33012Re: ngc7331 7 hrs Lum
- Oct 1, 2004
> 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.
> ..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...
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
> they have a noticeable star streak at least one pixel. in them dueThe 'burnout' is down to exposure time, and processing. Most people wanting
> 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...
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 starsIf he was sampling at 2.5arc seconds/pixel, the stars would no longer show
> would have been round...comprising fewer pixels....and produce a
> more pleasing photo.
the tracking errors, but would be noticeably undersampled, with the stars
only being 'round', if he artificially processed them with a blur to do
> --- In email@example.com, "Randy Nulman" <rj.nulman@v...>Best Wishes
> > 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"
> > since Ken's "seeing" was around 2 arcseconds)...this is almost
> > perfectly sampled. If anything, if the seeing was poorer, then
> > image would be "oversampled"...is that what you were trying to
> > say?...don't know, just curious and want to keep the facts
> > 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
> > case? (Not criticizing, just trying to understand <g>)
> > Randy Nulman
> > http://www.nulman.darkhorizons.org
> > --- In firstname.lastname@example.org, rhoowl@y... wrote:
> > >
> > > HI
> > >
> > > it's overall a good picture...but it appears undersampled....
> > >
> > > jamie
> > >
> > > --- In email@example.com, "Ken Levin" <klevin@a...>
> > wrote:
> > > > Took 7hrs (over 2 nights) to overcome the lights of Wash, DC.
> > > > some great advice from Randy Nulman, especially for colors and
> > > > Photoshop. thanks, Randy! used Astrodon RGB filters (better
> > > > response helped and only needed minor color tweaking), and
> > 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
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