                    > >
                                    > > Thanks for this - I can stop torturing Timm.
                                    > >
                                    > > Any idea how to get ATMOS to run the numbers for what will be
                                    the edge
                                    > > of the field?
                                    > >
                                    > > >
                                    > > > Scott Walker
                                    > > > ----- Original Message -----
                                    > > > From: Acme Zenith
                                    > > > To: William-Optics@yahoogroups.com
                                    > > > Sent: Friday, February 23, 2007 9:59 AM
                                    > > > Subject: [William-Optics] Re: ZS80FD - use at close range
                                    > > >
                                    > > >
                                    > > > --- In William-Optics@yahoogroups.com, "Timm Bottoni"
                                    <t.bottoni@>
                                    > > > wrote:
                                    > > > > the original full shots, JPEG format, from the basement
                                    > charts,shot
                                    > > > > about 35 feet away.
                                    > > > >
                                    > > > I really appreciate your entertaining my questions on this.
                                    I'm just
                                    > > > going to ramble with some possibly fuzzy math for a second.
                                    > > >
                                    > > > Based on a f/l of 555mm I make your horizontal angle of view
                                    > with the
                                    > > > 22mm wide sensor as 2.3 degrees. As you say the test chart is
                                    35
                                    > feet
                                    > > > away. tan(2.3/2) * 2 * 35 * 12 * 2.54 = 42.8. So I figure your
                                    > > > horizontal field there is 42.8cm.
                                    > > >
                                    > > > 428mm / 22mm equals an image magnification of about 20:1, the
                                    > maximum
                                    > > > for this telescope without extension. And it looks good, so
                                    this is
                                    > > > very useful info.
                                    > > >
                                    > > > I'd like to know how things looks if you drop in enough
                                    extension to
                                    > > > make a magnification of, say, 10:1. You'll probably need a
                                    new test
                                    > > > chart, to keep some pixel level detail (as in the chart that
                                    > went all
                                    > > > the way to extinction at the bottom of your image).
                                    > > >
                                    > >
                                    >
                                  • Scott Walker
                                    Hi Timm I should have really used the term focal surfaces not planes, since they are curved surfaces not planes.Sorry about the misuse of terms. Now about
                                    Message 17 of 17 , Feb 28, 2007
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                                      Hi Timm
                                       
                                       I should have really used the term focal surfaces not planes, since they are curved surfaces not planes.Sorry about the misuse of terms.
                                       
                                      Now about those surfaces(spheres I believe). If you look at your photos from Hawaii you will find that the stars in the center of the FOV are sharp and round. When you look at the stars near the edge of the FOV they are blurred and elongated. The width of the star image in both the long and the narrow directions are wider than a star of equal brightness in the middle of the FOV. This is due to the field curvature. If you were to take a series of shots where for each shot you moved the focus "in" a small amount, you would see the following: The star images near the edge would start to become smaller in both directions and the image brighter. This would continues until the size in what was the smaller direction stopped getting smaller. This would be the best focus for that direction. As you continued to focus inward what was the long direction will continue to get smaller and the other direction will now start to get larger. At the best focus for the average of the two the star image will be round but not as dense or as small as it was in the center. The best focus in what was the long direction will cause a star image that is elongated but 90 degree to the elongation seen when the scope was focused at the center of the FOV. Therefore changing the focus can greatly improve the sharpness of an off axis star, but it can not make it as good as the image in the center with the scope focused at the center.
                                       
                                      The above example shows what one will see due to the two different focal surfaces formed by a doublet. These two surfaces lie on the same point at the center of the FOV. They are called the Tangential focal surface and the Sagittal focal surface. For a doublet the Tangential surface is the most curved. It is what determines the sharpness in what was called the long direction above.
                                       
                                      I hope that help. Sorry again for the misuse of the term planes.
                                       
                                      Scott Walker 
                                      ----- Original Message -----
                                      Sent: Wednesday, February 28, 2007 12:21 PM
                                      Subject: [William-Optics] Re: ZS80FD - use at close range

                                      Hi Scott,

                                      Can you explain further what you mean by "at the edge, the two loanes
                                      of focus are not in focus at the same point"?

                                      I can easily try to take a few photos of my 1x1 cm grid in the
                                      basement, at different focus points. This might show what is being
                                      sought.

