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Re: [William Optics] WO FF test with Megrez 90mm APO

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  • rfrench_spice
    In the corner, the center, or both? Is it just the coma you re seeing? Rob
    Message 1 of 13 , Jul 31, 2007
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      In the corner, the center, or both?

      Is it just the coma you're seeing?

      Rob

      --- In William-Optics@yahoogroups.com, msxvlk <no_reply@...> wrote:
      >
      > The stars with the FF look pretty strange, they seem to be truncated.
      >
    • msxvlk
      The look strange in both center and corner, they don t suffer of field curvature or coma, they look like a part of them is missing. Silly question : did you
      Message 2 of 13 , Aug 3, 2007
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        The look strange in both center and corner, they don't suffer of field
        curvature or coma, they look like a part of them is missing. Silly
        question : did you used an extra dew shield when taking the image

        --- In William-Optics@yahoogroups.com, "rfrench_spice" <co-yahoo@...>
        wrote:
        >
        > In the corner, the center, or both?
        >
        > Is it just the coma you're seeing?
        >
        > Rob
        >
      • rfrench_spice
        Perhaps it was just a little out of focus. I have an image I took last night with very carefully focused stars. When I finish processing it I ll post it and
        Message 3 of 13 , Aug 4, 2007
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          Perhaps it was just a little out of focus. I have an image I took last
          night with very carefully focused stars. When I finish processing it
          I'll post it and tell me what you think.

          Last night I took flat field images of a twilight sky with and without
          the FF II with my Canon Rebel XTi. Without the FF II I get a
          vignetting of 1.0% between highest and lowest pixel level. With the FF
          I get 3.3%. The variation without the FF doesn't even really look like
          vignetting - it's not an obvious falloff towards the edge. But with
          the FF it's obvious vignetting.

          This difference magnifies greatly, of course, when you stretch the
          picture.

          Here is the fully stretched flat field with the FF II:

          <http://www.rfrench.org/lj/WOFF2-FF-flat-125-100.jpg>

          and here is the fully stretched flat field without the FF II:

          <http://www.rfrench.org/lj/WOFF2-noFF-flat-125-100.jpg>

          --- In William-Optics@yahoogroups.com, msxvlk <no_reply@...> wrote:
          >
          > The look strange in both center and corner, they don't suffer of
          field
          > curvature or coma, they look like a part of them is missing. Silly
          > question : did you used an extra dew shield when taking the image
          >
          > --- In William-Optics@yahoogroups.com, "rfrench_spice" <co-yahoo@>
          > wrote:
          > >
          > > In the corner, the center, or both?
          > >
          > > Is it just the coma you're seeing?
          > >
          > > Rob
          > >
          >
        • i_ween.
          Hi all, Here is a shot of the Moon on July 27th, 2007. I m not sure I had perfect focus. Also, the Moon was low in the sky and orange in color, so seeing
          Message 4 of 13 , Aug 4, 2007
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            Hi all,

            Here is a shot of the Moon on July 27th, 2007.

            I'm not sure I had perfect focus. Also, the Moon was low in the sky and orange
            in color, so "seeing" was probably a bit of an issue.

            Prime focus with a Canon 350D, unmodified.
            Custom white balance. Used camera settings when transferred to PS.
            Bit of Smart Sharpening in PS.

            Click on "get original picture" under the shot for full resolution.

            Any comments are welcome!!

            http://tinyurl.com/yumvtl

            Peace,
            -Derek Java
            --------------------------------------
            43°17'N, 73°39'W
            But optics sharp it needs, I ween,
            To see what is not to be seen.
            -- John Trumbull (American, 1750-1831)
            --------------------------------------
          • Scott Walker
            I am a little confused by your post. I would expect the corners to be about 61% the brightness of the center when using the FFII. This is what I measured on my
            Message 5 of 13 , Aug 5, 2007
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              I am a little confused by your post. I would expect the corners to be about 61% the brightness of the center when using the FFII. This is what I measured on my Canon 350 and the WOZS80FD. This vignetting is due to the small clear aperture of the FFII. I would expect the results with your setup to be similar. Both camera have the same size chip. A 61% brightness means that the numbers are about 25% lower in the corners than the center. This assumes a standard image format that uses a gamma of around 2.2. Say if the blue level average 200 in the center it would be about 150 in the corner. This accounts for the gamma in the system. Could you post the unstretched flats?
               
