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Re: ngc7331 7 hrs Lum

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  • Roger Hamlett
    ... Whoa.... The suggestion to sample at half the seeing , is a common _and wrong_ application of Nyquists theorem... Nyquist, shows that for sinusoidal
    Message 1 of 28 , Oct 1, 2004
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      > HI
      >
      > i sorry..i meant oversampled....
      >
      > with his location near washington dc this puts him near sea
      > level...probably the maximum seeing conditions there would be about
      > 4 or 5 arcseconds
      >
      >
      > according to apogee instruments.
      >
      > http://www.ccd.com/ccd113.html
      >
      > ..you should be imaging at 1/2 your seeing conditions...or 2 arc
      > seconds per pixel....or most likely 2.5 arc seconds per pixel...
      Whoa....
      The suggestion to sample at half the 'seeing', is a common _and wrong_
      application of Nyquists theorem...
      Nyquist, shows that for sinusoidal waveforms, the _minimum_ sampling
      'interval', is half the wavelength, to reproduce a particular frequency.
      When sampling with a camera, the pixel size is the sampling interval, and
      the seeing, may (possibly), be taken to represent the 'wavelength', so
      people then go on to say that you have to use 1/2 the seeing as the maximum
      sample size to not lose significant resolution.
      The first (biggest) problem, is that the image is a 2D structure, not a one
      dimensional structure, and the worst sampling, is diagonally across the
      pixel, not the width of the pixel. Hence ignoring anything else, the
      Nyquist criteria, actually requires you to sample (assuming square pixels),
      at 1/2.8th the seeing, if detail is not to be lost.
      There is though also a second problem. The light curve produced by a star,
      is not a nice sinusoid. The image of a star, will be close to the shape of
      the Airy disk, then 'spread' by a Gaussian noise function. If you do this,
      and then look at the measures of 'seeing', you will see that the edges on
      the star, have much sharper rise/falls than a sinusoid, and to reproduce
      these properly, requires a slightly higher sample rate. If you sit down,
      and calculate the effects, you find that you have to sample at just over
      1/3rd the 'seeing', to get all the available detail without oversampling.

      > at .57 seconds he is way oversampled...and his stars show this...
      No. 0.57arc seconds/pixel, is close to perfect sampling, for 2 arc second
      seeing.

      > they have a noticeable star streak at least one pixel. in them due
      > to periodic error (which is about 1 arcsecond in the
      > picture...actually quite excellent pec)....and they are overly large
      > because of being oversampled....also this lends itself to loss of
      > sensitivity..which is why i think the core is burned out...
      The 'burnout' is down to exposure time, and processing. Most people wanting
      to get detail in both the core and arms, will end up using two seperate
      exposures, and combining these, since the dynamic range, exceeds that
      genuinely available from the CCD.

      > if he were imaging at 2 1/2 arcseconds per pixel..then the stars
      > would have been round...comprising fewer pixels....and produce a
      > more pleasing photo.
      >
      > jamie
      If he was sampling at 2.5arc seconds/pixel, the stars would no longer show
      the tracking errors, but would be noticeably undersampled, with the stars
      only being 'round', if he artificially processed them with a blur to do
      so...

      > --- In ccd-newastro@yahoogroups.com, "Randy Nulman" <rj.nulman@v...>
      > wrote:
      > > Hi Jamie,
      > > Trying to understand why you said this is "undersampled"?
      > >
      > > With Ken's 17" RC at F7.5, his FL is about 3200mm. With 9 micron
      > > pixels, his image scale is around .57 arcsec/pixel...since most
      > > agree that 3.5 x the image scale is a good choice for "hi-res"
      > (and
      > > since Ken's "seeing" was around 2 arcseconds)...this is almost
      > > perfectly sampled. If anything, if the seeing was poorer, then
      > the
      > > image would be "oversampled"...is that what you were trying to
      > > say?...don't know, just curious and want to keep the facts
      > straight
      > > for the "newbies".
      > >
      > > Perhaps, one might consider this "oversampled" (not undersampled),
      > > but the long exposure times make up for this...the faint stufff is
      > > quite smooth...and there is a better chance of bringing out more
      > > detail under these circumstances...not to mention more "data" for
      > > deconvolution and other, similar, programs to work better with the
      > > data.
      > >
      > > My guess is you were trying to indicate something else...is that
      > the
      > > case? (Not criticizing, just trying to understand <g>)
      > >
      > > Randy Nulman
      > > http://www.nulman.darkhorizons.org
      > >
      > >
      > >
      > >
      > > --- In ccd-newastro@yahoogroups.com, rhoowl@y... wrote:
      > > >
      > > > HI
      > > >
      > > > it's overall a good picture...but it appears undersampled....
      > > >
      > > > jamie
      > > >
      > > > --- In ccd-newastro@yahoogroups.com, "Ken Levin" <klevin@a...>
      > > wrote:
      > > > > Took 7hrs (over 2 nights) to overcome the lights of Wash, DC.
      > Got
      > > > > some great advice from Randy Nulman, especially for colors and
      > > > > Photoshop. thanks, Randy! used Astrodon RGB filters (better
      > blue
      > > > > response helped and only needed minor color tweaking), and
      > SBIG
      > > 6303
      > > > > camera. Ken Levin, Silver Spring, MD
      > > > >
      > > > > http://webzoom.freewebs.com/klevin/7331Curves6.jpg
      > > > >
      > > > >
      > > > > (also check out Perseus A galaxy cluster at bottom of page:
      > > > >
      > > > > http://www.freewebs.com/klevin/galaxies.htm