                                      Thanks,

                                      Timm

                                      --- In William-Optics@ yahoogroups. com, "Scott Walker" <sdwalker@.. .>
                                      wrote:
                                      >
                                      > One can improve the focus on the edge but it is limited by the
                                      astigmatism that most telescope have off the center of the FOV. At
                                      the edge the two planes of focus are not in focus at the same point.
                                      >
                                      > Scott Walker
                                      > ----- Original Message -----
                                      > From: Acme Zenith
                                      > To: William-Optics@ yahoogroups. com
                                      > Sent: Tuesday, February 27, 2007 5:15 PM
                                      > Subject: [William-Optics] Re: ZS80FD - use at close range
                                      >
                                      >
                                      > --- In William-Optics@ yahoogroups. com, "Scott Walker" <sdwalker@>
                                      > wrote:
                                      > >
                                      > > The image at the edge of the FOV should be limited by the field
                                      > curvature.
                                      >
                                      > Yep, I gathered this, but it should still be possible to bring the
                                      > edges to focus at the expense of the centre. As I understand field
                                      > curvature if one took a photo of the inside of a sufficiently
                                      large
                                      > sphere(oid) all of it could be focus, but this is not the case
                                      with
                                      > spherical aberration. Hence my suggestion in testing to focus on
                                      an
                                      > edge.
                                      >
                                      > But now you also have me wondering what, for example, the .8x
                                      field
                                      > flattener does to spherical aberration!
                                      >
                                      > >
                                      > > Scott Walker
                                      > > ----- Original Message -----
                                      > > From: Acme Zenith
                                      > > To: William-Optics@ yahoogroups. com
                                      > > Sent: Monday, February 26, 2007 7:19 PM
                                      > > Subject: [William-Optics] Re: ZS80FD - use at close range
                                      > >
                                      > >
                                      > > --- In William-Optics@ yahoogroups. com, "Scott Walker"
                                      <sdwalker@>
                                      > > wrote:
                                      > > >
                                      > > > Per the ATMOS model a 80mm aperture 555mm focal length scope
                                      with
                                      > > perfect spherical aberration in green at infinite focus
                                      degrades to
                                      > > .1580 wave peak to valley when focused at 5550mm and to .06
                                      waves peak
                                      > > to valley at 11100mm. The close focusing causes under
                                      correction.
                                      > > These numbers are all at the center of the FOV. I think the
                                      degrading
                                      > > is much worse for faster systems.
                                      > >
                                      > > Thanks for this - I can stop torturing Timm.
                                      > >
                                      > > Any idea how to get ATMOS to run the numbers for what will be
                                      the edge
                                      > > of the field?
                                      > >
                                      > > >
                                      > > > Scott Walker
                                      > > > ----- Original Message -----
                                      > > > From: Acme Zenith
                                      > > > To: William-Optics@ yahoogroups. com
                                      > > > Sent: Friday, February 23, 2007 9:59 AM
                                      > > > Subject: [William-Optics] Re: ZS80FD - use at close range
                                      > > >
                                      > > >
                                      > > > --- In William-Optics@ yahoogroups. com, "Timm Bottoni"
                                      <t.bottoni@>
                                      > > > wrote:
                                      > > > > the original full shots, JPEG format, from the basement
                                      > charts,shot
                                      > > > > about 35 feet away.
                                      > > > >
                                      > > > I really appreciate your entertaining my questions on this.
                                      I'm just
                                      > > > going to ramble with some possibly fuzzy math for a second.
                                      > > >
                                      > > > Based on a f/l of 555mm I make your horizontal angle of view
                                      > with the
                                      > > > 22mm wide sensor as 2.3 degrees. As you say the test chart is
                                      35
                                      > feet
                                      > > > away. tan(2.3/2) * 2 * 35 * 12 * 2.54 = 42.8. So I figure your
                                      > > > horizontal field there is 42.8cm.
                                      > > >
                                      > > > 428mm / 22mm equals an image magnification of about 20:1, the
                                      > maximum
                                      > > > for this telescope without extension. And it looks good, so
                                      this is
                                      > > > very useful info.
                                      > > >
                                      > > > I'd like to know how things looks if you drop in enough
                                      extension to
                                      > > > make a magnification of, say, 10:1. You'll probably need a
                                      new test
                                      > > > chart, to keep some pixel level detail (as in the chart that
                                      > went all
                                      > > > the way to extinction at the bottom of your image).
                                      > > >
                                      > >
                                      >

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