Re: SBIG STL-11000 ¿ color or monochrome ?
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 :-)).
--- In email@example.com, "Roger Hamlett" <roger@t...>
> > HI Ron,something
> > 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,
> 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 thenext), to
> generate your luminance. For the next 'virtual pixel', you moveforward by
> one, and take the second and third from each row. For the nextline, you
> start with the first and second from the second row, and combinethis with
> the first and second from the third row. This works fine for signaldetails
> that are gently changing, but has problems when dealing with brightcolour
> changes at high spatial frequencies. If (as a really 'worst case'example),
> you had a grid of white dots, that hit the first, and then thethird pixel
> on the top line (happening to line up perfectly with the matrix),the
> system cannot distinguish this from a grid of red dots at the sameaccording to
> frequency!. The matrix gives spatial resolution, that varies
> the nature of the source, from the full resolution of the CCD, downto
> about half the resolution of the CCD, and 'averages' about 0.7* thethere
> The second problem is loss of sensitivity. Because the filters are
> all the time, you have the light losses associated with them,always
> present. The amount of loss, varies according to the colour of thelight
> being recorded, with green light (selected because this is the mostunfiltered
> sensitive colour for the human eye), getting close to 50% of the
> cameras sensitivity, but red or blue light, are effectivelyonly 'seen' by
> one pixel in four, and the sensitivity here is about 25%. Now thehuman
> visual system is much more 'worried' about changes in intensitythan
> changes in colour. This is why systems like TV, allocate themajority of
> their available bandwidth to the 'mono' image, and then arelatively small
> amount to the colours. The colour information, is effectively a lowmono
> accuracy 'wash', applied over the more detailed mono image. With a
> camera, you can take advantage of this, and take longer luminanceimages,
> using the full sensitivity, and resolution of the unfilteredcamera, then
> use less detailed (probably binned) colour images to apply thecolours to
> this. You are then making 'better use' of the imaging time, interms of the
> actual acquisition of data for you to see. So for a givenimaging 'time',
> you can actually record more of the important data.cannot
> The third problem is also because the filters are always there. You
> 'bin' the colour images, and retain the colour information. If yourCCD, is
> relatively oversampling, you are stuck with this (and the poorersignal to
> noise ratio this implies). This is one of the most serious 'costs'supplied.
> 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 thenormal
> astronomical imaging filters, giving in many cases, significantoverlap,
> and while the filter selection is designed to give a closeapproximation to
> what you would 'see', it is not necessarily a good choice for someobjects
> with unusual colours. If you elect to us a narrowband filter togive better
> signal to noise performance against the light in the sky, you arethen
> stuck with imaging through 'double' filters, and probably onlydetecting
> light using one pixel in four, at half the spatial resolution. :-(the
> 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 colourimaging,
> information at the same time. They are truly 'great' for planetary
> and for objects like comets, that move significantly against thesky
> background. They also give pleasing general pictures of mostobjects, and
> given the lack of extra wiring are a lovely solution if wanting togo
> 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 usedin one of
> two combinations, with the mono CCD, and a solar Ha filter, or withthe
> mono CCD, and a colour wheel for general imaging. However when I amgoing
> on holiday, I replace the CCD with the colour one, and combinedwith a
> short OAG, it makes about the shortest/simplest little colourimager,
> attached to a C5 OTA (which I think is one of the trulygreat "ability for
> it's size" optical tubes around), I have a imaging setup, that fitsin two
> small toolboxes (plus the tripod), and can be tucked into the bootof the
> car to get images from a better site than my normal one...site
> Best Wishes
> > OK, I have compared the response curves available at the SBIG
> > but Imonochrome
> > 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