      Best Wishes
    • rhoowl@yahoo.com
      roger you re trying to extrapolate a 2 dimensional system from a linear set of equations from two unrelated occurances. nyquists theorom deals with the
      Message 2 of 28 , Oct 1, 2004
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        roger

        you're trying to extrapolate a 2 dimensional system from a linear set
        of equations from two unrelated occurances. nyquists theorom deals
        with the wavelength of sound. do you have scientific method to show
        this correlation or sources?

        unless you can mathematically prove your statements of 1/2.8 of
        seeing i would think that the experts at apogee instruments who study
        these problems would have more credence.

        the point i am trying to make if that you don't have great seeing
        conditions. and i am sure the washington dc area probably averages
        between 3 and 5 arc seconds of good seeing conditions. when you get 2
        arc seconds the transparency is generally poor due to the heat and
        humidity stagnating. i think one would probably get better results
        overall if you live in conditions like this having a camera with 16
        micron pixels for focal lengths greater than 2000

        jamie


        --- In ccd-newastro@yahoogroups.com, "Roger Hamlett" <roger@t...>
        wrote:
        > > HI
        > >
        > > i sorry..i meant oversampled....
        > >
        > > with his location near washington dc this puts him near sea
        > > level...probably the maximum seeing conditions there would be
        about
        > > 4 or 5 arcseconds
        > >
        > >
        > > according to apogee instruments.
        > >
        > > http://www.ccd.com/ccd113.html
        > >
        > > ..you should be imaging at 1/2 your seeing conditions...or 2 arc
        > > seconds per pixel....or most likely 2.5 arc seconds per pixel...
        > Whoa....
        > The suggestion to sample at half the 'seeing', is a common _and
        wrong_
        > application of Nyquists theorem...
        > Nyquist, shows that for sinusoidal waveforms, the _minimum_ sampling
        > 'interval', is half the wavelength, to reproduce a particular
        frequency.
        > When sampling with a camera, the pixel size is the sampling
        interval, and
        > the seeing, may (possibly), be taken to represent the 'wavelength',
        so
        > people then go on to say that you have to use 1/2 the seeing as the
        maximum
        > sample size to not lose significant resolution.
        > The first (biggest) problem, is that the image is a 2D structure,
        not a one
        > dimensional structure, and the worst sampling, is diagonally across
        the
        > pixel, not the width of the pixel. Hence ignoring anything else, the
        > Nyquist criteria, actually requires you to sample (assuming square
        pixels),
        > at 1/2.8th the seeing, if detail is not to be lost.
        > There is though also a second problem. The light curve produced by
        a star,
        > is not a nice sinusoid. The image of a star, will be close to the
        shape of
        > the Airy disk, then 'spread' by a Gaussian noise function. If you
        do this,
        > and then look at the measures of 'seeing', you will see that the
        edges on
        > the star, have much sharper rise/falls than a sinusoid, and to
        reproduce
        > these properly, requires a slightly higher sample rate. If you sit
        down,
        > and calculate the effects, you find that you have to sample at just
        over
        > 1/3rd the 'seeing', to get all the available detail without
        oversampling.
        >
        > > at .57 seconds he is way oversampled...and his stars show this...
        > No. 0.57arc seconds/pixel, is close to perfect sampling, for 2 arc
        second
        > seeing.
        >
        > > they have a noticeable star streak at least one pixel. in them due
        > > to periodic error (which is about 1 arcsecond in the
        > > picture...actually quite excellent pec)....and they are overly
        large
        > > because of being oversampled....also this lends itself to loss of
        > > sensitivity..which is why i think the core is burned out...
        > The 'burnout' is down to exposure time, and processing. Most people
