Detector non-linearity

The detector non-linearity, as measured over a representative region of the array can be fitted with the function

$$f_{\rm T}=f_{\rm M}+a*f_{\rm M}^2+b*f_{\rm M}^3$$ (7.1)

where $f_{\rm M}$ is the the measured flux and $f_{\rm T}$ is the true flux.

For the readout modes which use the high bias voltage, equation gives a relatively poor description (not better than 0.5%) of the non-linearity at low flux levels (4000 ADU). We will provide a better description of the non-linearity soon. The co-efficients for the various readout modes are given in the table  .

 

Table: LW linearity

Detector Mode Name	a	b	Validity
UnCorrLowBias (UCR)	2.04e-6	0.75e-10	Before March 2001
UnCorrLowBias (UCR)	6.28e-7	1.04e-10	After March 2001
DoubleCorrHighBias (DCR-HB)	1.26e-6	1.00e-10	Before March 2001
DoubleCorrHighBias (DCR-HB)	1.05e-6	0.84e-10	After March 2001
DoubleCorrLowBias (DCR-LB)	2.78e-6	1.65e-10	-

 

These co-efficients have been measured over a region which is thought to be representative of the entire array. In detail, this will not be true as each pixel will behave differently.

To correct any given frame for non-linearity, one requires between one and two additional frames. For observations that do not use chopping, a dark frame is required. For observations that use chopping, both the dark frame and the half cycle frames are required. These frames allow you to determine the measured flux in each pixel, which can the be corrected for non-linearity.

The procedure depends on the data set. If you have half cycle frames for every exposure, then it is simplest to correct the half cycle frames directly. If you have only the half cycle frame of the first exposure, then the correction for the non-linearity is more complex.