CCD pixel-to-pixel variations

[Paul.Jorden at eev.com]

Posted to CCD-world:
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Hi Klaus, & everyone,

 > >QUESTION #1:
 > > Since no light is falling on the CCD during the dark frame
 > > exposure, what value of the PRNU would I use, since it
 > > shows such dramatic variations as a function of wavelength
 > > in the table above?
 > >
 >
 >    we have found that the pixel-to-pixel QE variation (i.e. PRNU)
 > and the pixel-to-pixel variation of thermal electron generation
 > (no acronym so far?) are different by an order of magnitude or
 > more. We have seen values for the latter of 20 to 30 percent
 > (standard deviation) at room temperature. PRNU values were
 > usually between a few and less then 1 percent as indicated
 > in the EEV table. These two effects should therefore be considered
 > and corrected separately.
 >
At EEV we use the term 'DSNU' (dark signal non uniformity). cf PRNU (pixel 
response non uniformity).
It is a measure of fluctuating dark signal generation, and in general is not 
related to PRNU (which is a measure of response fluctuations).  It should 
scale with temperature in the same way as global dark current.

In general DSNU is irrelevant, and not measured for cryogenic astronomy 
devices.

PRNU can be ~1% or less. (removed by flat field division)
DSNU will be ~10% or more. (removed by bias subtraction)

Regards,
Paul
_______________________________________________________________
Dr Paul Jorden,  CCD Applications, EEV
Waterhouse Lane, Chelmsford, Essex, CM1 2QU, UK
Tel:      44 (0) 1245 -453458 (direct), -493493 (switchboard)
Fax:      44 (0) 1245 453224
see also:-     http://www.ccd.eev.com/

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