"William D. Carroll" a écrit :
>
> Have you read the specs of the MX7C camera,
> it's QE with Filters is 70% from 470nM to 650nM. it's noise is in the
> 12e- range and a 90k well depth. The CCD is a Sony ICX249AK, These
> camera's are producing great images. If your goal is photometry then
> yes I agree with you I would not get a MX7C or any one-shot color camera
> instead use a B&W with a set of BVR filters if you want to do it right.
>
> If your goal is to take images and maybe some astrometry then the MX7C
> is a great camera to go with (I'm getting one soon I hope). it cost is
> great, features are great and if you want grayscale you can get it.
>
> Soon as I have mine I will post results of the true dark current, gain
> noise etc... but might be several more months
>
> Doug
>

Your message was intriguing to me, so I searched on the web for the data
sheet of this chip then for that of the camera.
I don't find any noise level in electrons in the chip data sheet, nor,
but maybe it is because I read fast, any well depth measurement.
I find a saturation level of 900 mv and a dark signal level of 2mv, but
at 60 degrees operating temperature (?), which is not interesting to us.
I don't find any QE curve or any other more scientific specifications,
only consumer electronics style specifications. Apparently the values
which you give, and given in the camera information are derived by the
manufacturer of the camera. In the camera info, we read indeed that the
"fundamental" QE is around 70%, whereas the "mean visual" QE is 45%...
What about the "real" QE ?
Using thinned CCDs and filters at observatories, you get usually very
rarely above 50% QE. You have to use specially designed interferential
filters to get such an efficiency. For information, the Space Telescope
WF/PC, it is in the best case, i.e. the R passband, around 20% QE. I
don't know where these 70% or 45% QE values are coming from, but they
have to be quite far from physical measurements made in laboratory who
are used to measure QE on detectors. I will of course apologize if
proven wrong.
Using dye filters (i.e. similar to wratten filters) and non thinned
CCDs, if you get above 40% QE in the best spectral region (and not over
the whole visible spectrum) you are lucky. Then comes the readout
system. Interline, don't even think about 70%, it would mean that full
frame CCDs could have at least 140% QE :-) . By definition half of the
CCD is occupied by transfert lines, there is no magic to it, you loose
half the signal, i.e. half the possible sensitivity. So it has to be
another case of musical watts peak to peak (i.e. where a 5 watt stereo
equipement can look as an 80 watt system on the paper ). Unless I am of
course totally mistaken, I would not bet above a 20% peak QE anywhere in
the spectral range you specify, if such curves have ever been measured
for this CCD.
I would not tempt astrometry with an interline CCD (the response varies
depending on where the source is, you would obtain a different relative
position from one image to another depending of the placement of the
image disk on the CCD, not even talking about the effects of the color
filters). I also think a direct color camera does not have the
flexibility of a black and white camera when it comes to colour imaging.
You are stuck with the colour filters put on the CCD by the manufacturer
(adjusted to daylight scenes), and with some luck as with some colour
films, it does not record spectral lines very well (what about OIII at
5007A or Halpha ?).
I'd be interested to hear about comparative results. All the direct CCD
colour images I have seen so far were not that great compared to
tricolor pictures obtained with filters (by the same, competent
observer).
Again a personal point of view, I would not buy it (or any other direct
colour imaging CCD camera) but if you like no hassle, direct colour
imaging of bright objects, and will likely never attempt any other type
of imaging, go for it. It seems this camera has a lot bells and whistle
to it, but I don't think it has the performance of a regular black and
white CCD camera. Again no magic, the price is interesting because the
CCDs are mass produced video CCD, not full frame devices. They may be
better than what was available a few years back, but still have the
inherent drawbacks of interline devices, and colour chips.
Alain
PS: I'll be away for the week end.

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