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Re: [atm_free] A newbie question on tube diameter

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  • Richard Schwartz
    The consequence of using NEWT is vignetting by an undersized diagonal. The degree of vignetting renders the scope useless for variable star brightness
    Message 1 of 20 , Jun 1, 2004
      The consequence of using NEWT is vignetting by an undersized diagonal.   The degree of vignetting renders the scope useless for variable star brightness estimates to commonly accepted standards of the AAVSO.
       
       
      ----- Original Message -----
      Sent: 5/29/2004 9:13:33 PM
      Subject: Re: [atm_free] A newbie question on tube diameter

      Prasad, Do a google search (or whatever search engine you might use) for a software program called NEWT. This program will give you all of the dimesions you need for building a newtonian reflector.
       
      The consequences of too small a diameter tube is vingetting of the light path.
       
      Neil
      ----- Original Message -----
      Sent: Saturday, May 29, 2004 9:00 PM
      Subject: [atm_free] A newbie question on tube diameter

      Group,

      Please advice me. I have been reading articles on the Internet and I note
      that most authors recommend that the tube diameter must be 2 inches larger
      than the objective diameter. I could not find any explanation on the
      reasons. What if it is just slightly (say 1/2") bigger than the objective? I
      have an 6" f4 objective and a 1-1/4" secondary. I want to place the
      secondary as far as possible from the primary mirror to reduce loss of
      illumination.

      Thanks you,
      -Prasad




    • Richard Schwartz
      unsubscrive atm ... The above is correct only for very high power long focus telescopes, not for low power wide field instruments, and not for telescopes used
      Message 2 of 20 , Jun 1, 2004
        unsubscrive atm

        > 2. The illumination at the edge of the fov has to be at least 70% (= an
        > illumination drop of 0.38 magnitudes).

        The above is correct only for very high power long focus telescopes, not
        for low power wide field instruments, and not for telescopes used with 35mm
        camera bodies to do astrophotography. I do not understand why you want to
        propagate this incorrect dogma. Variable star observers who work by
        visual comparison with nearby stars are accurate to 0.1 magnitude. You
        tolerate a level of vignetting that would render a telescope worthless for
        such an observation.

        . . . Richard
      • Richard Schwartz
        I have a program that will computer the light lost in your telescope from vignetting at the tube rim, the central obstruction of the diagonal, vignetting at
        Message 3 of 20 , Jun 1, 2004
          I have a program that will computer the light lost in your telescope from
          vignetting at the tube rim, the central obstruction of the diagonal,
          vignetting at the diagonal, and vignetting at the focuser rim. With this
          program, you could play with the numbers to get a wrokable solution. It
          runs on a Palm. I'm looking at converting it to an Excel spreadsheet
          (which will run on the Palm AND on a Pocket PC). But how to do it without
          macros....???

          . . . Richard


          > [Original Message]
          > From: Prasad Agrahar <prasad@...>
          > To: <atm_free@yahoogroups.com>
          > Date: 5/30/2004 10:02:18 AM
          > Subject: [atm_free] A newbie question on tube diameter
          >
          > Group,
          >
          > Please advice me. I have been reading articles on the Internet and I note
          > that most authors recommend that the tube diameter must be 2 inches larger
          > than the objective diameter. I could not find any explanation on the
          > reasons. What if it is just slightly (say 1/2") bigger than the
          objective? I
          > have an 6" f4 objective and a 1-1/4" secondary. I want to place the
          > secondary as far as possible from the primary mirror to reduce loss of
          > illumination.
          >
          > Thanks you,
          > -Prasad
          >
          >
          >
          >
          >
          >
          > Yahoo! Groups Links
          >
          >
          >
          >
        • Jan van gastel
          I assumed the telescope was for general visual observing. But you are right of course: for astrophotography and situations (mentioned by you) where one needs a
          Message 4 of 20 , Jun 1, 2004
            I assumed the telescope was for general visual observing. But you are right
            of course: for astrophotography and situations (mentioned by you) where one
            needs a (very) large fully illuminated field, the secondary could be too
            small.


