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DISCOVERY REPORT on 13.1 inch

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  • bigdob101
    Hello! I posted here two or three months ago, regarding my 13.1 inch Dob from Discovery. It seemed to me to be severely overcorrected (1/3-1/2 wave), so I
    Message 1 of 3 , Feb 1 5:24 AM
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      Hello!

      I posted here two or three months ago, regarding my 13.1 inch Dob from Discovery. It
      seemed to me to be severely overcorrected (1/3-1/2 wave), so I sent it back to Discovery
      to be evaluated. I sent it via USPS, and after waiting three weeks, it was returned to me! I
      called Discovery and found that every time they went to pick the mirror up, it was out for
      delivery. So, I resent the mirror via UPS. It took around a month for them to get back with
      me, but finally, here is what they reported. The average peak to valley was .19 wave with a
      Strehl of 95 and an RMS of .035. This was at the wavefront, with laser interferometry and
      computer aided fringe analysis. That sounded good, so I told them to send the mirror
      back. I thought perhaps I didn't let it cool long enough. Well, upon letting the scope cool
      down with a big fan blowing on the primary at high speed, for more than an hour, and
      collimating it very carefully, I can say that the star test is still TOTALLY unacceptable.
      There is something horrifyingly wrong. It tests so BAD--much worse that what an under-
      cooled primary should cause. The difference between inside and outside focus diff. discs is
      amazing, striking, horrifying. It is RAZOR sharp on one side, with the brightest ring on the
      outside, and quickly tapering off in brightness toward the center-- on the other side, very
      fuzzy, with rings of different brightness in different parts if the disc. I know from
      experience that this is not the way a 1/5 wave scope tests! I used eyepieces from Orion,
      (Sirius brand) as the two eyepieces discovery included with the scope are JUNK!!! The stars
      in one, are pinpoint only to about 1/3 of the way to the edge and from there on they flare
      dramatically. The other eyepiece suffers from SEVERE troubles (astigmatism, and I don't
      know what else); stars don't focus to pinpoints at all. Saturn was fuzzy, showing little
      more, (if any) detail than my 6 inch Newt. from Orion. Of course, deep sky objects are
      much brighter in the 13.1 inch Dob. The primary optic is very poorly ventilated. It
      appears that the mirror is taped to a circular piece of plywood with one, two-inch
      ventilation hole drilled in the center! Then there is a space of about an inch all between
      the periphery of the mirror and the tube. That seems like very bad design. Overall, I am
      not impressed. This is the worst star test I have ever seen. And Discovery claims the
      primary is 1/5 wave. If that is true, there must be a whopping problem somewhere else!
      What should I do? Could I be doing something wrong? Could there be a problem with the
      secondary? I have a mortal disgruntle with Discovery. They will NOT answer my e-mails
      and it is very difficult to reach them by phone—this is frustrating! Has anyone else had
      problems like this?

      Thanks for your advice,

      Randy
    • Don D'Egidio
      Randy, Do check the secondary. They had put to much batting in the one for my 17.5 TD which made it take a convex shape. I also had to send back my first
      Message 2 of 3 , Feb 1 6:10 AM
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        Randy,

        Do check the secondary. They had put to much batting in the one for my 17.5" TD which made it take a convex shape.

        I also had to send back my first mirror as it tested at 1/2.51 wave p-v and gave terrible star images. The replacement mirror tested at 1/4.85 wave, which was better, but combined with the secondary, is not diffraction limited.

        Don

        ----- Original Message -----
        From: "bigdob101" <rlsjrbox@...>
        To: <telescopes@yahoogroups.com>
        Sent: Wednesday, February 01, 2006 8:24
        Subject: [Telescopes] DISCOVERY REPORT on 13.1 inch


        Hello!

