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RE: [ccd-newastro] Re: FWHM and seeing

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  • Wodaski - Yahoo
    In order to measure seeing directly, you need exposures of 10ms or shorter. You will find that, typically, the longer the exposure, the greater the FWHM. If a
    Message 1 of 14 , Apr 1, 2006
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      In order to measure seeing directly, you need exposures of 10ms or shorter.

      You will find that, typically, the longer the exposure, the greater the
      FWHM. If a 1-second exposure has a FWHM of 2.0", then a 1-minute exposure
      might have a FWHM of 2.4", and a 10-minute exposure might have a FWHM of
      2.6". These are just estimate from memory, but the trend is about like
      that.

      Of course you need to be sampling with enough pixels to see the difference.

      To actually measure seeing, you need extremely short exposures and you need
      to be looking at how the FWHM varies. (In practice, many observatories use
      DIMM (dual image motion monitors) and scintillation monitors.) Longer
      exposures average in varying seeing conditions, which smothers there moment
      by moment information.

      And let's not forget that your telescope, mount and other physical
      components add their own bits of noise to the FWHM. The telescope optics add
      some scatter and optical aberrations. The mount has mount jitter or even
      worse problems while tracking. Guiding contributes its own errors. All of
      those things add some amount to the final FWHM of a long (>10ms) exposure.


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





      _____

      From: ccd-newastro@yahoogroups.com [mailto:ccd-newastro@yahoogroups.com] On
      Behalf Of Iver
      Sent: Saturday, April 01, 2006 6:38 PM
      To: ccd-newastro@yahoogroups.com
      Subject: [ccd-newastro] Re: FWHM and seeing


      This is interesting, it seems counter intuitive to me. I would have
      guessed that measuring a longer exposure would be the way to measure
      seeing, ie. if the sky is steady the stars would be smaller and if the
      sky is turbulent the stars would be larger. So would there be anything
      you could learn by measuring the FWHM of your final stacked image?
      Iver















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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 2 of 14 , Apr 1, 2006
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        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 3 of 14 , Apr 2, 2006
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          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 4 of 14 , Apr 2, 2006
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            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 5 of 14 , Apr 2, 2006
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              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 6 of 14 , Apr 2, 2006
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                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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