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33014Re: ngc7331 7 hrs Lum

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  • rhoowl@yahoo.com
    Oct 1, 2004
      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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