              The new FFIII has a much larger lens so that the lens aperture does not cause a vignet. One may still see some vignetting due to the baffles used in their scope. In my ZS80FD very little vignetting is seen in the corners when using the Canon 350 without a FF. With a perfect X0.8 FF I calculated about a 20% drop-off in the corners due to the vignetting due to the baffle. Remember with the X0.8 reducer the same corner is now a 25% large viewing angle for the scope. The 20% is based from memory but believed to be about right. Depending on the baffle design of the 90 your #'s may vary.
               
              Scott Walker
              ----- Original Message -----
              Sent: Saturday, August 04, 2007 8:52 AM
              Subject: Re: [William Optics] WO FF test with Megrez 90mm APO

              Perhaps it was just a little out of focus. I have an image I took last
              night with very carefully focused stars. When I finish processing it
              I'll post it and tell me what you think.

              Last night I took flat field images of a twilight sky with and without
              the FF II with my Canon Rebel XTi. Without the FF II I get a
              vignetting of 1.0% between highest and lowest pixel level. With the FF
              I get 3.3%. The variation without the FF doesn't even really look like
              vignetting - it's not an obvious falloff towards the edge. But with
              the FF it's obvious vignetting.

              This difference magnifies greatly, of course, when you stretch the
              picture.

              Here is the fully stretched flat field with the FF II:

              <http://www.rfrench. org/lj/WOFF2- FF-flat-125- 100.jpg>

              and here is the fully stretched flat field without the FF II:

              <http://www.rfrench. org/lj/WOFF2- noFF-flat- 125-100.jpg>

              --- In William-Optics@ yahoogroups. com, msxvlk <no_reply@.. .> wrote:
              >
              > The look strange in both center and corner, they don't suffer of
              field
              > curvature or coma, they look like a part of them is missing. Silly
              > question : did you used an extra dew shield when taking the image
              >
              > --- In William-Optics@ yahoogroups. com, "rfrench_spice" <co-yahoo@>
              > wrote:
              > >
              > > In the corner, the center, or both?
              > >
              > > Is it just the coma you're seeing?
              > >
              > > Rob
              > >
              >

            • Scott Walker
              Hi Group I made a quick measurement of the vignetting with the new FFIII and the WOZS80FD with the baffle inplace. There is very little vignetting in the
              Message 6 of 13 , Aug 7, 2007
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                Hi Group
                 
                I made a quick measurement of the vignetting with the new FFIII and the WOZS80FD with the baffle inplace. There is very little vignetting in the corners based on the measures, only about an average of 11% drop off in brightness after gamma correction. I used a sun lit white wall as my source. The scope was focused at infinite and the scope was about 5 ft from the wall. I wanted to correct my memory of about 20% that was posted below. The camera was a Canon 350.
                 
                Sorry about the poor memory
                 
                Scott Walker 
                ----- Original Message -----
                Sent: Sunday, August 05, 2007 7:15 PM
                Subject: Re: [William Optics] WO FF test with Megrez 90mm APO

                I am a little confused by your post. I would expect the corners to be about 61% the brightness of the center when using the FFII. This is what I measured on my Canon 350 and the WOZS80FD. This vignetting is due to the small clear aperture of the FFII. I would expect the results with your setup to be similar. Both camera have the same size chip. A 61% brightness means that the numbers are about 25% lower in the corners than the center. This assumes a standard image format that uses a gamma of around 2.2. Say if the blue level average 200 in the center it would be about 150 in the corner. This accounts for the gamma in the system. Could you post the unstretched flats?
                 
