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focus problems

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  • rudieallison
    I am a newbie to imaging and am having a problem I have not seen addressed in the forum. I am using an ST8E camera with a SBIG 5 filter wheel. I have to
    Message 1 of 4 , Aug 3, 2011
      I am a newbie to imaging and am having a problem I have not seen addressed in the forum. I am using an ST8E camera with a SBIG 5 filter wheel. I have to refocus every time I change filters. Since I have not found this addressed in Ron's book nor in the CCDSoft owners manual I have to assume this is not normally necessary. I took the filter wheel apart to make certain the filters were all screwed down all the way and they are. The greatest difference is between the Schuller(?) luminence filter and the RGB filters (from SBIG) but there is even a little difference between each color. This tweeking certainly adds a great deal of time to the whole process. Any ideas??
    • Mike Dodd
      ... Yes, that is fairly common. Even with top-of-the-line parfocal filters, refocusing is recommended for each filter. Some automation software, like ACP, will
      Message 2 of 4 , Aug 3, 2011
        rudieallison wrote:
        > I am a newbie to imaging and am having a problem I have not seen
        > addressed in the forum. I am using an ST8E camera with a SBIG 5
        > filter wheel. I have to refocus every time I change filters.

        Yes, that is fairly common. Even with top-of-the-line parfocal filters,
        refocusing is recommended for each filter.

        Some automation software, like ACP, will build and use an "offset table"
        that contains the number of steps +/- from the luminance filter to move
        your motorized focuser. This allows automated refocusing (if the offset
        table is accurate).

        If you have a motorized focuser, you could implement the same thing
        manually. Just record the number of steps from luminance for each color
        and, when you change filters during an imaging session,
        click-click-click your focuser the needed steps to reach that filter's
        offset.
        --
        Mike

        Mike Dodd
        http://astronomy.mdodd.com
        Louisa County, Virginia USA N37.58.23 W77.56.24
      • Thouse
        Adding to what Mike said, normally you would take an average of your filters. Focus each one 5 ~ 8 times and average the offset from the Lum / clear. Tom ...
        Message 3 of 4 , Aug 3, 2011
          Adding to what Mike said, normally you would take an average of your
          filters.
          Focus each one 5 ~ 8 times and average the offset from the Lum / clear.

          Tom

          On 8/3/2011 5:00 PM, Mike Dodd wrote:
          >
          > rudieallison wrote:
          > > I am a newbie to imaging and am having a problem I have not seen
          > > addressed in the forum. I am using an ST8E camera with a SBIG 5
          > > filter wheel. I have to refocus every time I change filters.
          >
          > Yes, that is fairly common. Even with top-of-the-line parfocal filters,
          > refocusing is recommended for each filter.
          >
          > Some automation software, like ACP, will build and use an "offset table"
          > that contains the number of steps +/- from the luminance filter to move
          > your motorized focuser. This allows automated refocusing (if the offset
          > table is accurate).
          >
          > If you have a motorized focuser, you could implement the same thing
          > manually. Just record the number of steps from luminance for each color
          > and, when you change filters during an imaging session,
          > click-click-click your focuser the needed steps to reach that filter's
          > offset.
          > --
          > Mike
          >
          > Mike Dodd
          > http://astronomy.mdodd.com
          > Louisa County, Virginia USA N37.58.23 W77.56.24
          >
          >


          [Non-text portions of this message have been removed]
        • Ron Wodaski
          That s normal. You will indeed find small differences between the LRGB filter, especially if you have a fast focal ratio. And the focus difference between the
          Message 4 of 4 , Aug 3, 2011
            That's normal. You will indeed find small differences between the LRGB filter, especially if you have a fast focal ratio.

            And the focus difference between the Schuller and the LRGB is also expected; they use completely different prescriptions, and the thickness (and probably refractive index) of the various filters will alter the focus.

            Ron Wodaski



            On Aug 3, 2011, at 5:42 PM, rudieallison wrote:

            > I am a newbie to imaging and am having a problem I have not seen addressed in the forum. I am using an ST8E camera with a SBIG 5 filter wheel. I have to refocus every time I change filters. Since I have not found this addressed in Ron's book nor in the CCDSoft owners manual I have to assume this is not normally necessary. I took the filter wheel apart to make certain the filters were all screwed down all the way and they are. The greatest difference is between the Schuller(?) luminence filter and the RGB filters (from SBIG) but there is even a little difference between each color. This tweeking certainly adds a great deal of time to the whole process. Any ideas??
            >
            >
            >
            > ------------------------------------
            >
            >
            >
            >
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