        wanting
        > to get detail in both the core and arms, will end up using two
        seperate
        > exposures, and combining these, since the dynamic range, exceeds
        that
        > genuinely available from the CCD.
        >
        > > if he were imaging at 2 1/2 arcseconds per pixel..then the stars
        > > would have been round...comprising fewer pixels....and produce a
        > > more pleasing photo.
        > >
        > > jamie
        > If he was sampling at 2.5arc seconds/pixel, the stars would no
        longer show
        > the tracking errors, but would be noticeably undersampled, with the
        stars
        > only being 'round', if he artificially processed them with a blur
        to do
        > so...
        >
        > > --- In ccd-newastro@yahoogroups.com, "Randy Nulman"
        <rj.nulman@v...>
        > > wrote:
        > > > Hi Jamie,
        > > > Trying to understand why you said this is "undersampled"?
        > > >
        > > > With Ken's 17" RC at F7.5, his FL is about 3200mm. With 9
        micron
        > > > pixels, his image scale is around .57 arcsec/pixel...since most
        > > > agree that 3.5 x the image scale is a good choice for "hi-res"
        > > (and
        > > > since Ken's "seeing" was around 2 arcseconds)...this is almost
        > > > perfectly sampled. If anything, if the seeing was poorer, then
        > > the
        > > > image would be "oversampled"...is that what you were trying to
        > > > say?...don't know, just curious and want to keep the facts
        > > straight
        > > > for the "newbies".
        > > >
        > > > Perhaps, one might consider this "oversampled" (not
        undersampled),
        > > > but the long exposure times make up for this...the faint stufff
        is
        > > > quite smooth...and there is a better chance of bringing out more
        > > > detail under these circumstances...not to mention more "data"
        for
        > > > deconvolution and other, similar, programs to work better with
        the
        > > > data.
        > > >
        > > > My guess is you were trying to indicate something else...is that
        > > the
        > > > case? (Not criticizing, just trying to understand <g>)
        > > >
        > > > Randy Nulman
        > > > http://www.nulman.darkhorizons.org
        > > >
        > > >
        > > >
        > > >
        > > > --- In ccd-newastro@yahoogroups.com, rhoowl@y... wrote:
        > > > >
        > > > > HI
        > > > >
        > > > > it's overall a good picture...but it appears undersampled....
        > > > >
        > > > > jamie
        > > > >
        > > > > --- In ccd-newastro@yahoogroups.com, "Ken Levin" <klevin@a...>
        > > > wrote:
        > > > > > Took 7hrs (over 2 nights) to overcome the lights of Wash,
        DC.
        > > Got
        > > > > > some great advice from Randy Nulman, especially for colors
        and
        > > > > > Photoshop. thanks, Randy! used Astrodon RGB filters (better
        > > blue
        > > > > > response helped and only needed minor color tweaking), and
        > > SBIG
        > > > 6303
        > > > > > camera. Ken Levin, Silver Spring, MD
        > > > > >
        > > > > > http://webzoom.freewebs.com/klevin/7331Curves6.jpg
        > > > > >
        > > > > >
        > > > > > (also check out Perseus A galaxy cluster at bottom of page:
        > > > > >
        > > > > > http://www.freewebs.com/klevin/galaxies.htm
        >
        > Best Wishes
      • rhoowl@yahoo.com
        hi actually...i m looking to buy a better camera than what i have now....and i[m looking into this issue very closely...this came up as a point....so i started
        Message 3 of 28 , Oct 1, 2004
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          hi

          actually...i'm looking to buy a better camera than what i have
          now....and i[m looking into this issue very closely...this came up as
          a point....so i started looking around at other images....

          i found some similiar type images..one i found was some photo by
          robert gendler...with a 12.5 rc at f9....and a fli dream machine with
          24 micron pixels....his images appear to be undersampled....

          looking for a sample of this telescope with a camera of about 16
          micron pixels..