                The new FFIII has a much larger lens so that the lens aperture does not cause a vignet. One may still see some vignetting due to the baffles used in their scope. In my ZS80FD very little vignetting is seen in the corners when using the Canon 350 without a FF. With a perfect X0.8 FF I calculated about a 20% drop-off in the corners due to the vignetting due to the baffle. Remember with the X0.8 reducer the same corner is now a 25% large viewing angle for the scope. The 20% is based from memory but believed to be about right. Depending on the baffle design of the 90 your #'s may vary.
                 
                Scott Walker
                ----- Original Message -----
                Sent: Saturday, August 04, 2007 8:52 AM
                Subject: Re: [William Optics] WO FF test with Megrez 90mm APO

                Perhaps it was just a little out of focus. I have an image I took last
                night with very carefully focused stars. When I finish processing it
                I'll post it and tell me what you think.

                Last night I took flat field images of a twilight sky with and without
                the FF II with my Canon Rebel XTi. Without the FF II I get a
                vignetting of 1.0% between highest and lowest pixel level. With the FF
                I get 3.3%. The variation without the FF doesn't even really look like
                vignetting - it's not an obvious falloff towards the edge. But with
                the FF it's obvious vignetting.

                This difference magnifies greatly, of course, when you stretch the
                picture.

                Here is the fully stretched flat field with the FF II:

                <http://www.rfrench. org/lj/WOFF2- FF-flat-125- 100.jpg>

                and here is the fully stretched flat field without the FF II:

                <http://www.rfrench. org/lj/WOFF2- noFF-flat- 125-100.jpg>

                --- In William-Optics@ yahoogroups. com, msxvlk <no_reply@.. .> wrote:
                >
                > The look strange in both center and corner, they don't suffer of
                field
                > curvature or coma, they look like a part of them is missing. Silly
                > question : did you used an extra dew shield when taking the image
                >
                > --- In William-Optics@ yahoogroups. com, "rfrench_spice" <co-yahoo@>
                > wrote:
                > >
                > > In the corner, the center, or both?
                > >
                > > Is it just the coma you're seeing?
                > >
                > > Rob
                > >
                >

              • rfrench_spice
                OK, I don t know what I did before, but looking at the unstretched files now I see exactly the behavior you re expecting. Max pixel is 7963, min pixel is
                Message 7 of 13 , Aug 7, 2007
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                  OK, I don't know what I did before, but looking at the unstretched
                  files now I see exactly the behavior you're expecting. Max pixel is
                  7963, min pixel is 5788, for a change of 27%.

                  Rob

                  > ----- Original Message -----
                  > From: Scott Walker
                  > To: William-Optics@yahoogroups.com
                  > Sent: Sunday, August 05, 2007 7:15 PM
                  > Subject: Re: [William Optics] WO FF test with Megrez 90mm APO
                  >
                  >
                  >
                  > I am a little confused by your post. I would expect the corners
                  to be about 61% the brightness of the center when using the FFII.
                  This is what I measured on my Canon 350 and the WOZS80FD. This
                  vignetting is due to the small clear aperture of the FFII. I would
                  expect the results with your setup to be similar. Both camera have
                  the same size chip. A 61% brightness means that the numbers are about
                  25% lower in the corners than the center. This assumes a standard
                  image format that uses a gamma of around 2.2. Say if the blue level
                  average 200 in the center it would be about 150 in the corner. This
                  accounts for the gamma in the system. Could you post the unstretched
                  flats?
                • Scott Walker
                  Rob Thanks for checking. I am assuming you are looking at the true raw data which is linear so I would expect about a 39% change. I reread my post and it
                  Message 8 of 13 , Aug 8, 2007
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                    Rob
                     
                    Thanks for checking. I am assuming you are looking at the true "raw" data which is linear so I would expect about a 39% change. I reread my post and it looks like I round the wrong way with my gamma correction. With a gamma of 2.2, I would expect the corners to be about 15 % lower. Anyway we are both seeing darker corners. I believe with "flat" correction neither a 27 or 39% roll-off in the extreme corners is a big deal. I did measure my ZS80FD with the new FFIII  today and got between 11 and 15 %(linear not with gamma) average  brightness roll-off in the corners. It is pretty subtle and gentle. I might use the FFIII for bird photos, so it will be nice not to have to correct for the dark corners, though it is easy in Photoshop. 
                     
