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Re: [APML] OT: CCD sensitivity [was: first CCD images]
Matt,
In fact, light pollution _is_ noise; The fact that light is discrete in
nature introduces the so-called 'photon shot noise'; If you had a perfect
sensor that would give a count of zero when 'exposed' to darkness and
exposed it to a stationary flat light source that gives a count
corresponding to N electrons (absorbed photons), you will not get a count
of N each time, but rather a count which has a rms error of sqrt(N). This
applies both to light-generated electrons, and to thermally generated
electrons (dark current). Even if you substract the 'N', you will still be
left with the sqrt(N) noise.
Radu
On Mon, 25 Mar 2002, Matt BenDaniel wrote:
> Loke,
>
> One of the most widely misunderstood concepts in astrophotography is noise.
> Light pollution is NOT noise in an engineering sense. Noise is a component
> that introduces a statistical variation to the signal. Light pollution is
> unwanted bias. It can be eliminated in a CCD image by a flat field. A flat
> field cannot eliminate noise. Gradients asides, light pollution reduces the
> dynamic range (S/N ratio) of an image. A light pollution gradient can be
> subtracted if you model it correctly (and we can do it intuitively Photoshop).
>
> The way to compensate for the loss of S/N due to light pollution is to
> collect more photon flux to increase the S/N. CCD's higher QE enables the
> capture of more photon flux per unit time, regaining more S/N lost by light
> pollution.
>
> You can fight light pollution with film by taking lots of images and
> stacking. But it takes longer and is more work.
>
> As an aside, a dark current is not noise either, because it can be
> subtracted. The source of noise for CCD is primarily readout noise. For
> film, the noise is mainly the grain.
>
> Biases include: light pollution, dark current, film fogging, and of course
> CCD bias.
>
> Matt
>
> At 02:46 PM 3/25/2002, lkt wrote:
>
> > The QE part would only make a difference in exposure lengths, that is
> >the higher the QE the shorter the exposures for the same amount of
> >exposure depth. The main question is, what makes CCDs seem almost immune
> >to light pollution vs. film. Of course, if you campare only CCD images
> >between themselves, one with light pollution and one without, you can
> >see the linear effect of degraded SNR. There must be something about
> >film which makes is much more susceptible to light pollution. Maybe its
> >the toe of the response or the logarithmic sensitivity (your 2nd point).
> >Your 3rd point about lower noise levels in CCDs is also not an issue. I
> >am talking about a case where the dominant noise component is light
> >pollution such as the moon etc and not any noise due to film grain or
> >CCD electronics.
> >
> >Loke
> >
> >
> >
> >Matt BenDaniel wrote:
> > >
> > > Loke,
> > >
> > > I'm newer at CCD than you are, but I have been reading a lot. Here's my
> > > understanding:
> > >
> > > CCD's have:
> > > * much higher quantum efficiency
> > > * linear response and essentially no toe
> > > * lower noise levels (basically readout noise) yielding higher dynamic
> > range
> > > * a more automatable exposure process, making stacking easier
> > >
> > > The noise that we fight in film is mostly grain size and uneven
> > distribution.
> > >
> > > Matt
> > >
> > > At 01:36 PM 3/25/2002, lkt wrote:
> > >
> > > >Yes, CCDs are quite impressive at recording light aren't they? I have
> > > >often wondered what the explanation of this effect is. That is, can this
> > > >be due to the linearity of CCDs in recording light versus film's
> > > >logarithmic response? Or possibly that film records light at a non
> > > >uniform monotonic rate as a function of image intensity.
> > > >
> > > >Loke
> > > >
> > > >
> > > >
> > > > > Tony Hallas wrote:
> > > > >
> > > > > Hi,
> > > > >
> > > > > Being a real "film guy" I thought it would be interesting for you
> > > > > to hear my impressions of the CCD camera I have on loan from SBIG...
> > > > >
> > > > > Picture this first: Cold front has just cleared out...
> > > > > everything is dripping wet... 3/4 moon is lighting up the haze in the
> > > > > air (aka as major dew event) to the extent that all I can see is the
> > > > > Big Dipper and a few other bright stars... in the finder scope I can
> > > > > just see the triangle that defines where M-51 is... no sign of the
> > > > > galaxy.
> > > > >
> > > > > The ST-10 takes a one minute exposure... one minute!
> > > > > As in 60 seconds... to my utter surprise most of the galaxy is
> > > > > there... including some of the faint tendrils coming off the
> > > > > companion... it is very noisy from low S/N.
> > > > >
> > > > > I took 3 - 10 minute shots with dark frames but no bias and no
> > > > > flat field just to see what it would record out of the haze...
> > > > > although there was information in the core when it was a native file,
> > > > > when I brought it over to PS as a Tiff file it got lost so the cores
> > > > > are all burned out.
> > > > >
> > > > > The first image is the 60 second image, the second is the 3 - 10
> > > > > minute shots run through PW and a little clean up because I didn't
> > > > > take a flat field. These are my first CCD images in many years so
> > > > > don't be too critical! : - ) What is amazing is the conditions under
> > > > > which they were taken... bright, hazy, moonlit night.
> > > > >
> > > > > http://www.astrophoto.com/CCD.htm
> > > > >
> > > > > For what it's worth,
> > > > >
> > > > > Tony
> > > > >
> > > > >
> > > > >
> > > > >
> > > >
> > > >-- APML Archives at <http://astro.umsystem.edu/apml/> ---
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> > >
> > > --
> > > Matt BenDaniel
> > > matt@starmatt.com
> > > http://starmatt.com
> > >
> > > -- APML Archives at <http://astro.umsystem.edu/apml/> ---
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> >
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>
> --
> Matt BenDaniel
> matt@starmatt.com
> http://starmatt.com
>
>
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>
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Radu Corlan Snail Mail: Bucuresti sect. 1,
rcorlan@pcnet.ro str. Argentina nr. 28, 71206 Romania
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Use Linux!
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