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Re: [sct-user] Re: Secondary questions

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  • Kerry Koppert
    A spherical primary doesn t have an optical axis. Kerry Koppert ... From: ancient.sull To: sct-user@yahoogroups.com Sent: Monday, August 01, 2011 2:36 PM
    Message 1 of 13 , Aug 1 12:28 AM
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      A spherical primary doesn't have an optical axis.

      Kerry Koppert

      ----- Original Message -----
      From: ancient.sull
      To: sct-user@yahoogroups.com
      Sent: Monday, August 01, 2011 2:36 PM
      Subject: [sct-user] Re: Secondary questions



      --- In sct-user@yahoogroups.com, William Hamblen <wrhamblen@...> wrote:

      > You can check
      > the secondary on a Celestron or Meade schmidt-cassegrain by simply
      > measuring the position of the secondary with a machinist's rule

      That will tell you where the secondary is relative to the walls of the tube, but not where the secondary is relative to the optical axis of the primary mirror. I.e. the primary mirror or the back of the OTA could be tilted.

      In any case it's a general question. What effect does an off center secondary have? Assymetric illumination of the field? And what parts of that can be fixed with collimation.

      > The Takahashi collimating telescope screws onto a special
      > fitting on their telescopes. How do you use one with another
      > manufacturer's telescope?

      With all screw-thread adapters.

      Drew S.





      [Non-text portions of this message have been removed]
    • ancient.sull
      ... Let me try to say this a different way: I have a primary mirror which is pointing in a certain direction . If that certain direction doesn t intersect
      Message 2 of 13 , Aug 1 7:31 AM
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        --- In sct-user@yahoogroups.com, "Kerry Koppert" <kkoppert@...> wrote:
        >
        > A spherical primary doesn't have an optical axis.
        >
        > Kerry Koppert

        Let me try to say this a different way:

        I have a primary mirror which is "pointing in a certain direction". If that "certain direction" doesn't intersect the center of the secondary mirror, but instead hits the secondary a bit off center, say 20% off center, and if I collimate as well as possible then

        What effect does that have on overall illumination of the chip? Is the illumination assymetric?

        What effect does that have on star shape? Are the stars elongated more on one side of the chip than the other (assymetrically elongated in that part of the chip even though I am collimated on a star at the center of the chip)

        Drew S.
      • John Mahony
        ... Mostly, yes.  But the pivot point (on most SCTs) is at the center rear of the metal plate that the sec is attached to.  The plate is around 1/4 thick,
        Message 3 of 13 , Aug 1 3:06 PM
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          ----- Original Message -----

          > From: ancient.sull <ancient.sull@...>
          >
          > What effect does it have if your secondary is off center a little (maybe by 10
          > or 20% of it's width), assuming you have collimated with the secondary in
          > that position?
          >
          > The secondary in a standard SCT is not spherical, right?
          >
          > Is it true that when you collimate you are tilting, not shifting, the secondary?

          Mostly, yes.  But the pivot point (on most SCTs) is at the center rear of the metal plate that the sec is attached to.  The plate is around 1/4" thick, and the glass sec is about 1/2" thick, so the front center does shift sideways a bit when you tilt it on the pivot point.
           
          > Mine is off center when viewed through a tack collimation scope.

          "tack" or "Tak"?

          Sounds like you're looking through the rear port.  But you've got three optical components, and the rear port, and their axes will all vary slightly due to manufacturing tolerances.  There's a case in the Mapug archives where a guy noticed his sec was off-center on the mounting plate.  So he "fixed" it by removing the sec and remounting in the center.  But then he found he couldn't get a good collimation.  Apparently Meade had deliberately mounted it off-center to compensate for other misalignments.

          -John
        • John Mahony
          Yes it does.  In fact, it has an infinite number of them.  Any line that s locally perpendicular to the surface (perpendicular to a tangent to the surface,
          Message 4 of 13 , Aug 1 3:12 PM
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            Yes it does.  In fact, it has an infinite number of them.  Any line that's "locally perpendicular" to the surface (perpendicular to a tangent to the surface, and intersecting at the point where that tangent meets the surface- or IOW- any "radius" of the sphere) could be taken as an optical axis.  But for the sake of even illumination, it's best to use the one that intersects the primary at the center.