        I posted here two or three months ago, regarding my 13.1 inch Dob from Discovery. It
        seemed to me to be severely overcorrected (1/3-1/2 wave), so I sent it back to Discovery
        to be evaluated. I sent it via USPS, and after waiting three weeks, it was returned to me! I
        called Discovery and found that every time they went to pick the mirror up, it was out for
        delivery. So, I resent the mirror via UPS. It took around a month for them to get back with
        me, but finally, here is what they reported. The average peak to valley was .19 wave with a
        Strehl of 95 and an RMS of .035. This was at the wavefront, with laser interferometry and
        computer aided fringe analysis. That sounded good, so I told them to send the mirror
        back. I thought perhaps I didn't let it cool long enough. Well, upon letting the scope cool
        down with a big fan blowing on the primary at high speed, for more than an hour, and
        collimating it very carefully, I can say that the star test is still TOTALLY unacceptable.
        There is something horrifyingly wrong. It tests so BAD--much worse that what an under-
        cooled primary should cause. The difference between inside and outside focus diff. discs is
        amazing, striking, horrifying. It is RAZOR sharp on one side, with the brightest ring on the
        outside, and quickly tapering off in brightness toward the center-- on the other side, very
        fuzzy, with rings of different brightness in different parts if the disc. I know from
        experience that this is not the way a 1/5 wave scope tests! I used eyepieces from Orion,
        (Sirius brand) as the two eyepieces discovery included with the scope are JUNK!!! The stars
        in one, are pinpoint only to about 1/3 of the way to the edge and from there on they flare
        dramatically. The other eyepiece suffers from SEVERE troubles (astigmatism, and I don't
        know what else); stars don't focus to pinpoints at all. Saturn was fuzzy, showing little
        more, (if any) detail than my 6 inch Newt. from Orion. Of course, deep sky objects are
        much brighter in the 13.1 inch Dob. The primary optic is very poorly ventilated. It
        appears that the mirror is taped to a circular piece of plywood with one, two-inch
        ventilation hole drilled in the center! Then there is a space of about an inch all between
        the periphery of the mirror and the tube. That seems like very bad design. Overall, I am
        not impressed. This is the worst star test I have ever seen. And Discovery claims the
        primary is 1/5 wave. If that is true, there must be a whopping problem somewhere else!
        What should I do? Could I be doing something wrong? Could there be a problem with the
        secondary? I have a mortal disgruntle with Discovery. They will NOT answer my e-mails
        and it is very difficult to reach them by phone-this is frustrating! Has anyone else had
        problems like this?

        Thanks for your advice,

        Randy








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      • Richard Runcie
        Randy, As others have posted check your secondary. Also you may want to mount your primary on another mirror support mount. Also for piece of mind, I would
        Message 3 of 3 , Feb 3 10:44 AM
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          Randy,

          As others have posted check your secondary. Also you may want to mount your primary on another mirror support mount. Also for piece of mind, I would check with Woden Optics. They offer free testing of primaries up to 18"'s of aperture. An independent test would definitely help your next decision if it is not your secondary.

          Good Luck and let us know how you make out!

          Clear and steady skies, Rick Runcie

          bigdob101 <rlsjrbox@...> wrote:
          Hello!

          I posted here two or three months ago, regarding my 13.1 inch Dob from Discovery. It
          seemed to me to be severely overcorrected (1/3-1/2 wave), so I sent it back to Discovery
          to be evaluated. I sent it via USPS, and after waiting three weeks, it was returned to me! I
          called Discovery and found that every time they went to pick the mirror up, it was out for
          delivery. So, I resent the mirror via UPS. It took around a month for them to get back with
          me, but finally, here is what they reported. The average peak to valley was .19 wave with a
          Strehl of 95 and an RMS of .035. This was at the wavefront, with laser interferometry and
          computer aided fringe analysis. That sounded good, so I told them to send the mirror
          back. I thought perhaps I didn't let it cool long enough. Well, upon letting the scope cool
          down with a big fan blowing on the primary at high speed, for more than an hour, and
          collimating it very carefully, I can say that the star test is still TOTALLY unacceptable.
          There is something horrifyingly wrong. It tests so BAD--much worse that what an under-
          cooled primary should cause. The difference between inside and outside focus diff. discs is
          amazing, striking, horrifying. It is RAZOR sharp on one side, with the brightest ring on the
          outside, and quickly tapering off in brightness toward the center-- on the other side, very
          fuzzy, with rings of different brightness in different parts if the disc. I know from
          experience that this is not the way a 1/5 wave scope tests! I used eyepieces from Orion,
          (Sirius brand) as the two eyepieces discovery included with the scope are JUNK!!! The stars
          in one, are pinpoint only to about 1/3 of the way to the edge and from there on they flare
          dramatically. The other eyepiece suffers from SEVERE troubles (astigmatism, and I don't
          know what else); stars don't focus to pinpoints at all. Saturn was fuzzy, showing little
          more, (if any) detail than my 6 inch Newt. from Orion. Of course, deep sky objects are
          much brighter in the 13.1 inch Dob. The primary optic is very poorly ventilated. It
          appears that the mirror is taped to a circular piece of plywood with one, two-inch
          ventilation hole drilled in the center! Then there is a space of about an inch all between
          the periphery of the mirror and the tube. That seems like very bad design. Overall, I am
          not impressed. This is the worst star test I have ever seen. And Discovery claims the
          primary is 1/5 wave. If that is true, there must be a whopping problem somewhere else!
          What should I do? Could I be doing something wrong? Could there be a problem with the
          secondary? I have a mortal disgruntle with Discovery. They will NOT answer my e-mails
          and it is very difficult to reach them by phone—this is frustrating! Has anyone else had
          problems like this?

          Thanks for your advice,

          Randy








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