          jamie


          --- In ccd-newastro@yahoogroups.com, "Roger Hamlett" <roger@t...>
          wrote:
          > > HI
          > >
          > > i sorry..i meant oversampled....
          > >
          > > with his location near washington dc this puts him near sea
          > > level...probably the maximum seeing conditions there would be
          about
          > > 4 or 5 arcseconds
          > >
          > >
          > > according to apogee instruments.
          > >
          > > http://www.ccd.com/ccd113.html
          > >
          > > ..you should be imaging at 1/2 your seeing conditions...or 2 arc
          > > seconds per pixel....or most likely 2.5 arc seconds per pixel...
          > Whoa....
          > The suggestion to sample at half the 'seeing', is a common _and
          wrong_
          > application of Nyquists theorem...
          > Nyquist, shows that for sinusoidal waveforms, the _minimum_ sampling
          > 'interval', is half the wavelength, to reproduce a particular
          frequency.
          > When sampling with a camera, the pixel size is the sampling
          interval, and
          > the seeing, may (possibly), be taken to represent the 'wavelength',
          so
          > people then go on to say that you have to use 1/2 the seeing as the
          maximum
          > sample size to not lose significant resolution.
          > The first (biggest) problem, is that the image is a 2D structure,
          not a one
          > dimensional structure, and the worst sampling, is diagonally across
          the
          > pixel, not the width of the pixel. Hence ignoring anything else, the
          > Nyquist criteria, actually requires you to sample (assuming square
          pixels),
          > at 1/2.8th the seeing, if detail is not to be lost.
          > There is though also a second problem. The light curve produced by
          a star,
          > is not a nice sinusoid. The image of a star, will be close to the
          shape of
          > the Airy disk, then 'spread' by a Gaussian noise function. If you
          do this,
          > and then look at the measures of 'seeing', you will see that the
          edges on
          > the star, have much sharper rise/falls than a sinusoid, and to
          reproduce
          > these properly, requires a slightly higher sample rate. If you sit
          down,
          > and calculate the effects, you find that you have to sample at just
          over
          > 1/3rd the 'seeing', to get all the available detail without
          oversampling.
          >
          > > at .57 seconds he is way oversampled...and his stars show this...
          > No. 0.57arc seconds/pixel, is close to perfect sampling, for 2 arc
          second
          > seeing.
          >
          > > they have a noticeable star streak at least one pixel. in them due
          > > to periodic error (which is about 1 arcsecond in the
          > > picture...actually quite excellent pec)....and they are overly
          large
          > > because of being oversampled....also this lends itself to loss of
          > > sensitivity..which is why i think the core is burned out...
          > The 'burnout' is down to exposure time, and processing. Most people
          wanting
          > to get detail in both the core and arms, will end up using two
          seperate
          > exposures, and combining these, since the dynamic range, exceeds
          that
          > genuinely available from the CCD.
          >
          > > if he were imaging at 2 1/2 arcseconds per pixel..then the stars
          > > would have been round...comprising fewer pixels....and produce a
          > > more pleasing photo.
          > >
          > > jamie
          > If he was sampling at 2.5arc seconds/pixel, the stars would no
          longer show
          > the tracking errors, but would be noticeably undersampled, with the
          stars
          > only being 'round', if he artificially processed them with a blur
          to do
          > so...
          >
          > > --- In ccd-newastro@yahoogroups.com, "Randy Nulman"
          <rj.nulman@v...>
          > > wrote:
          > > > Hi Jamie,
          > > > Trying to understand why you said this is "undersampled"?
          > > >
          > > > With Ken's 17" RC at F7.5, his FL is about 3200mm. With 9
          micron
          > > > pixels, his image scale is around .57 arcsec/pixel...since most
          > > > agree that 3.5 x the image scale is a good choice for "hi-res"
          > > (and
          > > > since Ken's "seeing" was around 2 arcseconds)...this is almost
          > > > perfectly sampled. If anything, if the seeing was poorer, then
          > > the
          > > > image would be "oversampled"...is that what you were trying to
          > > > say?...don't know, just curious and want to keep the facts
          > > straight
          > > > for the "newbies".
          > > >
          > > > Perhaps, one might consider this "oversampled" (not
          undersampled),
          > > > but the long exposure times make up for this...the faint stufff
          is
          > > > quite smooth...and there is a better chance of bringing out more
          > > > detail under these circumstances...not to mention more "data"
          for
          > > > deconvolution and other, similar, programs to work better with
          the
          > > > data.
          > > >
          > > > My guess is you were trying to indicate something else...is that
          > > the
          > > > case? (Not criticizing, just trying to understand <g>)
          > > >
          > > > Randy Nulman
          > > > http://www.nulman.darkhorizons.org
          > > >
          > > >
          > > >
          > > >
          > > > --- In ccd-newastro@yahoogroups.com, rhoowl@y... wrote:
          > > > >
          > > > > HI
          > > > >
          > > > > it's overall a good picture...but it appears undersampled....
          > > > >
          > > > > jamie
          > > > >
          > > > > --- In ccd-newastro@yahoogroups.com, "Ken Levin" <klevin@a...>
          > > > wrote:
          > > > > > Took 7hrs (over 2 nights) to overcome the lights of Wash,
          DC.
          > > Got
          > > > > > some great advice from Randy Nulman, especially for colors
          and
          > > > > > Photoshop. thanks, Randy! used Astrodon RGB filters (better
          > > blue
          > > > > > response helped and only needed minor color tweaking), and
          > > SBIG
          > > > 6303
          > > > > > camera. Ken Levin, Silver Spring, MD
          > > > > >
          > > > > > http://webzoom.freewebs.com/klevin/7331Curves6.jpg
          > > > > >
          > > > > >
          > > > > > (also check out Perseus A galaxy cluster at bottom of page:
          > > > > >
          > > > > > http://www.freewebs.com/klevin/galaxies.htm
          >
          > Best Wishes
        • Wodaski Yahoo
          Roger s discussion of the Nyquist theorem, and the sampling required to get all the available information, is accurate. It matches materials I have read
          Message 4 of 28 , Oct 1, 2004
          • 0 Attachment
            Roger's discussion of the Nyquist theorem, and the sampling required to get
            all the available information, is accurate. It matches materials I have read
            elsewhere on the subject.