            -John



            ----- Original Message -----
            > From: Kerry Koppert <kkoppert@...>
            >
            > A spherical primary doesn't have an optical axis.
            >
            > Kerry Koppert
            >
            >   ----- Original Message -----
            >   From: ancient.sull
            >   To: sct-user@yahoogroups.com
            >   Sent: Monday, August 01, 2011 2:36 PM
            >   Subject: [sct-user] Re: Secondary questions
            >
            >
            >    
            >   --- In sct-user@yahoogroups.com, William Hamblen <wrhamblen@...> wrote:
            >
            >   > You can check
            >   > the secondary on a Celestron or Meade schmidt-cassegrain by simply
            >   > measuring the position of the secondary with a machinist's rule
            >
            >   That will tell you where the secondary is relative to the walls of the tube,
            > but not where the secondary is relative to the optical axis of the primary
            > mirror. I.e. the primary mirror or the back of the OTA could be tilted.
            >
            >   In any case it's a general question. What effect does an off center
            > secondary have? Assymetric illumination of the field? And what parts of that can
            > be fixed with collimation.
            >
            >   > The Takahashi collimating telescope screws onto a special
            >   > fitting on their telescopes. How do you use one with another
            >   > manufacturer's telescope?
            >
            >   With all screw-thread adapters.
            >
            >   Drew S.
          • Rod Mollise
            Which for all practical purposed is _exactly the same thing_. ;-) Peace, Rod Mollise Rod s new book: _Choosing and Using a New CAT_ Time to waste? Waste it
            Message 5 of 13 , Aug 1 4:03 PM
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              Which for all practical purposed is _exactly the same thing_. ;-)


              Peace,
              Rod Mollise
              Rod's new book:
              _Choosing and Using a New CAT_
              Time to waste? Waste it with Uncle Rod's Astro Blog:

              http://uncle-rods.blogspot.com/





              -----Original Message-----
              From: sct-user@yahoogroups.com [mailto:sct-user@yahoogroups.com] On Behalf
              Of John Mahony
              Sent: Monday, August 01, 2011 5:13 PM
              To: sct-user@yahoogroups.com
              Subject: Re: [sct-user] Re: Secondary questions

              Yes it does.  In fact, it has an infinite number of them. 



              ----- Original Message -----
              > From: Kerry Koppert <kkoppert@...>
              >
              > A spherical primary doesn't have an optical axis.
              >
              > Kerry Koppert
              >
              >   ----- Original Message -----
              >   From: ancient.sull
              >   To: sct-user@yahoogroups.com
              >   Sent: Monday, August 01, 2011 2:36 PM
              >   Subject: [sct-user] Re: Secondary questions
              >
              >
              >    
              >   --- In sct-user@yahoogroups.com, William Hamblen <wrhamblen@...> wrote:
              >
              >   > You can check
              >   > the secondary on a Celestron or Meade schmidt-cassegrain by simply
              >   > measuring the position of the secondary with a machinist's rule
              >
              >   That will tell you where the secondary is relative to the walls of the
              tube,
              > but not where the secondary is relative to the optical axis of the primary

              > mirror. I.e. the primary mirror or the back of the OTA could be tilted.
              >
              >   In any case it's a general question. What effect does an off center
              > secondary have? Assymetric illumination of the field? And what parts of
              that can
              > be fixed with collimation.
              >
              >   > The Takahashi collimating telescope screws onto a special
              >   > fitting on their telescopes. How do you use one with another
              >   > manufacturer's telescope?
              >
              >   With all screw-thread adapters.
              >
              >   Drew S.


              ------------------------------------

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            • Kerry Koppert
              I m sorry but you re just making the same invalid assumption in different words. ANY LINE THRU THE CENTRE OF A SPHERE is an optical axis. Take a spherical
              Message 6 of 13 , Aug 1 4:58 PM
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                I'm sorry but you're just making the same invalid assumption in different words. ANY LINE THRU THE CENTRE OF A SPHERE is an optical axis. Take a spherical primay and a spherical secondary, draw a line betwen the centres and that line is a common axis of symettry or optical axis. This line may not coincide with the optical axis of the eyepiece, and we have to ask can they be made coincident? The answer is no so I guess the rest of reasoning holds.

                Kerry Koppert

                ----- Original Message -----
                From: ancient.sull
                To: sct-user@yahoogroups.com
                Sent: Tuesday, August 02, 2011 2:31 AM
                Subject: [sct-user] Re: Secondary questions



                --- In sct-user@yahoogroups.com, "Kerry Koppert" <kkoppert@...> wrote:
                >
                > A spherical primary doesn't have an optical axis.
                >
                > Kerry Koppert

                Let me try to say this a different way:

                I have a primary mirror which is "pointing in a certain direction". If that "certain direction" doesn't intersect the center of the secondary mirror, but instead hits the secondary a bit off center, say 20% off center, and if I collimate as well as possible then

                What effect does that have on overall illumination of the chip? Is the illumination assymetric?

                What effect does that have on star shape? Are the stars elongated more on one side of the chip than the other (assymetrically elongated in that part of the chip even though I am collimated on a star at the center of the chip)

                Drew S.





                [Non-text portions of this message have been removed]
              • John Mahony
                No.  Not just any line through the surface is an axis. -John
                Message 7 of 13 , Aug 1 6:24 PM
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                  No.  Not just any line through the surface is an axis.