                    Scott Walker
                    ----- Original Message -----
                    Sent: Tuesday, August 07, 2007 10:09 PM
                    Subject: Re: [William Optics] WO FF test with Megrez 90mm APO

                    OK, I don't know what I did before, but looking at the unstretched
                    files now I see exactly the behavior you're expecting. Max pixel is
                    7963, min pixel is 5788, for a change of 27%.

                    Rob

                    > ----- Original Message -----
                    > From: Scott Walker
                    > To: William-Optics@ yahoogroups. com
                    > Sent: Sunday, August 05, 2007 7:15 PM
                    > Subject: Re: [William Optics] WO FF test with Megrez 90mm APO
                    >
                    >
                    >
                    > I am a little confused by your post. I would expect the corners
                    to be about 61% the brightness of the center when using the FFII.
                    This is what I measured on my Canon 350 and the WOZS80FD. This
                    vignetting is due to the small clear aperture of the FFII. I would
                    expect the results with your setup to be similar. Both camera have
                    the same size chip. A 61% brightness means that the numbers are about
                    25% lower in the corners than the center. This assumes a standard
                    image format that uses a gamma of around 2.2. Say if the blue level
                    average 200 in the center it would be about 150 in the corner. This
                    accounts for the gamma in the system. Could you post the unstretched
                    flats?

                  • rfrench_spice
                    Scott - Could you educate me here? I think that the original RAW file is going to be linear. At what point is some sort of gamma correction applied? During
                    Message 9 of 13 , Aug 8, 2007
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                      Scott -

                      Could you educate me here? I think that the original RAW file is going
                      to be linear. At what point is some sort of gamma correction applied?
                      During the Bayer interpolation? Or some later stage that I can't think
                      of?

                      The numbers that I'm quoting are after a daylight white-balanced
                      conversion of RAW to FITS.

                      Rob
                    • Scott Walker
                      Rob I am not an expert here but I will let you know what I think I know. First, one has to be careful with the term RAW . I believe the true RAW files
                      Message 10 of 13 , Aug 8, 2007
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                        Rob
                         
                        I am not an expert here but I will let you know what I think I know. First, one has to be careful with the term "RAW". I believe the true "RAW" files stored on the memory card of DSLR are close to the linear 12-bit data from the cameras A/D. This file is likely not the true A/D outputs but probably has some minor calibration corrections, but is pretty close to the A/D outputs. When Photoshop user look at this file it is not seen in it true RAW form but rather the corrections for the Bayer mask has been made and a Gamma correction has been made. In astro photo programs the RAW form may be used for a lot of the calculations since linear data is much easier to work with. I do not know all the different formats used for astro photography. For most displays the data needs to be gamma corrected for it to look right. I too have been confused about what is real happening in some astro photo programs. To better understand your program take a daylight picture of some differnent colored paper. Have a black ,grey and white piece and some additional colors. I use a color chart but construction paper will work. Process this picture through your astro photo program. It will give you a much better idea of what the different steps of the program is doing. With astro photos we often do not know what the true color of what we are looking at or even what its shape is, so it is hard to tell what the program is doing. It is common to wildly change the contrast and offset in an astro image to bring out details which is all great. By working with an image that you know what its truely looks like, you will get a better idea of what you are really doing.
                         
                        I hope this helps
                         
                        Scott Walker
                        ----- Original Message -----
                        Sent: Wednesday, August 08, 2007 9:16 AM
                        Subject: Re: [William Optics] WO FF test with Megrez 90mm APO

                        Scott -

                        Could you educate me here? I think that the original RAW file is going
                        to be linear. At what point is some sort of gamma correction applied?
                        During the Bayer interpolation? Or some later stage that I can't think
                        of?

                        The numbers that I'm quoting are after a daylight white-balanced
                        conversion of RAW to FITS.

                        Rob

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