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Focal reducers for 105/6.2

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  • Craig Stark
    I do a bit of small-chip CCD imaging using a 105/6.2. I know I m not in the league of many here, but I have fun seeing what I can get out of inexpensive CCD
    Message 1 of 5 , Oct 11, 2004
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      I do a bit of small-chip CCD imaging using a 105/6.2. I know I'm not
      in the league of many here, but I have fun seeing what I can get out
      of inexpensive CCD gear (here, a SAC8 camera -- 3.7 x 4.9 mm chip
      size). I've worked a lot with an 8" SCT and had success with simple
      focal reducers (single 120mm f/l lens).

      The TMB on its own does wonderfully. The shot of M11 on this page was
      a 5 panel montage and the stars are within my tolerance of "nice":
      http://home.earthlink.net/~celstark/id3.html

      When coupled with the simple reducer, however, the picture is
      different. The Veil shot on there was a quick test of the setup from
      my light polluted back yard. It's a 3 panel montage with the reducer
      in there and the stars near each panel edge look like crud. It looks
      like coma there and as I rack in and out of focus, I see astigmatism.
      I've previously looked for abberations using this reducing lens on my
      f/10 SCT and found nothing really problematic (see bottom of:
      http://home.earthlink.net/~celstark/id7.html if you like).

      Testing in daylight of a target (a "Do not enter" sign 100' away) both
      on and off-axis confirmed off-axis problems (I've got 2 of these
      lenses "A" and "B" with one showing better performance than the other):
      http://home.earthlink.net/~celstark/oct10_FR_test_results.jpg

      This all is a long lead up to my two questions:
      1) Out of curiosity, why would this show up more on the TMB than on
      the SCT? The f-ratio is a likely culprit, but I have seen people use
      this same reducer on cheap 80mm f/5 scopes without problem (I've yet
      to test my examples of the reducer in one)

      2) Out of practicality, what reducer lens would do better than this
      one? Yes, the TMB one would be fantastic, but I have no need at all
      to go to medium format sizes and no desire to spend more on the
      reducing lens than I did on my camera. Cost knocks that one out.
      Televue has two 3-element 0.8x ones for $265. Vixen has a 0.75x for
      the ED102SS (also a 4" f/6) at $255. Meade and Celestron have a 0.63x
      and Meade has a 0.3x (actual numbers will vary with spacing of
      course), both of which are talked about on f/6.3 SCTs at least
      (~$150). Anyone have any thoughts on how these would perform? The
      Meade / Celestron are supposedly reasonable (esp. given my imaging
      gear) but I don't know how they would perform given the difference in
      optics. Any other suggestions?

      Thanks for any help,

      Craig
    • erdmanpe
      People often try to use generic reducers, but the results are mixed. Any positive lens will act as a reducer, and will also somewhat reduce positive curvature
      Message 2 of 5 , Oct 11, 2004
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        People often try to use generic reducers, but the results are mixed.
        Any positive lens will act as a reducer, and will also somewhat reduce
        positive curvature of field. But if the reducer is not well matched
        to the optical system, the field curvature will not be fully
        corrected, and other aberations will also be introduced. Perhaps you
        are experiencing this problem.

        Your sample image with the Veil seems to show dramatic differences
        between the stars on the left side and those on the right side of the
        image. Something isn't aligned well--either the optical axis and the
        mechanical axis, or there is a tilt in a component.

        You do get good images without the reducer, so I would suspect somehow
        the mounting of the reducer is at fault. Also, with such a strong,
        simple lens, serious aberations are almost surely introduced.

