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Re: FWHM and seeing

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  • Dennis Persyk
    Hello Iver, I believe you are measuring relative, not absolute, seeing by that technique. Absolute seeing measurement requires a DIMM setup. You are measuring
    Message 1 of 14 , Apr 1, 2006
      Hello Iver,

      I believe you are measuring relative, not absolute, seeing by that
      technique. Absolute seeing measurement requires a DIMM setup.

      You are measuring the quadrature addition of seeing plus the PSF of
      your scope. If you had perfect seeing (you were out in space), you
      would still measure a finite FWHM. This is of little concern if your
      goal is to see how your local seeing varies from one night to the
      next. But if you are considering gauging your seeing by what others
      report for FWHM, then I think you will run into considerable
      confusion.

      Clear skies,

      Dennis Persyk
      Igloo Observatory Home Page http://dpersyk.home.att.net
      Hampshire, IL

      New Images http://home.att.net/~dpersyk/new.htm

      --- In ccd-newastro@yahoogroups.com, "Iver" <iriise@...> wrote:
      >
      > I have been using FWHM for focus for a while but just as a focus
      aid. I keep reading about it used as a measurement of seeing. I
      assume you just multiply your FWHM value by your image scale? So if
      you have 3 arc sec FWHM is that equal to 3 arc sec seeing?
      > Just looking for more info. on this.
      > Iver
      >
    • Randy Nulman
      Dennis, I agree with your comments below. Trying to compare relative seeing from one location to the next is difficult without the right standards. However,
      Message 2 of 14 , Apr 2, 2006
        Dennis,
        I agree with your comments below. Trying to compare relative seeing
        from one location to the next is difficult without the right
        standards. However, as I pointed out in my first reply, one can
        come close to DIMM measurements when looking at an average of "short
        exposure" focus images (1 second or less) if they are averaged out.

        Also, Ron Wodaski found that, when using the AO-L unit at 10hz or
        better, his FWHM came very, very close to the seeing measurements as
        reported by NM Skies when he took his hi-res Cone image.

        Bottom line, is that "seeing" as measured by FWHM on very short
        exposures does, indeed, come reasonably close and ultimately becomes
        a good standard of comparison for one night vs the other for any
        particular location. Perhaps, extremely "quick" exposures where the
        FWHM (if proper sampled and done with a larger instrument) can very
        closely match even the DIMM measurements one may get from a
        different site?

        Ron has worked hard on seeing issues and I would love to have his
        input, as well as yours, regarding the above assumptions.

        Regards,
        Randy


        --- In ccd-newastro@yahoogroups.com, "Dennis Persyk" <dpersyk@...>
        wrote:
        >
        > Hello Iver,
        >
        > I believe you are measuring relative, not absolute, seeing by that
        > technique. Absolute seeing measurement requires a DIMM setup.
        >
        > You are measuring the quadrature addition of seeing plus the PSF
        of
        > your scope. If you had perfect seeing (you were out in space), you
        > would still measure a finite FWHM. This is of little concern if
        your
        > goal is to see how your local seeing varies from one night to the
        > next. But if you are considering gauging your seeing by what
        others
        > report for FWHM, then I think you will run into considerable
        > confusion.
        >
        > Clear skies,
        >
        > Dennis Persyk
        > Igloo Observatory Home Page http://dpersyk.home.att.net
        > Hampshire, IL
        >
        > New Images http://home.att.net/~dpersyk/new.htm
        >
        > --- In ccd-newastro@yahoogroups.com, "Iver" <iriise@> wrote:
        > >
        > > I have been using FWHM for focus for a while but just as a focus
        > aid. I keep reading about it used as a measurement of seeing. I
        > assume you just multiply your FWHM value by your image scale? So
        if
        > you have 3 arc sec FWHM is that equal to 3 arc sec seeing?
        > > Just looking for more info. on this.
        > > Iver
        > >
        >
      • Iver
        Dennis, you sometimes refer to a Pickering scale that you use to asses your seeing, I assume that is poorly a visual method? Can that scale be quantified in
        Message 3 of 14 , Apr 2, 2006
          Dennis, you sometimes refer to a Pickering scale that you use to asses
          your seeing, I assume that is poorly a visual method? Can that scale be
          quantified in arc sec.? If so have you ever compared it with your FWHM
          measurements?
          Iver
        • Wodaski - Yahoo
          The most trustworthy way to measure seeing at two spots is to use the same equipment to do it. Ron Wodaski http://www.newastro.com
          Message 4 of 14 , Apr 2, 2006
            The most trustworthy way to measure seeing at two spots is to use the same
            equipment to do it.