            With respect to seeing, I don't think one can assume the seeing will be one
            thing or another on any give night. As noted by the person who actually took
            the images, he had seeing of about 2 arcseconds, so your estimate isn't
            correct for that person on that night.

            Group policy is that posters should do their homework on issues before
            posting. Roger did his homework, and I want to make sure that readers
            understand that they can rely on Roger's numbers and conclusions.


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

            -----Original Message-----
            From: rhoowl@... [mailto:rhoowl@...]
            Sent: Friday, October 01, 2004 6:33 AM
            To: ccd-newastro@yahoogroups.com
            Subject: [ccd-newastro] Re: ngc7331 7 hrs Lum

            roger

            you're trying to extrapolate a 2 dimensional system from a linear set
            of equations from two unrelated occurances. nyquists theorom deals
            with the wavelength of sound. do you have scientific method to show
            this correlation or sources?

            unless you can mathematically prove your statements of 1/2.8 of
            seeing i would think that the experts at apogee instruments who study
            these problems would have more credence.

            the point i am trying to make if that you don't have great seeing
            conditions. and i am sure the washington dc area probably averages
            between 3 and 5 arc seconds of good seeing conditions. when you get 2
            arc seconds the transparency is generally poor due to the heat and
            humidity stagnating. i think one would probably get better results
            overall if you live in conditions like this having a camera with 16
            micron pixels for focal lengths greater than 2000

