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Issues with flats

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  • Mark Striebeck
    When I take images with my reducer, I get some pretty strong vignetting in the corners, like here:
    Message 1 of 7 , Feb 2, 2015
      When I take images with my reducer, I get some pretty strong vignetting in the corners, like here:


      But when I calibrate the images, the corners often don't corrected well. Either too much or too little:


      (see lower left corner).

      I tried to take different flats (more subs, sky flats, LED panel flats) but always with the same results.

      Any idea what this could be? I wonder if my image train is sagging and the pattern is slightly different when I take the flats (where the scope is pointed straight upwards).

      Thanks
            MarkS
    • Ron Wodaski
      It’s impossible to say much about it from JPG images - one really needs to look at the brightness levels carefully to try to understand what’s going on.
      Message 2 of 7 , Feb 2, 2015
        It’s impossible to say much about it from JPG images - one really needs to look at the brightness levels carefully to try to understand what’s going on.

        That is pretty severe vignetting; it would be interesting to know how many stops of vignetting you have. Anything over half a stop is getting much harder to cure, and even if it is cured, you will have significantly poorer signal to noise ratio in the corrected areas, which can affect color balance especially (there will be more color noise). 

        So basically you have a hard problem, and, yes, it’s quite possible that for some reason you are not getting even illumination of the flats matching your nighttime exposures. I’m not sure flex is a likely candidate; off-axis illumination seems a more likely candidate. Rotating the camera might provide some insights.

        Ron Wodaski



        On Feb 2, 2015, at 3:47 AM, Mark Striebeck mark.striebeck@... [ccd-newastro] <ccd-newastro@yahoogroups.com> wrote:


        When I take images with my reducer, I get some pretty strong vignetting in the corners, like here:


        But when I calibrate the images, the corners often don't corrected well. Either too much or too little:


        (see lower left corner).

        I tried to take different flats (more subs, sky flats, LED panel flats) but always with the same results.

        Any idea what this could be? I wonder if my image train is sagging and the pattern is slightly different when I take the flats (where the scope is pointed straight upwards).

        Thanks
              MarkS


      • stan_ccd
        It would seem that you are using excessive reduction. But regardless of that issue, there are at least 4 reasons that flat-fielding can fail to correct
        Message 3 of 7 , Feb 2, 2015

          It would seem that you are using excessive reduction. But regardless of that issue, there are at least 4 reasons that flat-fielding can fail to correct illumination issues:


          1) Improper calibration.  Both flat and the raw image must be properly dark subtracted for the flattening math to work correctly.  Additionally, master flats (and darks) must be properly constructed; in particular the construction of a master flat can be defective due to incorrect normalization.  Also, overly right flats can exceed camera linearity, resulting in defective flattening.


          2) Light leaks and scatter within the scape and/or camera block behave differently under different ambient conditions, Thus flats made via flat-box or sky flat can easily fail to replicate the focal plane dynamics of the raw image.


          3) Real sky gradients cannot be corrected via flat fielding. This has nothing to do with the problem discussed (i.e. does not affect vignette of dust spots) but is worth mentioning.


          4) A non-linear (defective) camera will fail to flatten correctly. This is unusual but does occasionally occur and can be fixed via the manufacturer.


          Stan

        • stan_ccd
          sorry for the typos (too early in the morning!) Obviously the corrections are: overly right flats can exceed camera linearity should be overly bright ...
          Message 4 of 7 , Feb 2, 2015

            sorry for the typos (too early in the morning!)

            Obviously the corrections are:


            "overly right flats can exceed camera linearity" should be "overly bright ..."

            "scatter within the scape and/or camera" should be "scatter within the scope"

          • stan_ccd
            P.S. It is most likely caused by light scatter due to over-reduction (#2 above). Significant FL reduction changes light path dynamics and violates baffling
            Message 5 of 7 , Feb 2, 2015
              P.S. It is most likely caused by light scatter due to over-reduction (#2 above).  Significant FL reduction changes light path dynamics and violates baffling design.  Additionally, many reducers have significant back-side reflection that is a nightmare for flattening.  This is prevalent in strong reducers (< 0.5x), e.g. the 0.33x SCT reducer is simply awful (in so many ways).
            • Mark Striebeck
              Thanks guys, What sounds like the most possible issue is off-axis illumination or light leaks. I am imaging from our backyard and there is plenty of light
              Message 6 of 7 , Feb 2, 2015
                Thanks guys,

                What sounds like the most possible issue is off-axis illumination or light leaks. I am imaging from our backyard and there is plenty of light coming from various directions. Let me look into that.

                With regards to the vignetting itself: the reducer should be able to create an image of the size of my chip (it's a Takahashi Super Reducer for my TOA-130). But I don't see this strong vignetting with the flattener.

                     MarkS

                On Mon, Feb 2, 2015 at 8:06 AM, stan_ccd@... [ccd-newastro] <ccd-newastro@yahoogroups.com> wrote:
                 

                P.S. It is most likely caused by light scatter due to over-reduction (#2 above).  Significant FL reduction changes light path dynamics and violates baffling design.  Additionally, many reducers have significant back-side reflection that is a nightmare for flattening.  This is prevalent in strong reducers (< 0.5x), e.g. the 0.33x SCT reducer is simply awful (in so many ways).


              • Ron Wodaski
                The super reducer should be able to provide a 65mm diameter flat field (although the info I found said nothing about illumination, but generally reducers are
                Message 7 of 7 , Feb 2, 2015
                  The super reducer should be able to provide a 65mm diameter flat field (although the info I found said nothing about illumination, but generally reducers are able to provide no more than one stop different center to edge of the flat field, if they are good quality reducers). That is said to equate to a flat field image circle of 4.78º.

                  Your image solves as being 2.7 x 1.79 degrees and is 8”/pixel in image scale. You should be nowhere near the limits of the field.

                  Some things to consider:

                  * Is there a baffle that is showing up inside the FOV with the reducer (e.g., a dew shield)?

                  * Are the elements in the reducer correctly oriented (front to back) and in the correct order? You would likely need to contact your dealer to get this kind of information. I know of at least two Tak reducers that were not put together right.

                  * Is the reducer mounted in the exact correct position recommended by Takahashi? Are the correct spacers in place? 

                  * Is the reducer itself oriented correctly, front to back?

                  In other words, consider every possible way in which the reducer might not be optically perfect.

                  Ron Wodaski



                  On Feb 2, 2015, at 8:53 AM, Mark Striebeck mark.striebeck@... [ccd-newastro] <ccd-newastro@yahoogroups.com> wrote:


                  Thanks guys,

                  What sounds like the most possible issue is off-axis illumination or light leaks. I am imaging from our backyard and there is plenty of light coming from various directions. Let me look into that.

                  With regards to the vignetting itself: the reducer should be able to create an image of the size of my chip (it's a Takahashi Super Reducer for my TOA-130). But I don't see this strong vignetting with the flattener.

                       MarkS

                  On Mon, Feb 2, 2015 at 8:06 AM, stan_ccd@... [ccd-newastro] <ccd-newastro@yahoogroups.com> wrote:
                   

                  P.S. It is most likely caused by light scatter due to over-reduction (#2 above).  Significant FL reduction changes light path dynamics and violates baffling design.  Additionally, many reducers have significant back-side reflection that is a nightmare for flattening.  This is prevalent in strong reducers (< 0.5x), e.g. the 0.33x SCT reducer is simply awful (in so many ways).




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