        Peter

        --- In tmboptical@yahoogroups.com, "Craig Stark" <celstark@e...> wrote:
        >
        > I do a bit of small-chip CCD imaging using a 105/6.2. I know I'm not
        > in the league of many here, but I have fun seeing what I can get out
        > of inexpensive CCD gear (here, a SAC8 camera -- 3.7 x 4.9 mm chip
        > size). I've worked a lot with an 8" SCT and had success with simple
        > focal reducers (single 120mm f/l lens).
        >
        > The TMB on its own does wonderfully. The shot of M11 on this page was
        > a 5 panel montage and the stars are within my tolerance of "nice":
        > http://home.earthlink.net/~celstark/id3.html
        >
        > When coupled with the simple reducer, however, the picture is
        > different. The Veil shot on there was a quick test of the setup from
        > my light polluted back yard. It's a 3 panel montage with the reducer
        > in there and the stars near each panel edge look like crud. It looks
        > like coma there and as I rack in and out of focus, I see astigmatism.
        > I've previously looked for abberations using this reducing lens on my
        > f/10 SCT and found nothing really problematic (see bottom of:
        > http://home.earthlink.net/~celstark/id7.html if you like).
        >
        > Testing in daylight of a target (a "Do not enter" sign 100' away) both
        > on and off-axis confirmed off-axis problems (I've got 2 of these
        > lenses "A" and "B" with one showing better performance than the other):
        > http://home.earthlink.net/~celstark/oct10_FR_test_results.jpg
        >
        > This all is a long lead up to my two questions:
        > 1) Out of curiosity, why would this show up more on the TMB than on
        > the SCT? The f-ratio is a likely culprit, but I have seen people use
        > this same reducer on cheap 80mm f/5 scopes without problem (I've yet
        > to test my examples of the reducer in one)
        >
        > 2) Out of practicality, what reducer lens would do better than this
        > one? Yes, the TMB one would be fantastic, but I have no need at all
        > to go to medium format sizes and no desire to spend more on the
        > reducing lens than I did on my camera. Cost knocks that one out.
        > Televue has two 3-element 0.8x ones for $265. Vixen has a 0.75x for
        > the ED102SS (also a 4" f/6) at $255. Meade and Celestron have a 0.63x
        > and Meade has a 0.3x (actual numbers will vary with spacing of
        > course), both of which are talked about on f/6.3 SCTs at least
        > (~$150). Anyone have any thoughts on how these would perform? The
        > Meade / Celestron are supposedly reasonable (esp. given my imaging
        > gear) but I don't know how they would perform given the difference in
        > optics. Any other suggestions?
        >
        > Thanks for any help,
        >
        > Craig
      • Adrian Catterall
        I don t know much about this particular reducer that you are using, but it is clearly not designed for your scope. They talk about using it for SCTs and
        Message 3 of 5 , Oct 11, 2004
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          I don't know much about this particular reducer that you are using, but
          it is clearly not designed for your scope. They talk about using it for
          SCTs and refractors, but they are completely different designs of
          optics. I did try using as Meade focal 0.63x reducer on a refractor just
          out of interest and it gave horrible astigmatic stars. TMB don't produce
          reducers, only the flatteners for wide field imaging. I have used the
          Astrophysics 0.75x reducer with good success and also tried the Televue
          one you mention - this works well as long as you have the distance from
          chip to reducer correct, but it does have a narrow barrel and introduces
          vignetting for larger camera's (such as ST10)..

          Adrian




          -----Original Message-----
          From: Craig Stark [mailto:celstark@...]
          Sent: 11 October 2004 17:42
          To: tmboptical@yahoogroups.com
          Subject: [tmboptical] Focal reducers for 105/6.2



          I do a bit of small-chip CCD imaging using a 105/6.2. I know I'm not
          in the league of many here, but I have fun seeing what I can get out
          of inexpensive CCD gear (here, a SAC8 camera -- 3.7 x 4.9 mm chip
          size). I've worked a lot with an 8" SCT and had success with simple
          focal reducers (single 120mm f/l lens).

          The TMB on its own does wonderfully. The shot of M11 on this page was
          a 5 panel montage and the stars are within my tolerance of "nice":
          http://home.earthlink.net/~celstark/id3.html

          When coupled with the simple reducer, however, the picture is
          different. The Veil shot on there was a quick test of the setup from
          my light polluted back yard. It's a 3 panel montage with the reducer
          in there and the stars near each panel edge look like crud. It looks
          like coma there and as I rack in and out of focus, I see astigmatism.
          I've previously looked for abberations using this reducing lens on my
          f/10 SCT and found nothing really problematic (see bottom of:
          http://home.earthlink.net/~celstark/id7.html if you like).

          Testing in daylight of a target (a "Do not enter" sign 100' away) both
          on and off-axis confirmed off-axis problems (I've got 2 of these
          lenses "A" and "B" with one showing better performance than the other):
          http://home.earthlink.net/~celstark/oct10_FR_test_results.jpg

          This all is a long lead up to my two questions:
          1) Out of curiosity, why would this show up more on the TMB than on
          the SCT? The f-ratio is a likely culprit, but I have seen people use
          this same reducer on cheap 80mm f/5 scopes without problem (I've yet
          to test my examples of the reducer in one)

          2) Out of practicality, what reducer lens would do better than this
          one? Yes, the TMB one would be fantastic, but I have no need at all
          to go to medium format sizes and no desire to spend more on the
          reducing lens than I did on my camera. Cost knocks that one out.
          Televue has two 3-element 0.8x ones for $265. Vixen has a 0.75x for
          the ED102SS (also a 4" f/6) at $255. Meade and Celestron have a 0.63x
          and Meade has a 0.3x (actual numbers will vary with spacing of
          course), both of which are talked about on f/6.3 SCTs at least
          (~$150). Anyone have any thoughts on how these would perform? The
          Meade / Celestron are supposedly reasonable (esp. given my imaging
          gear) but I don't know how they would perform given the difference in
          optics. Any other suggestions?

          Thanks for any help,

          Craig








          Yahoo! Groups Links
        • Robert Anderson
          To add to Adrian s comments, I use Optec s .62 reducer which is designed for refractors only with good success. In fact, with this reducer on the new TMB 175 I
          Message 4 of 5 , Oct 12, 2004
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            To add to Adrian's comments, I use Optec's .62 reducer which is
            designed for refractors only with good success.