                  -John



                  ----- Original Message -----
                  > From: Rod Mollise <rmollise@...>
                  >
                  > Which for all practical purposed is _exactly the same thing_. ;-)
                  >
                  >
                  > Peace,
                  > Rod Mollise
                  > Rod's new book:
                  > _Choosing and Using a New CAT_
                  > Time to waste? Waste it with Uncle Rod's Astro Blog:
                  >
                  > http://uncle-rods.blogspot.com/
                  >
                  >
                  >
                  >
                  >
                  > -----Original Message-----
                  > From: sct-user@yahoogroups.com [mailto:sct-user@yahoogroups.com] On Behalf
                  > Of John Mahony
                  > Sent: Monday, August 01, 2011 5:13 PM
                  > To: sct-user@yahoogroups.com
                  > Subject: Re: [sct-user] Re: Secondary questions
                  >
                  > Yes it does.  In fact, it has an infinite number of them. 
                  >
                  >
                  >
                  > ----- Original Message -----
                  >> From: Kerry Koppert <kkoppert@...>
                  >>
                  >> A spherical primary doesn't have an optical axis.
                  >>
                  >> Kerry Koppert
                  >>
                  >>   ----- Original Message -----
                  >>   From: ancient.sull
                  >>   To: sct-user@yahoogroups.com
                  >>   Sent: Monday, August 01, 2011 2:36 PM
                  >>   Subject: [sct-user] Re: Secondary questions
                  >>
                  >>
                  >>    
                  >>   --- In sct-user@yahoogroups.com, William Hamblen <wrhamblen@...>
                  > wrote:
                  >>
                  >>   > You can check
                  >>   > the secondary on a Celestron or Meade schmidt-cassegrain by simply
                  >>   > measuring the position of the secondary with a machinist's rule
                  >>
                  >>   That will tell you where the secondary is relative to the walls of the
                  > tube,
                  >> but not where the secondary is relative to the optical axis of the primary
                  >
                  >> mirror. I.e. the primary mirror or the back of the OTA could be tilted.
                  >>
                  >>   In any case it's a general question. What effect does an off center
                  >> secondary have? Assymetric illumination of the field? And what parts of
                  > that can
                  >> be fixed with collimation.
                  >>
                  >>   > The Takahashi collimating telescope screws onto a special
                  >>   > fitting on their telescopes. How do you use one with another
                  >>   > manufacturer's telescope?
                  >>
                  >>   With all screw-thread adapters.
                  >>
                  >>   Drew S.
                  >
                  >
                  > ------------------------------------
                  >
                  > Visit the sct-user home page at:
                  >
                  >
                  >
                  > http://skywatch.brainiac.com/SCThpYahoo! Groups Links
                  >
                  >
                  >
                  >
                  >
                  > ------------------------------------
                  >
                  > Visit the sct-user home page at:
                  >
                  >
                  >
                  > http://skywatch.brainiac.com/SCThpYahoo! Groups Links
                  >
                  >
                  >
                • John Mahony
                  But the primary is only part of the surface of one of those spheres, so if that line through the centers of the spheres doesn t intersect the center of the
                  Message 8 of 13 , Aug 1 6:28 PM
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                    But the primary is only part of the surface of one of those spheres, so if that line through the centers of the spheres doesn't intersect the center of the primary, you get uneven illumination, which is one of the issues Drew was asking about.

                    -John



                    ----- Original Message -----
                    > From: Kerry Koppert <kkoppert@...>
                    >
                    > I'm sorry but you're just making the same invalid assumption in
                    > different words. ANY LINE THRU THE CENTRE OF A SPHERE is an optical axis. Take a
                    > spherical primay and a spherical secondary, draw a line betwen the centres and
                    > that line is a common axis of symettry or optical axis. This line may not
                    > coincide with the optical axis of the eyepiece, and we have to ask can they be
                    > made coincident? The answer is no so I guess the rest of reasoning holds.
                    >
                    > Kerry Koppert
                    >
                    >   ----- Original Message -----
                    >   From: ancient.sull
                    >   To: sct-user@yahoogroups.com
                    >   Sent: Tuesday, August 02, 2011 2:31 AM
                    >   Subject: [sct-user] Re: Secondary questions
                    >
                    >
                    >    
                    >   --- In sct-user@yahoogroups.com, "Kerry Koppert"
                    > <kkoppert@...> wrote:
                    >   >
                    >   > A spherical primary doesn't have an optical axis.
                    >   >
                    >   > Kerry Koppert
                    >
                    >   Let me try to say this a different way:
                    >
                    >   I have a primary mirror which is "pointing in a certain direction".
                    > If that "certain direction" doesn't intersect the center of the
                    > secondary mirror, but instead hits the secondary a bit off center, say 20% off
                    > center, and if I collimate as well as possible then
                    >
                    >   What effect does that have on overall illumination of the chip? Is the
                    > illumination assymetric?
                    >
                    >   What effect does that have on star shape? Are the stars elongated more on one
                    > side of the chip than the other (assymetrically elongated in that part of the
                    > chip even though I am collimated on a star at the center of the chip)
                    >
                    >   Drew S.
                    >
                    >
                    >
                    >  
                    >
                    > [Non-text portions of this message have been removed]
                    >
                    >
                    >
                    > ------------------------------------
                    >
                    > Visit the sct-user home page at:
                    >
                    >
                    >
                    > http://skywatch.brainiac.com/SCThpYahoo! Groups Links
                    >
                    >
                    >
                  • Rod Mollise
                    Yes. Most assuredly, John. Peace, Rod Mollise Rod s new book: _Choosing and Using a New CAT_ Time to waste? Waste it with Uncle Rod s Astro Blog:
                    Message 9 of 13 , Aug 2 2:45 AM
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                      Yes. Most assuredly, John.