            jamie


            --- In ccd-newastro@yahoogroups.com, "Roger Hamlett" <roger@t...>
            wrote:
            > > HI
            > >
            > > i sorry..i meant oversampled....
            > >
            > > with his location near washington dc this puts him near sea
            > > level...probably the maximum seeing conditions there would be
            about
            > > 4 or 5 arcseconds
            > >
            > >
            > > according to apogee instruments.
            > >
            > > http://www.ccd.com/ccd113.html
            > >
            > > ..you should be imaging at 1/2 your seeing conditions...or 2 arc
            > > seconds per pixel....or most likely 2.5 arc seconds per pixel...
            > Whoa....
            > The suggestion to sample at half the 'seeing', is a common _and
            wrong_
            > application of Nyquists theorem...
            > Nyquist, shows that for sinusoidal waveforms, the _minimum_ sampling
            > 'interval', is half the wavelength, to reproduce a particular
            frequency.
            > When sampling with a camera, the pixel size is the sampling
            interval, and
            > the seeing, may (possibly), be taken to represent the 'wavelength',
            so
            > people then go on to say that you have to use 1/2 the seeing as the
            maximum
            > sample size to not lose significant resolution.
            > The first (biggest) problem, is that the image is a 2D structure,
            not a one
            > dimensional structure, and the worst sampling, is diagonally across
            the
            > pixel, not the width of the pixel. Hence ignoring anything else, the
            > Nyquist criteria, actually requires you to sample (assuming square
            pixels),
            > at 1/2.8th the seeing, if detail is not to be lost.
            > There is though also a second problem. The light curve produced by
            a star,
            > is not a nice sinusoid. The image of a star, will be close to the
            shape of
            > the Airy disk, then 'spread' by a Gaussian noise function. If you
            do this,
            > and then look at the measures of 'seeing', you will see that the
            edges on
            > the star, have much sharper rise/falls than a sinusoid, and to
            reproduce
            > these properly, requires a slightly higher sample rate. If you sit
            down,
            > and calculate the effects, you find that you have to sample at just
            over
            > 1/3rd the 'seeing', to get all the available detail without
            oversampling.
            >
            > > at .57 seconds he is way oversampled...and his stars show this...
            > No. 0.57arc seconds/pixel, is close to perfect sampling, for 2 arc
            second
            > seeing.
            >
            > > they have a noticeable star streak at least one pixel. in them due
            > > to periodic error (which is about 1 arcsecond in the
            > > picture...actually quite excellent pec)....and they are overly
            large
            > > because of being oversampled....also this lends itself to loss of
            > > sensitivity..which is why i think the core is burned out...
            > The 'burnout' is down to exposure time, and processing. Most people
            wanting
            > to get detail in both the core and arms, will end up using two
            seperate
            > exposures, and combining these, since the dynamic range, exceeds
            that
            > genuinely available from the CCD.
            >
            > > if he were imaging at 2 1/2 arcseconds per pixel..then the stars
            > > would have been round...comprising fewer pixels....and produce a
            > > more pleasing photo.
            > >
            > > jamie
            > If he was sampling at 2.5arc seconds/pixel, the stars would no
            longer show
            > the tracking errors, but would be noticeably undersampled, with the
            stars
            > only being 'round', if he artificially processed them with a blur
            to do
            > so...
            >
            > > --- In ccd-newastro@yahoogroups.com, "Randy Nulman"
            <rj.nulman@v...>
            > > wrote:
            > > > Hi Jamie,
            > > > Trying to understand why you said this is "undersampled"?
            > > >
            > > > With Ken's 17" RC at F7.5, his FL is about 3200mm. With 9
            micron
            > > > pixels, his image scale is around .57 arcsec/pixel...since most
            > > > agree that 3.5 x the image scale is a good choice for "hi-res"
            > > (and
            > > > since Ken's "seeing" was around 2 arcseconds)...this is almost
            > > > perfectly sampled. If anything, if the seeing was poorer, then
            > > the
            > > > image would be "oversampled"...is that what you were trying to
            > > > say?...don't know, just curious and want to keep the facts
            > > straight
            > > > for the "newbies".
            > > >
            > > > Perhaps, one might consider this "oversampled" (not
            undersampled),
            > > > but the long exposure times make up for this...the faint stufff
            is
            > > > quite smooth...and there is a better chance of bringing out more
            > > > detail under these circumstances...not to mention more "data"
            for
            > > > deconvolution and other, similar, programs to work better with
            the
            > > > data.
            > > >
            > > > My guess is you were trying to indicate something else...is that
            > > the
            > > > case? (Not criticizing, just trying to understand <g>)
            > > >
            > > > Randy Nulman
            > > > http://www.nulman.darkhorizons.org
            > > >
            > > >
            > > >
            > > >
            > > > --- In ccd-newastro@yahoogroups.com, rhoowl@y... wrote:
            > > > >
            > > > > HI
            > > > >
            > > > > it's overall a good picture...but it appears undersampled....
            > > > >
            > > > > jamie
            > > > >
            > > > > --- In ccd-newastro@yahoogroups.com, "Ken Levin" <klevin@a...>
            > > > wrote:
            > > > > > Took 7hrs (over 2 nights) to overcome the lights of Wash,
            DC.
            > > Got
            > > > > > some great advice from Randy Nulman, especially for colors
            and
            > > > > > Photoshop. thanks, Randy! used Astrodon RGB filters (better
            > > blue
            > > > > > response helped and only needed minor color tweaking), and
            > > SBIG
            > > > 6303
            > > > > > camera. Ken Levin, Silver Spring, MD
            > > > > >
            > > > > > http://webzoom.freewebs.com/klevin/7331Curves6.jpg
            > > > > >
            > > > > >
            > > > > > (also check out Perseus A galaxy cluster at bottom of page:
            > > > > >
            > > > > > http://www.freewebs.com/klevin/galaxies.htm
            >
            > Best Wishes






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          • Douglas B. George
            ... Actually, to properly represent that shape without significant aliasing would require much higher sampling than that. Non-sinusoidal shapes are
            Message 5 of 28 , Oct 1, 2004
            • 0 Attachment
              Roger Hamlett wrote:

