CCD-world: CTE and linearity of KAF-4201

[Anton Norup Soerensen]

The following was posted to CCD-world:


We have by now installed two Kodak KAF-4201 CCDs in astronomical cameras.
In most respects they behave nicely, but they appear to a problem with vertical
CTE and low level linearity.

Our first experience with poor VCTE was due to the unusually large capacitance
of the electrodes on this CCD. When our clock voltage driver was adapted to 
this, the VCTE improved, but still left a lot to be desired. The best estimate
of CTE was obtained by examining the tails of hot pixels in dark exposures.
>From 10.000e- pixels, a CTI of 10^-5 was found. At lower levels, the VCTE
quickly degrades: At 1000e-, CTI is 10^-4. At 500e-, 6*10^-3.
Horizontal CTE was indistinguishable from perfect.
This was seen in both chips, although only quantitatively examined in one of 
them. 

Linearity was measured by plotting counts per second versus exposure time,
using a stable light source. Shutter delay correction was applied.
The output was found to be linear to within +/- 1% in the range 2000e- to
120.000e-. At lower levels the response starts to diverge and has changed by
6% at a level of 150e-.
Using Poisson statistics to determine gain versus exposure level leads to
much larger deviations, but this is believed to be due to the poor CTE 
modifying the noise properties.

If you want more details on the measured performance of the chip, you may
fetch a report at:
http://www.astro.ku.dk/~ijaf/schmidt/KAF_schmidt.ps.gz

So, do you consider the VCTE and linearity behavior typical for these chips,
or might we be doing something wrong?

Thanks,

Anton Norup Sorensen
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