Subject: CCD response

Date: Tue, 9 Oct 2001 08:07:18 +0200 (MEST)

Andrew,    cc to: Michael and Lennart with a question

I am trying to derive the astrometric accuracy for the proposed
sampling of bright stars. But this appears to be impossible in a
reliable way with the present simple and unrealistic
assumption of "linearity and then saturation".

As you pointed out a realistic assumption would be linearity and then
gradual flattening. The flattening could be tailored to some extent to
cover a larger range of counts.

I wonder which assumption was made for the Fig.3.14 in the GSR ?
The assumptions are not clearly given in the GSR, nor in the GAIA 
Concept & Technology Study Report in connection with Fig.3.3/8.

Perhaps Michael or Lennart could tell us something about the
assumptions ?

The details given in GSR Sect.3.7 CCD Details are as follows.
Sect.3.7.4: maximum charge 330 000 e- for a conversion factor of
3uV/e-. It is also said that the conversion factor could be 
switchable, depending on the information from the ASM. I do not 
think this is a good idea because we are usually measuring many 
stars at a time on each CCD, so which stars should be taken to 
control the conversion factor?

In Fig.8.8 of GSR a saturation limit of 500 000 e- per star for 
the BBP is assumed. But this means much fewer counts per pixel. 
So this figure is consistent with the 330 000.

Could you perhaps send me two realistic responses for the 9x27
um pixel in the Astro focal plane ?  One with linear, another with
gradual flattening.

Is it correct when I assume that a charge exceeding the above 330 000
e- in a pixel will flow into the two neighbouring pixels in the 
scan direction ?
Will this make high precision astrometry impossible ? 
Or could we still use the non-saturated pixel ? 
That is all to say: can the charge content in these pixel be understood
quantitatively ?

The neighbouring pixels across scan would *not* be affected (?).

In case of non-linear response the charge still stay in the pixel until
saturation is reached (?)

We may have to divide the present two central patches of 2 pixel 
height into 4 patches of only 1 pixel height in order to cope 
with the steep slope of the PSF near the centre.
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Andrew, could you please answer the easiest questions by about 17 Oct.,
the date I mentioned for comments to CUO-100 when I distributed the
draft last week. I could then include such new knowledge in an updated
version.

Best regards   Erik
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