            Ron Wodaski
            http://www.newastro.com <http://www.newastro.com/>





            _____

            From: ccd-newastro@yahoogroups.com [mailto:ccd-newastro@yahoogroups.com] On
            Behalf Of Randy Nulman
            Sent: Sunday, April 02, 2006 11:10 AM
            To: ccd-newastro@yahoogroups.com
            Subject: [ccd-newastro] Re: FWHM and seeing


            Dennis,
            I agree with your comments below. Trying to compare relative seeing
            from one location to the next is difficult without the right
            standards. However, as I pointed out in my first reply, one can
            come close to DIMM measurements when looking at an average of "short
            exposure" focus images (1 second or less) if they are averaged out.

            Also, Ron Wodaski found that, when using the AO-L unit at 10hz or
            better, his FWHM came very, very close to the seeing measurements as
            reported by NM Skies when he took his hi-res Cone image.

            Bottom line, is that "seeing" as measured by FWHM on very short
            exposures does, indeed, come reasonably close and ultimately becomes
            a good standard of comparison for one night vs the other for any
            particular location. Perhaps, extremely "quick" exposures where the
            FWHM (if proper sampled and done with a larger instrument) can very
            closely match even the DIMM measurements one may get from a
            different site?

            Ron has worked hard on seeing issues and I would love to have his
            input, as well as yours, regarding the above assumptions.

            Regards,
            Randy


            --- In ccd-newastro@yahoogroups.com, "Dennis Persyk" <dpersyk@...>
            wrote:
            >
            > Hello Iver,
            >
            > I believe you are measuring relative, not absolute, seeing by that
            > technique. Absolute seeing measurement requires a DIMM setup.
            >
            > You are measuring the quadrature addition of seeing plus the PSF
            of
            > your scope. If you had perfect seeing (you were out in space), you
            > would still measure a finite FWHM. This is of little concern if
            your
            > goal is to see how your local seeing varies from one night to the
            > next. But if you are considering gauging your seeing by what
            others
            > report for FWHM, then I think you will run into considerable
            > confusion.
            >
            > Clear skies,
            >
            > Dennis Persyk
            > Igloo Observatory Home Page http://dpersyk.home.att.net
            > Hampshire, IL
            >
            > New Images http://home.att.net/~dpersyk/new.htm
            >
            > --- In ccd-newastro@yahoogroups.com, "Iver" <iriise@> wrote:
            > >
            > > I have been using FWHM for focus for a while but just as a focus
            > aid. I keep reading about it used as a measurement of seeing. I
            > assume you just multiply your FWHM value by your image scale? So
            if
            > you have 3 arc sec FWHM is that equal to 3 arc sec seeing?
            > > Just looking for more info. on this.
            > > Iver
            > >
            >











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          • Dennis Persyk
            Hi Iver, I never make FWHM measurements. I am a retired physicist and have long since stopped doing science with my scope. Now I just take pretty pictures. My
            Message 5 of 14 , Apr 2, 2006
              Hi Iver,

              I never make FWHM measurements. I am a retired physicist and have
              long since stopped doing science with my scope. Now I just take
              pretty pictures.

              My mentor taught me Pickering seeing quantification. He had a
              dedicated Pickering scope
              http://schmidling.netfirms.com/seeing.htm

              I quantify seeing by noting the standard deviation in successive
              readings in the half-flux diameter parameter in Maxim DL's focus
              routine. I find this to be a good measure of relative seeing at my
              Midwest USA location that has notoriously poor seeing.

              Clear skies,

              Dennis
              Igloo Observatory Home Page http://dpersyk.home.att.net
              Hampshire, IL

              New Images http://home.att.net/~dpersyk/new.htm

              --- In ccd-newastro@yahoogroups.com, "Iver" <iriise@...> wrote:
              >
              >
              > Dennis, you sometimes refer to a Pickering scale that you use to
              asses your seeing, I assume that is poorly a visual method? Can that
              scale be quantified in arc sec.? If so have you ever compared it
              with your FWHM measurements?
              > Iver
              >
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