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Re: SBIG STL-11000 ¿ color or monochrome ?

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  • Rainer
    Hi, Thanks to everybody now I am more confused as before :-)) No really, thanks a lot for all those explanations as you have given me now more food for thought
    Message 1 of 2 , Mar 1, 2005
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      Thanks to everybody now I am more confused as before :-))

      No really, thanks a lot for all those explanations as you have given
      me now more food for thought :-)) and I can begin to save money for
      my SBIG STL-11000 either Monochrome or Color :-)).

      regards Rainer

      --- In ccd-newastro@yahoogroups.com, "Roger Hamlett" <roger@t...>
      > > HI Ron,
      > >
      > > Thanks for that explanation but still I miss the advantage of
      > > getting a
      > > color camera or a monochrome camera with a filter wheel.
      > >
      > > What is the advantage of the monochrome ccd camera against the one
      > > shot
      > > color ccd camera ?
      > There are a number of 'costs' to the colour camera. The first is in
      > resolution. The CCD, is covered with a matrix of colour filters,
      > like:
      > R G R G R G.....
      > G B G B G B.....
      > R G R G R G...
      > Now to get a 'luminance' value, you take the whole group of four
      > (first and second from the top row then first and second from the
      next), to
      > generate your luminance. For the next 'virtual pixel', you move
      forward by
      > one, and take the second and third from each row. For the next
      line, you
      > start with the first and second from the second row, and combine
      this with
      > the first and second from the third row. This works fine for signal
      > that are gently changing, but has problems when dealing with bright
      > changes at high spatial frequencies. If (as a really 'worst case'
      > you had a grid of white dots, that hit the first, and then the
      third pixel
      > on the top line (happening to line up perfectly with the matrix),
      > system cannot distinguish this from a grid of red dots at the same
      > frequency!. The matrix gives spatial resolution, that varies
      according to
      > the nature of the source, from the full resolution of the CCD, down
      > about half the resolution of the CCD, and 'averages' about 0.7* the
      > resolution.
      > The second problem is loss of sensitivity. Because the filters are
      > all the time, you have the light losses associated with them,
      > present. The amount of loss, varies according to the colour of the
      > being recorded, with green light (selected because this is the most
      > sensitive colour for the human eye), getting close to 50% of the
      > cameras sensitivity, but red or blue light, are effectively
      only 'seen' by
      > one pixel in four, and the sensitivity here is about 25%. Now the
      > visual system is much more 'worried' about changes in intensity
      > changes in colour. This is why systems like TV, allocate the
      majority of
      > their available bandwidth to the 'mono' image, and then a
      relatively small
      > amount to the colours. The colour information, is effectively a low
      > accuracy 'wash', applied over the more detailed mono image. With a
      > camera, you can take advantage of this, and take longer luminance
      > using the full sensitivity, and resolution of the unfiltered
      camera, then
      > use less detailed (probably binned) colour images to apply the
      colours to
      > this. You are then making 'better use' of the imaging time, in
      terms of the
      > actual acquisition of data for you to see. So for a given
      imaging 'time',
      > you can actually record more of the important data.
      > The third problem is also because the filters are always there. You
      > 'bin' the colour images, and retain the colour information. If your
      CCD, is
      > relatively oversampling, you are stuck with this (and the poorer
      signal to
      > noise ratio this implies). This is one of the most serious 'costs'
      > associated with the one shot cameras.
      > The fourth problem, is that you are stuck with the filters
      > Generally, these do not give the sharp cutoff associated with the
      > astronomical imaging filters, giving in many cases, significant
      > and while the filter selection is designed to give a close
      approximation to
      > what you would 'see', it is not necessarily a good choice for some
      > with unusual colours. If you elect to us a narrowband filter to
      give better
      > signal to noise performance against the light in the sky, you are
      > stuck with imaging through 'double' filters, and probably only
      > light using one pixel in four, at half the spatial resolution. :-(
      > So what are 'one shot' cameras good for then?.
      > The answer here is the simplicity of not having a filter wheel, and
      > lack of cabling involved, combined with recording all the colour
      > information at the same time. They are truly 'great' for planetary
      > and for objects like comets, that move significantly against the
      > background. They also give pleasing general pictures of most
      objects, and
      > given the lack of extra wiring are a lovely solution if wanting to
      > mobile.
      > As a classic 'example', I have a SVX-H9, and also the second CCD,
      > convert it to colour use. For 95% of the time, the camera is used
      in one of
      > two combinations, with the mono CCD, and a solar Ha filter, or with
      > mono CCD, and a colour wheel for general imaging. However when I am
      > on holiday, I replace the CCD with the colour one, and combined
      with a
      > short OAG, it makes about the shortest/simplest little colour
      > attached to a C5 OTA (which I think is one of the truly
      great "ability for
      > it's size" optical tubes around), I have a imaging setup, that fits
      in two
      > small toolboxes (plus the tripod), and can be tucked into the boot
      of the
      > car to get images from a better site than my normal one...
      > Best Wishes
      > > OK, I have compared the response curves available at the SBIG
      > > but I
      > > honestly do not see an advantage in this case of the monochrome to
      > > the one
      > > shot color camera.
      > >
      > > Am I missing something ?
      > >
      > > I see an advantage in the time saving. If I take 4 images each one
      > LRGB or
      > > take one single RGB image, in the time I take 1 with the
      > > I take 4
      > > with the single shot.
      > >
      > > regards Rainer
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