## 33014Re: ngc7331 7 hrs Lum

Expand Messages
• 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
> > 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
• Show all 28 messages in this topic