              > Whoa....
              > The suggestion to sample at half the 'seeing', is a common _and wrong_
              > application of Nyquists theorem...
              > Nyquist, shows that for sinusoidal waveforms, the _minimum_ sampling
              > 'interval', is half the wavelength, to reproduce a particular frequency.
              > When sampling with a camera, the pixel size is the sampling interval, and
              > the seeing, may (possibly), be taken to represent the 'wavelength', so
              > people then go on to say that you have to use 1/2 the seeing as the maximum
              > sample size to not lose significant resolution.
              > The first (biggest) problem, is that the image is a 2D structure, not a one
              > dimensional structure, and the worst sampling, is diagonally across the
              > pixel, not the width of the pixel. Hence ignoring anything else, the
              > Nyquist criteria, actually requires you to sample (assuming square pixels),
              > at 1/2.8th the seeing, if detail is not to be lost.
              > There is though also a second problem. The light curve produced by a star,
              > is not a nice sinusoid. The image of a star, will be close to the shape of
              > the Airy disk, then 'spread' by a Gaussian noise function. If you do this,
              > and then look at the measures of 'seeing', you will see that the edges on
              > the star, have much sharper rise/falls than a sinusoid, and to reproduce
              > these properly, requires a slightly higher sample rate. If you sit down,
              > and calculate the effects, you find that you have to sample at just over
              > 1/3rd the 'seeing', to get all the available detail without oversampling.

              Actually, to properly represent that shape without significant aliasing
              would require much higher sampling than that. Non-sinusoidal shapes are
              represented by harmonics that are 2x, 3x, 4x... the fundamental
              frequency of the sinusoid.

              In the real world, compromises have to be made in terms of image/signal
              fidelity and noise. When the sampling is sufficient that the "noise"
              due to aliasing is smaller than the noise from other sources (photon
              shot noise, read noise, etc.) then you might consider it to be "ideally
              sampled".

              But of course bright stars have a different SNR than faint ones, so
              there probably isn't an "ideal sampling rate" for the whole image.
              Instead you have to compromise based upon what you are trying to
              achieve. If you are doing profile fitting photometry, you'll want very
              high sampling. If you're doing supernova hunting, you might make a
              different choice.

              In reality, the "1/3 the seeing" guideline is a simply pragmatic choice
              of sampling for "pretty picture" imaging. It isn't necessarily the
              optimum in any theoretical perspective.

              And I'll also point out that the Nyquist Sampling Criterion has more to
              it than just the minimum sampling rate. The criterion requires that, in
              order to reproduce the original signal/image, that you:

              1. Filter out any components that are above 1/2 the sampling rate (this
              is impractical in optical systems; however, the Gaussian shape is fairly
              close to sinusoidal so the residual error is not huge. It does suggest
              however, that if you're undersampling you might want to defocus the
              telescope a bit).

              2. Apply a "reconstruction filter" when you output the signal/image.
              The reconstruction filter provides the necessary interpolation between
              the individual samples, to reconstruct the original image.

              This last step seems to be completely ignored by astroimagers. Partly
              this is due to technological limitations (see below), but I think it's
              also due to ignorance of how Nyquist Sampling Criterion actually works.

              In the digital domain, the only practical way to perform a resampling
              filter is to upsample the data, then low-pass filter it to 1/2 the
              original sampling rate. That's what the Double Size function in MaxIm
              DL does. (Of course a better reconstruction filter can be had using
              even higher sampling and appropriate sin(x)/x correction, like CD
              players do, but that isn't practical for imaging.)

              The technological limitation I mentioned comes from the fact that you
              might not have enough pixels on your computer screen to display the
              resampled image. However, it is a useful exercise if you are planning
              on printing on a high resolution media, or if your images are smaller
              than your screen size.

              Doug

              -----------------------------------

              Doug George
              dgeorge@...

              Diffraction Limited
              Makers of Cyanogen Imaging Products
              http://www.cyanogen.com