                      Peace,
                      Rod Mollise
                      Rod's new book:
                      _Choosing and Using a New CAT_
                      Time to waste? Waste it with Uncle Rod's Astro Blog:

                      http://uncle-rods.blogspot.com/




                      -----Original Message-----
                      From: sct-user@yahoogroups.com [mailto:sct-user@yahoogroups.com] On Behalf
                      Of John Mahony
                      Sent: Monday, August 01, 2011 8:25 PM
                      To: sct-user@yahoogroups.com
                      Subject: Re: [sct-user] Re: Secondary questions

                      No.  Not just any line through the surface is an axis.

                      -John



                      ----- Original Message -----
                      > From: Rod Mollise <rmollise@...>
                      >
                      > Which for all practical purposed is _exactly the same thing_. ;-)
                      >
                      >
                      > Peace,
                      > Rod Mollise
                      > Rod's new book:
                      > _Choosing and Using a New CAT_
                      > Time to waste? Waste it with Uncle Rod's Astro Blog:
                      >
                      > http://uncle-rods.blogspot.com/
                      >
                      >
                      >
                      >
                      >
                      > -----Original Message-----
                      > From: sct-user@yahoogroups.com [mailto:sct-user@yahoogroups.com] On Behalf
                      > Of John Mahony
                      > Sent: Monday, August 01, 2011 5:13 PM
                      > To: sct-user@yahoogroups.com
                      > Subject: Re: [sct-user] Re: Secondary questions
                      >
                      > Yes it does.  In fact, it has an infinite number of them. 
                      >
                      >
                      >
                      > ----- Original Message -----
                      >> From: Kerry Koppert <kkoppert@...>
                      >>
                      >> A spherical primary doesn't have an optical axis.
                      >>
                      >> Kerry Koppert
                      >>
                      >>   ----- Original Message -----
                      >>   From: ancient.sull
                      >>   To: sct-user@yahoogroups.com
                      >>   Sent: Monday, August 01, 2011 2:36 PM
                      >>   Subject: [sct-user] Re: Secondary questions
                      >>
                      >>
                      >>    
                      >>   --- In sct-user@yahoogroups.com, William Hamblen <wrhamblen@...>
                      > wrote:
                      >>
                      >>   > You can check
                      >>   > the secondary on a Celestron or Meade schmidt-cassegrain by simply
                      >>   > measuring the position of the secondary with a machinist's rule
                      >>
                      >>   That will tell you where the secondary is relative to the walls of the
                      > tube,
                      >> but not where the secondary is relative to the optical axis of the
                      primary
                      >
                      >> mirror. I.e. the primary mirror or the back of the OTA could be tilted.
                      >>
                      >>   In any case it's a general question. What effect does an off center
                      >> secondary have? Assymetric illumination of the field? And what parts of
                      > that can
                      >> be fixed with collimation.
                      >>
                      >>   > The Takahashi collimating telescope screws onto a special
                      >>   > fitting on their telescopes. How do you use one with another
                      >>   > manufacturer's telescope?
                      >>
                      >>   With all screw-thread adapters.
                      >>
                      >>   Drew S.
                      >
                      >
                      > ------------------------------------
                      >
                      > Visit the sct-user home page at:
                      >
                      >
                      >
                      > http://skywatch.brainiac.com/SCThpYahoo! Groups Links
                      >
                      >
                      >
                      >
                      >
                      > ------------------------------------
                      >
                      > Visit the sct-user home page at:
                      >
                      >
                      >
                      > http://skywatch.brainiac.com/SCThpYahoo! Groups Links
                      >
                      >
                      >


                      ------------------------------------

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