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Imager and Autoguider not in focus?

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  • Umair
    Hello, When i try to autoguide from ST9XE..i could not get both imager and autoguider to the same focus..only one remains in good focus and other looses it
    Message 1 of 6 , Jun 2, 2008
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      Hello,

      When i try to autoguide from ST9XE..i could not get both imager
      and autoguider to the same focus..only one remains in good focus and
      other looses it all..is there a problem or am i missing something here?

      Regards,
      Umair
      C14 and ST9XE
      Losmandy Titan with Gemini
      0.63 Reducer and CFW8 filter wheel
    • Rainer
      Hi Umair, Are you talking of a camera with dual chip ? If yes then something is wrong with the camera eg. somehow one chip has a different focusing plane
      Message 2 of 6 , Jun 2, 2008
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        Hi Umair,

        Are you talking of a camera with dual chip ? If yes then something is
        wrong with the camera eg. somehow one chip has a different focusing
        plane because maybe due to something the chip moved.

        saludos Rainer

        >
        >
        >
        > Hello,
        >
        > When i try to autoguide from ST9XE..i could not get both imager
        > and autoguider to the same focus..only one remains in good focus and
        > other looses it all..is there a problem or am i missing something
        here?
        >
        > Regards,
        > Umair
        > C14 and ST9XE
        > Losmandy Titan with Gemini
        > 0.63 Reducer and CFW8 filter wheel
        >
      • Ron Brant
        As previously stated, this will vary with field curvature. I have noticed it on several of my cameras, especially when using focal reducers. rb ... From:
        Message 3 of 6 , Jun 2, 2008
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          As previously stated, this will vary with field curvature. I have noticed it on several of my cameras, especially when using focal reducers.
          rb
          ----- Original Message -----
          From: Rainer
          To: ccd-newastro@yahoogroups.com
          Sent: Monday, June 02, 2008 10:37 AM
          Subject: [ccd-newastro] Re: Imager and Autoguider not in focus?


          Hi Umair,

          Are you talking of a camera with dual chip ? If yes then something is
          wrong with the camera eg. somehow one chip has a different focusing
          plane because maybe due to something the chip moved.

          saludos Rainer

          >
          >
          >
          > Hello,
          >
          > When i try to autoguide from ST9XE..i could not get both imager
          > and autoguider to the same focus..only one remains in good focus and
          > other looses it all..is there a problem or am i missing something
          here?
          >
          > Regards,
          > Umair
          > C14 and ST9XE
          > Losmandy Titan with Gemini
          > 0.63 Reducer and CFW8 filter wheel
          >





          [Non-text portions of this message have been removed]
        • Frank S Barnes III
          Hi Umair, What is the distance from the 0.63 reducer to the main chip? If this spacing is too far, it could produce the effect you are experiencing. ... Clear
          Message 4 of 6 , Jun 2, 2008
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            Hi Umair,

            What is the distance from the 0.63 reducer to the main chip? If
            this spacing is too far, it could produce the effect you are
            experiencing.


            >On 2 Jun 2008 22:03:02 -0000, ccd-newastro@yahoogroups.com wrote:

            >________________________________________________________________________
            >3a. Imager and Autoguider not in focus?
            > Posted by: "Umair" umairasim9@... umairasim9
            > Date: Mon Jun 2, 2008 7:52 am ((PDT))
            >
            >
            >
            > Hello,
            >
            > When i try to autoguide from ST9XE..i could not get both imager
            >and autoguider to the same focus..only one remains in good focus and
            >other looses it all..is there a problem or am i missing something here?
            >
            > Regards,
            > Umair
            > C14 and ST9XE
            > Losmandy Titan with Gemini
            > 0.63 Reducer and CFW8 filter wheel
            >


            Clear Skies ......

            Klaatu Barada Nikto ...