              25 Conover Street
              Ottawa, Ontario,
              Canada, K2G 4C3

              Phone: (613) 225-2732
              Fax: (613) 225-9688

              -----------------------------------
            • Roger Hamlett
              ... frequency. ... and ... maximum ... one ... pixels), ... star, ... of ... this, ... on ... reproduce ... down, ... over ... oversampling. ... Agreed. Good
              Message 6 of 28 , Oct 2, 2004
              • 0 Attachment
                > Roger Hamlett wrote:
                >
                > > Whoa....
                > > The suggestion to sample at half the 'seeing', is a common _and wrong_
                > > application of Nyquists theorem...
                > > Nyquist, shows that for sinusoidal waveforms, the _minimum_ sampling
                > > 'interval', is half the wavelength, to reproduce a particular
                frequency.
                > > When sampling with a camera, the pixel size is the sampling interval,
                and
                > > the seeing, may (possibly), be taken to represent the 'wavelength', so
                > > people then go on to say that you have to use 1/2 the seeing as the
                maximum
                > > sample size to not lose significant resolution.
                > > The first (biggest) problem, is that the image is a 2D structure, not a
                one
                > > dimensional structure, and the worst sampling, is diagonally across the
                > > pixel, not the width of the pixel. Hence ignoring anything else, the
                > > Nyquist criteria, actually requires you to sample (assuming square
                pixels),
                > > at 1/2.8th the seeing, if detail is not to be lost.
                > > There is though also a second problem. The light curve produced by a
                star,
                > > is not a nice sinusoid. The image of a star, will be close to the shape
                of
                > > the Airy disk, then 'spread' by a Gaussian noise function. If you do
                this,
                > > and then look at the measures of 'seeing', you will see that the edges
                on
                > > the star, have much sharper rise/falls than a sinusoid, and to
                reproduce
                > > these properly, requires a slightly higher sample rate. If you sit
                down,
                > > and calculate the effects, you find that you have to sample at just
                over
                > > 1/3rd the 'seeing', to get all the available detail without
                oversampling.
                >
                > Actually, to properly represent that shape without significant aliasing
                > would require much higher sampling than that. Non-sinusoidal shapes are
                > represented by harmonics that are 2x, 3x, 4x... the fundamental
                > frequency of the sinusoid.
                Agreed. Good old Fourier analysis...
                I 'simplified', and shortened what could become rather a long comment, to
                give the general 1/3rd seeing 'rule'. It works better than might be
                thought, because the fundamental, is not actually at the frequency
                represented by the 'seeing', and with the normal 'seeing' measures, it
                results in the inclusion of the next harmonic for the image date, giving
                quite a close overall approximation.

                > In the real world, compromises have to be made in terms of image/signal
                > fidelity and noise. When the sampling is sufficient that the "noise"
                > due to aliasing is smaller than the noise from other sources (photon
                > shot noise, read noise, etc.) then you might consider it to be "ideally
                > sampled".
                >
                > But of course bright stars have a different SNR than faint ones, so
                > there probably isn't an "ideal sampling rate" for the whole image.
                > Instead you have to compromise based upon what you are trying to
                > achieve. If you are doing profile fitting photometry, you'll want very
                > high sampling. If you're doing supernova hunting, you might make a
                > different choice.
                >
                > In reality, the "1/3 the seeing" guideline is a simply pragmatic choice
                > of sampling for "pretty picture" imaging. It isn't necessarily the
                > optimum in any theoretical perspective.
                >
                > And I'll also point out that the Nyquist Sampling Criterion has more to
                > it than just the minimum sampling rate. The criterion requires that, in
                > order to reproduce the original signal/image, that you:
                >
                > 1. Filter out any components that are above 1/2 the sampling rate (this
                > is impractical in optical systems; however, the Gaussian shape is fairly
                > close to sinusoidal so the residual error is not huge. It does suggest
                > however, that if you're undersampling you might want to defocus the
                > telescope a bit).
                Yes.

                > 2. Apply a "reconstruction filter" when you output the signal/image.
                > The reconstruction filter provides the necessary interpolation between
                > the individual samples, to reconstruct the original image.
                >
                > This last step seems to be completely ignored by astroimagers. Partly
                > this is due to technological limitations (see below), but I think it's
                > also due to ignorance of how Nyquist Sampling Criterion actually works.
                >
                > In the digital domain, the only practical way to perform a resampling
                > filter is to upsample the data, then low-pass filter it to 1/2 the
                > original sampling rate. That's what the Double Size function in MaxIm
                > DL does. (Of course a better reconstruction filter can be had using
                > even higher sampling and appropriate sin(x)/x correction, like CD
                > players do, but that isn't practical for imaging.)
                >
                > The technological limitation I mentioned comes from the fact that you
                > might not have enough pixels on your computer screen to display the
                > resampled image. However, it is a useful exercise if you are planning
                > on printing on a high resolution media, or if your images are smaller
                > than your screen size.
                >
                > Doug
                Reconstruction, is quite commonly being done on binned images (most people
                do upsample on these), but as you say, is commonly being ignored.

                Best Wishes
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