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Re: gradients & normalization

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  • jfeldhou
    this is good to know. more questions... do I need to switch to 8 bit to do the combine in photoshop? I was unable to get layers, nor Apply image to work in
    Message 1 of 7 , Feb 1, 2002
      this is good to know.
      more questions...

      do I need to switch to 8 bit to do the combine in photoshop?
      I was unable to get layers, nor "Apply image" to work in
      15/16 bit mode? is switching to 8 bits before doing advanced
      processing (ddp,unsharp,L-R) a bad idea?
      is switching to 8 bits a bad idea in general?

      thanks, jim


      --- In ccd-newastro@y..., "Ron Wodaski" <ronw@n...> wrote:
      > You are dealing with two very different reference systems here. I've
      never
      > had success this way, at least not without considerable mental
      gymnastics to
      > try to get the two files to have similar levels.
      >
      > The simplest solution is to apply the gradient removal to the image
      in
      > Photoshop. Photoshop just doesn't seem to understand the 16-bit
      format when
      > it comes to saving. I've heard that it actually only saves 15 bits,
      not 16,
      > and if this is true it would be just about impossible to use
      Photoshop to
      > create a gradient removal image in the way you describe.
      >
      > Since I had so much trouble with the approach, I only documented the
      > all-Photoshop methods in the book. I just can't recommend this
      approach
      > because Photoshop seems intent on making it not work.
      >
      > Ron Wodaski
      > The New CCD Astronomy
      > http://www.newastro.com
      >
      > -----Original Message-----
      > From: jfeldhou [mailto:jfeldhou@y...]
      > Sent: Thursday, January 31, 2002 5:05 PM
      > To: ccd-newastro@y...
      > Subject: [ccd-newastro] gradients & normalization
      >
      >
      > is it a bad idea to attempt to produce a gradient
      > from an image that has been normalized during stacking?
      >
      > I attempted to do the following procedure, with very
      > poor results. I had 11x600s luminence images,
      > preprocessed, stacked(sigmamean). M51 occupied most
      > of the corner quadrant, the rest stars of varying bloat.
      >
      > I saved the stacked fits image as 16 bit tiff, then
      > opened it in photoshop. I used the dust&scratches
      > tool at (12,16) cause some stars were big. Then I clone
      > tooled out any remaining stars, and the M51 portion.
      > after a gausian blur 19, I copied the 16 bit tif back
      > to my PC, changed it back to fits, and attempt to
      > subtract the blur from my original M51 stack.
      > I attempted to subtract the blur, after multiplying
      > the blurred pixels by .8 then .7 then .5 .4
      >
      > no matter my choice, the target image was erradicated.
      > my blur wasn't so good in the area of m51, but I had
      > hoped I'd see some worthwhile results on the rest of
      > the target frame, and then I could make a better blur.
      >
      > is there an obvious place where I went wrong?
      > thanks, jim
      >
      >
      >
      > To unsubscribe from this group, send an email to:
      > ccd-newastro-unsubscribe@e...
      >
      >
      >
      > Your use of Yahoo! Groups is subject to
      http://docs.yahoo.com/info/terms/
    • Mark Jenkins
      Hi Jim, The short answer is yes. You cannot perform many operations in PS without changing to 8 bit mode. In PS 6 certain filters are available in 16bit like
      Message 2 of 7 , Feb 1, 2002
        Hi Jim,

        The short answer is yes. You cannot perform many operations in PS
        without changing to 8 bit mode.

        In PS 6 certain filters are available in 16bit like Unsharp Mask, and
        Gaussian Blur and a few others.

        As many histogram adjustments as possible should be done in 16 bit but
        some tweaking of the histogram can be done in 8 bit mode if needed.

        Switching to 8 bit is not necessarily a "bad idea" simply because it is
        necessary.

        HTH

        Mark Jenkins
        markj@...

        On Friday, February 1, 2002, at 02:39 AM, jfeldhou wrote:

        > this is good to know.
        > more questions...
        >
        > do I need to switch to 8 bit to do the combine in photoshop?
        > I was unable to get layers, nor "Apply image" to work in
        > 15/16 bit mode? is switching to 8 bits before doing advanced
        > processing (ddp,unsharp,L-R) a bad idea?
        > is switching to 8 bits a bad idea in general?
      • Matt Russell
        Mark, I thought when you trimmed back to 8 bits in Photoshop, you actually lose a lot of data? Can someone clairify this for me? Thanks, Matt
        Message 3 of 7 , Feb 1, 2002
          Mark,

          I thought when you trimmed back to 8 bits in Photoshop, you actually lose a
          lot of data? Can someone clairify this for me?

          Thanks,

          Matt
        • Ron Wodaski
          When you convert to 8 bits, you are rescaling the data to 256 levels instead of 65,000. So there is a big loss in precision. HOWEVER: for most images, the vast
          Message 4 of 7 , Feb 1, 2002
            When you convert to 8 bits, you are rescaling the data to 256 levels instead
            of 65,000. So there is a big loss in precision. HOWEVER: for most images,
            the vast bulk of the useful data is actually packed into a very, very small
            range of values (some small percentage of those 65,000 values, as evidenced
            by the thin clump of data at the left end of the raw histogram). Histogram
            adjustments, linear and non-linear, have the effect of discarding specially
            the portions of the data that are less interesting. After you complete this
            process, you have carefully decided yourself what to discard and what to
            keep. Converting to 8 bits after making these decisions means that you lose
            very little meaningful data (if any) when you convert to 8 bits.

            So they key is converting to 8 bits of data at the right moment. See another
            message I am posting about the right order for making changes and converting
            to 8 bits.

