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Definitions or calculations with GAIN and SATUR_LEVEL are wrong
07-13-2014, 11:59 (This post was last modified: 07-13-2014 12:10 by bmv.)
Post: #1
Definitions or calculations with GAIN and SATUR_LEVEL are wrong
Definitions or calculations of GAIN and SATUR_LEVEL are wrong

From:
http://www.stsci.edu/hst/acs/documents/h..._Ch42.html
Quote: 4.1.2 A-to-D Conversion
The analog information (electrons) accumulated in the CCD are converted into data numbers (DN) by the analog-to-digital converter (ADC). Both ACS CCD cameras employ 16-bit ADCs, which can produce a maximum of 65,535 DN. If the gain conversion factor g is expressed in e-/DN, the largest number of electrons representable by these ADCs is given by g * 65,535. Any charge packet containing more than g * 65,535 electrons will reach the digital saturation limit (see Section 4.7.4). The analog-to-digital converter produces only discrete output levels. This means that a range of analog inputs can produce the same digital output. This round-off error is called quantizing noise. It can be shown (Janesick J. R., 2001, Scientific Charge-Coupled Devices, SPIE Press.) that quantizing noise QN is constant for a given gain setting when expressed in DN: QN(DN) = 1/sqrt(12) = 0.288675. Quantizing noise can be converted into noise electrons as: QN(e-) = 0.288675 * g.
The measured read noise (RN; reported in Tables 4.1, 4.2, and 4.3) is the quadrature sum of this quantizing noise and the intrinsic read noise (IRN) associated with a particular readout amplifier:
RN = (IRN2 +(0.288675 * g)2)1/2

Let's make two experiments:
1) Small GAIN
GAIN 0.01 # gain (e-/ADU)
WELL_CAPACITY 0 # full well capacity in e- (0 = infinite)
SATUR_LEVEL 65535 # saturation level (ADU)
READOUT_NOISE 1.0 # read-out noise (e-)

Level of digital saturation limit has to be GAIN*SATUR_LEVEL=655.35 e-
On the model image it always 65535

2) Large GAIN
GAIN 100 # gain (e-/ADU)
WELL_CAPACITY 0 # full well capacity in e- (0 = infinite)
SATUR_LEVEL 65535 # saturation level (ADU)
READOUT_NOISE 1.0 # read-out noise (e-)

Level of digital saturation limit has to be GAIN*SATUR_LEVEL=6553500 e-
On the model image it always 65535

3-4) Same for RON on model image
For Small GAIN Resultant noise in e- has to be small, and for Large GAIN noise in e- has to be small.
At present all on the contrariwise.

Everything higher told truly for the following definitions:
GAIN - gain (e-/ADU)
SATUR_LEVEL - saturation level (ADU)
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