            Jan
            http://home.wanadoo.nl/jhm.vangastel/Astronomy/



            ----- Original Message -----
            From: "Richard Schwartz" <richas@...>
            To: <atm_free@yahoogroups.com>
            Sent: Tuesday, June 01, 2004 1:40 PM
            Subject: Re: [atm_free] A newbie question on tube diameter


            > unsubscrive atm
            >
            > > 2. The illumination at the edge of the fov has to be at least 70% (= an
            > > illumination drop of 0.38 magnitudes).
            >
            > The above is correct only for very high power long focus telescopes, not
            > for low power wide field instruments, and not for telescopes used with
            35mm
            > camera bodies to do astrophotography. I do not understand why you want
            to
            > propagate this incorrect dogma. Variable star observers who work by
            > visual comparison with nearby stars are accurate to 0.1 magnitude. You
            > tolerate a level of vignetting that would render a telescope worthless for
            > such an observation.
            >
            > . . . Richard
            >
            >
            >
            >
            >
            >
            >
            > Yahoo! Groups Links
            >
            >
            >
            >
            >
            >
          • jkolling
            http://home.att.net/~dale.keller/atm/newtonians/newtsoft/newtsoft.htm http://members.aol.com/ronwin20/
            Message 5 of 20 , Jun 1, 2004
              http://home.att.net/~dale.keller/atm/newtonians/newtsoft/newtsoft.htm
              http://members.aol.com/ronwin20/


              Richard Schwartz wrote:

              > The consequence of using NEWT is vignetting by an undersized diagonal.
              > The degree of vignetting renders the scope useless for variable star
              > brightness estimates to commonly accepted standards of the AAVSO.
              >
              >
              >
              > ----- Original Message -----
              > From: Neil Carroll <mailto:ncarz@...>
              > To: atm_free@yahoogroups.com <mailto:atm_free@yahoogroups.com>
              > Sent: 5/29/2004 9:13:33 PM
              > Subject: Re: [atm_free] A newbie question on tube diameter
              >
              > Prasad, Do a google search (or whatever search engine you might use)
              > for a software program called NEWT. This program will give you all
              > of the dimesions you need for building a newtonian reflector.
              >
              > The consequences of too small a diameter tube is vingetting of the
              > light path.
              >
              > Neil
              >
              > ----- Original Message -----
              > From: Prasad Agrahar <mailto:prasad@...>
              > To: atm_free@yahoogroups.com <mailto:atm_free@yahoogroups.com>
              > Sent: Saturday, May 29, 2004 9:00 PM
              > Subject: [atm_free] A newbie question on tube diameter
              >
              > Group,
              >
              > Please advice me. I have been reading articles on the Internet
              > and I note
              > that most authors recommend that the tube diameter must be 2
              > inches larger
              > than the objective diameter. I could not find any explanation on the
              > reasons. What if it is just slightly (say 1/2") bigger than the
              > objective? I
              > have an 6" f4 objective and a 1-1/4" secondary. I want to place the
              > secondary as far as possible from the primary mirror to reduce
              > loss of
              > illumination.
              >
              > Thanks you,
              > -Prasad
              >
              >
              >
              >
              >
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            • Prasad Agrahar
              Hi Group Honestly, I did not expect to receive such large response from all of you. I am overwhelmed and this is going to keep me occupied for a lot more time
              Message 6 of 20 , Jun 1, 2004
                Hi Group
                 
                Honestly, I did not expect to receive such large response from all of you. I am overwhelmed and this is going to keep me occupied for a lot more time trying to understand it all. I am grateful to all of you who responded and I will have more questions very soon.
                 
                Thank you once again,
                -Prasad
                 
              • Richard Schwartz
                Be sure to ingore all of the replies (except mine, of course!). They don t know anything. Only I am right. . . . Richard ... From: Prasad Agrahar To:
                Message 7 of 20 , Jun 1, 2004
                  Be sure to ingore all of the replies (except mine, of course!).   They don't know anything.  Only I am right.
                   
                  . . . Richard
                   
                   
                  ----- Original Message -----
                  Sent: 6/1/2004 11:02:46 PM
                  Subject: Re: [atm_free] A newbie question on tube diameter

                  Hi Group
                   
                  Honestly, I did not expect to receive such large response from all of you. I am overwhelmed and this is going to keep me occupied for a lot more time trying to understand it all. I am grateful to all of you who responded and I will have more questions very soon.
                   
                  Thank you once again,
                  -Prasad
                   

                • Jerry
                  Oh, come on Richard. Your advise was ridiculous. He can look up the magnitudes. Jerry ... From: Richard Schwartz [mailto:richas@earthlink.net] Be sure to
                  Message 8 of 20 , Jun 1, 2004

                                Oh, come on Richard.  Your advise was ridiculous.  He can look up the magnitudes.

                     

                                Jerry

                     

                    -----Original Message-----
                    From: Richard Schwartz [mailto:richas@...]
                     

                    Be sure to ingore all of the replies (except mine, of course!).   They don't know anything.  Only I am right.

                     

                    . . . Richard

                     

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