            Ron Wodaski
            The New CCD Astronomy
            http://www.newastro.com

            -----Original Message-----
            From: Matt Russell [mailto:matthewr@...]
            Sent: Friday, February 01, 2002 9:11 AM
            To: 'ccd-newastro@yahoogroups.com'
            Subject: [ccd-newastro] Re: gradients & normalization


            Mark,

            I thought when you trimmed back to 8 bits in Photoshop, you actually lose a
            lot of data? Can someone clairify this for me?

            Thanks,

            Matt


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            ccd-newastro-unsubscribe@egroups.com



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          • Ron Wodaski
            You want to postpone the change to 8 bits to as late in the process as possible, for the reasons outlined in another reply I posted today. With respect to the
            Message 5 of 7 , Feb 1, 2002
              You want to postpone the change to 8 bits to as late in the process as
              possible, for the reasons outlined in another reply I posted today. With
              respect to the questions:

              * Yes, you need to eventually switch to 8 bits. Some Photoshop tools only
              work in 8-bit mode, as you've discovered. And you must convert to 8 bits to
              save as JPG anyway.

              * The order in which you do certain processing steps is critical. I rarely,
              if ever, use DDP if I am going to use Photoshop. The histogram tools in
              Photoshop allow you to do a much more precise manual form of DDP. And you
              can use Photoshop's unsharp mask tool in conjunction with a selection to
              sharpen only the portions of the image with good enough S/N for sharpening
              to be effective. Specialized operations like deconvolution should be done on
              the 16-bit file BEFORE you ever bring it into Photoshop. Once you have the
              image in Photoshop, you should finish with it there. Go as far as you can
              and convert to 8-bit mode at the last possible moment, when you have no
              further use for the 16-bit tools. If you do this, the data left when do the
              conversion will suffer minimal loss during the conversion to 8 bits.

              So:

              * Perform specialized processing (deconvolution and other astro-only types
              of tools) to the 16-bit file BEFORE you ever open it in Photoshop. Always
              retain your raw data in case you need to start over!!!

              * Open the image in Photoshop and do linear and non-linear histogram
              changes. This discards less interesting portions of the data, and retains
              the most interesting portions.

              * Convert to 8-bit mode, and now you can use of the tools that were
              unavailable to you while still in 16-bit mode. This includes things like
              gradient removal and combining L and RGB images to create LRGB images. See
              the summary tutorial for creating LRGB images on the book web site's
              tutorial page.

              Ron Wodaski
              The New CCD Astronomy
              http://www.newastro.com

              -----Original Message-----
              From: jfeldhou [mailto:jfeldhou@...]
              Sent: Friday, February 01, 2002 12:40 AM
              To: ccd-newastro@yahoogroups.com
              Subject: [ccd-newastro] Re: gradients & normalization


              this is good to know.
              more questions...

              do I need to switch to 8 bit to do the combine in photoshop?
              I was unable to get layers, nor "Apply image" to work in
              15/16 bit mode? is switching to 8 bits before doing advanced
              processing (ddp,unsharp,L-R) a bad idea?
              is switching to 8 bits a bad idea in general?

              thanks, jim


              --- In ccd-newastro@y..., "Ron Wodaski" <ronw@n...> wrote:
              > You are dealing with two very different reference systems here. I've
              never
              > had success this way, at least not without considerable mental
              gymnastics to
              > try to get the two files to have similar levels.
              >
              > The simplest solution is to apply the gradient removal to the image
              in
              > Photoshop. Photoshop just doesn't seem to understand the 16-bit
              format when
              > it comes to saving. I've heard that it actually only saves 15 bits,
              not 16,
              > and if this is true it would be just about impossible to use
              Photoshop to
              > create a gradient removal image in the way you describe.
              >
              > Since I had so much trouble with the approach, I only documented the
              > all-Photoshop methods in the book. I just can't recommend this
              approach
              > because Photoshop seems intent on making it not work.
              >
              > Ron Wodaski
              > The New CCD Astronomy
              > http://www.newastro.com
              >
              > -----Original Message-----
              > From: jfeldhou [mailto:jfeldhou@y...]
              > Sent: Thursday, January 31, 2002 5:05 PM
              > To: ccd-newastro@y...
              > Subject: [ccd-newastro] gradients & normalization
              >
              >
              > is it a bad idea to attempt to produce a gradient
              > from an image that has been normalized during stacking?
              >
              > I attempted to do the following procedure, with very
              > poor results. I had 11x600s luminence images,
              > preprocessed, stacked(sigmamean). M51 occupied most
              > of the corner quadrant, the rest stars of varying bloat.
              >
              > I saved the stacked fits image as 16 bit tiff, then
              > opened it in photoshop. I used the dust&scratches
              > tool at (12,16) cause some stars were big. Then I clone
              > tooled out any remaining stars, and the M51 portion.
              > after a gausian blur 19, I copied the 16 bit tif back
              > to my PC, changed it back to fits, and attempt to
              > subtract the blur from my original M51 stack.
              > I attempted to subtract the blur, after multiplying
              > the blurred pixels by .8 then .7 then .5 .4
              >
              > no matter my choice, the target image was erradicated.
              > my blur wasn't so good in the area of m51, but I had
              > hoped I'd see some worthwhile results on the rest of
              > the target frame, and then I could make a better blur.
              >
              > is there an obvious place where I went wrong?
              > thanks, jim
              >
              >
              >
              > To unsubscribe from this group, send an email to:
              > ccd-newastro-unsubscribe@e...
              >
              >
              >
              > Your use of Yahoo! Groups is subject to
              http://docs.yahoo.com/info/terms/



              To unsubscribe from this group, send an email to:
              ccd-newastro-unsubscribe@egroups.com



              Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
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