            In fact, with this reducer on the new TMB 175 I am getting, it will
            mimic the FOV of my TMB 115 but with higher light gathering
            capability.

            The only issue is that I am just about at the limit of this reducer
            on my current camera which has a 13mm diagonal chip, (SXV-9) and
            though the Optec reducer is flat out to 18mm. When I upgrade to the
            next generation of cameras, the FOV provided by the chip will be in
            the 25-35mm range and I will start all over again looking for a good
            reducer/flattener.

            Cheers

            Bob

            --- In tmboptical@yahoogroups.com, "Adrian Catterall" <adrian@a...>
            wrote:
            > I don't know much about this particular reducer that you are using,
            but
            > it is clearly not designed for your scope. They talk about using it
            for
            > SCTs and refractors, but they are completely different designs of
            > optics. I did try using as Meade focal 0.63x reducer on a refractor
            just
            > out of interest and it gave horrible astigmatic stars. TMB don't
            produce
            > reducers, only the flatteners for wide field imaging. I have used
            the
            > Astrophysics 0.75x reducer with good success and also tried the
            Televue
            > one you mention - this works well as long as you have the distance
            from
            > chip to reducer correct, but it does have a narrow barrel and
            introduces
            > vignetting for larger camera's (such as ST10)..
            >
            > Adrian
            >
            >
            >
            >
            > -----Original Message-----
            > From: Craig Stark [mailto:celstark@e...]
            > Sent: 11 October 2004 17:42
            > To: tmboptical@yahoogroups.com
            > Subject: [tmboptical] Focal reducers for 105/6.2
            >
            >
            >
            > I do a bit of small-chip CCD imaging using a 105/6.2. I know I'm
            not
            > in the league of many here, but I have fun seeing what I can get out
            > of inexpensive CCD gear (here, a SAC8 camera -- 3.7 x 4.9 mm chip
            > size). I've worked a lot with an 8" SCT and had success with simple
            > focal reducers (single 120mm f/l lens).
            >
            > The TMB on its own does wonderfully. The shot of M11 on this page
            was
            > a 5 panel montage and the stars are within my tolerance of "nice":
            > http://home.earthlink.net/~celstark/id3.html
            >
            > When coupled with the simple reducer, however, the picture is
            > different. The Veil shot on there was a quick test of the setup
            from
            > my light polluted back yard. It's a 3 panel montage with the
            reducer
            > in there and the stars near each panel edge look like crud. It
            looks
            > like coma there and as I rack in and out of focus, I see
            astigmatism.
            > I've previously looked for abberations using this reducing lens on
            my
            > f/10 SCT and found nothing really problematic (see bottom of:
            > http://home.earthlink.net/~celstark/id7.html if you like).
            >
            > Testing in daylight of a target (a "Do not enter" sign 100' away)
            both
            > on and off-axis confirmed off-axis problems (I've got 2 of these
            > lenses "A" and "B" with one showing better performance than the
            other):
            > http://home.earthlink.net/~celstark/oct10_FR_test_results.jpg
            >
            > This all is a long lead up to my two questions:
            > 1) Out of curiosity, why would this show up more on the TMB than on
            > the SCT? The f-ratio is a likely culprit, but I have seen people
            use
            > this same reducer on cheap 80mm f/5 scopes without problem (I've yet
            > to test my examples of the reducer in one)
            >
            > 2) Out of practicality, what reducer lens would do better than this
            > one? Yes, the TMB one would be fantastic, but I have no need at all
            > to go to medium format sizes and no desire to spend more on the
            > reducing lens than I did on my camera. Cost knocks that one out.
            > Televue has two 3-element 0.8x ones for $265. Vixen has a 0.75x for
            > the ED102SS (also a 4" f/6) at $255. Meade and Celestron have a
            0.63x
            > and Meade has a 0.3x (actual numbers will vary with spacing of
            > course), both of which are talked about on f/6.3 SCTs at least
            > (~$150). Anyone have any thoughts on how these would perform? The
            > Meade / Celestron are supposedly reasonable (esp. given my imaging
            > gear) but I don't know how they would perform given the difference
            in
            > optics. Any other suggestions?
            >
            > Thanks for any help,
            >
            > Craig
            >
            >
            >
            >
            >
            >
            >
            >
            > Yahoo! Groups Links
          • Craig Stark
            I d like to thank you all for your help and suggestions. One thing that is becoming clear is that my two versions of the same reducer differ considerably in
            Message 5 of 5 , Oct 12, 2004
            • 0 Attachment
              I'd like to thank you all for your help and suggestions. One thing
              that is becoming clear is that my two versions of the same reducer
              differ considerably in this respect not only on the TMB but also on
              the SCT. Another thing that is becoming clear is that a different
              reducer will be warranted when I step up in CCD size. Hopefully, I
              will arrive at a workable solution for my current 6mm diagonal chip --
              that's not really asking for a very large clean area and with
              4"/pixel, it's not like it needs to be pinpoint sharp either.

              Thanks again,

              Craig
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