            Frank S (Sandy) Barnes III
            TwinOaks Observatory
            http://www.skyimager.com
            SBarnes@...
          • Roger Hamlett
            ... To some extent, it is normal, but the question is how much .... Basically, unless the scope s image is _completely_ flat, there will always be a slight
            Message 5 of 6 , Jun 3, 2008
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              ________________________________________________________________________
              > Hello,
              >
              > When i try to autoguide from ST9XE..i could not get both imager
              > and autoguider to the same focus..only one remains in good focus and
              > other looses it all..is there a problem or am i missing something here?
              >
              > Regards,
              > Umair
              > C14 and ST9XE
              > Losmandy Titan with Gemini
              > 0.63 Reducer and CFW8 filter wheel
              To some extent, it is normal, but the question is 'how much'....
              Basically, unless the scope's image is _completely_ flat, there will
              always be a slight focus difference on the guider image, from the 'main'
              image. This will also be present to a lesser extent across the main
              image field, which is why it may well be worth electing to focus on a
              star perhaps 1/3rd 'away' from the centre of the field to get a better
              'compromise' across the field. The guider will function well, with a
              slightly defocussed image (in some cases, slightly _better_!). However
              if the difference if major, then it implies the internal alignment of
              the two CCD's has been compromised. SBIG will re-align the chips for
              you. Remember also, that the guide chip, has much smaller pixels than
              the main chip on your camera, so will show errors more.
              So, start by 'quantifying' the problem. Given a star in the centre of
              the main CCD field, focus as well as possible. What is the FWHM?.
              Correct it to measure in arc seconds, rather than pixels. Do this for
              all the measurements. Now, what is the FWHM, of a star, right in the
              corner of the main CCD?. Now get the FWHM of a star close to the bottom
              of the guide CCD. I'd expect to see perhaps a 10 to 20% 'growth', from
              centre to edge, and perhaps another 5% to the tracking CCD, so figures
              like (depending on seeing), perhaps 2.5arc/sec 'centre', 3arc/sec
              'corner', and 3.2arc/sec on the guider. With your relative CCD pixel
              sizes, when working in pixels, just perhaps 1.8 pixels fwhm on the main
              chip, but over 5 pixels on the guider!...
              If the step is much more than this, then look at one more thing. Focus a
              star at the top of the main chip, and compare it's fwhm, with one right
              at the bottom of the main chip. If there is any significant difference,
              then there is a tilt problem in your imaging train.
              I there is no tilt, and the jump is more than the sort of figure I'm
              suggesting, then you sould send the camer in for re-alignment.

              Best Wishes
            • Umair
              Thanks for your suggestions..i will see into it now. Umair ... here? ... main ... Focus a
              Message 6 of 6 , Jun 3, 2008
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                Thanks for your suggestions..i will see into it now.

                Umair

                --- In ccd-newastro@yahoogroups.com, Roger Hamlett <roger@...> wrote:
                >
                >
                ________________________________________________________________________
                > > Hello,
                > >
                > > When i try to autoguide from ST9XE..i could not get both imager
                > > and autoguider to the same focus..only one remains in good focus and
                > > other looses it all..is there a problem or am i missing something
                here?
                > >
                > > Regards,
                > > Umair
                > > C14 and ST9XE
                > > Losmandy Titan with Gemini
                > > 0.63 Reducer and CFW8 filter wheel
                > To some extent, it is normal, but the question is 'how much'....
                > Basically, unless the scope's image is _completely_ flat, there will
                > always be a slight focus difference on the guider image, from the
                'main'
                > image. This will also be present to a lesser extent across the main
                > image field, which is why it may well be worth electing to focus on a
                > star perhaps 1/3rd 'away' from the centre of the field to get a better
                > 'compromise' across the field. The guider will function well, with a
                > slightly defocussed image (in some cases, slightly _better_!). However
                > if the difference if major, then it implies the internal alignment of
                > the two CCD's has been compromised. SBIG will re-align the chips for
                > you. Remember also, that the guide chip, has much smaller pixels than
                > the main chip on your camera, so will show errors more.
                > So, start by 'quantifying' the problem. Given a star in the centre of
                > the main CCD field, focus as well as possible. What is the FWHM?.
                > Correct it to measure in arc seconds, rather than pixels. Do this for
                > all the measurements. Now, what is the FWHM, of a star, right in the
                > corner of the main CCD?. Now get the FWHM of a star close to the bottom
                > of the guide CCD. I'd expect to see perhaps a 10 to 20% 'growth', from
                > centre to edge, and perhaps another 5% to the tracking CCD, so figures
                > like (depending on seeing), perhaps 2.5arc/sec 'centre', 3arc/sec
                > 'corner', and 3.2arc/sec on the guider. With your relative CCD pixel
                > sizes, when working in pixels, just perhaps 1.8 pixels fwhm on the main
                > chip, but over 5 pixels on the guider!...
                > If the step is much more than this, then look at one more thing.
                Focus a
                > star at the top of the main chip, and compare it's fwhm, with one right
                > at the bottom of the main chip. If there is any significant difference,
                > then there is a tilt problem in your imaging train.
                > I there is no tilt, and the jump is more than the sort of figure I'm
                > suggesting, then you sould send the camer in for re-alignment.
                >
                > Best Wishes
                >
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