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detector linearity 


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LWarm linearity frames are taken bimonthly. The stack is submitted to the isaac_img_detlin recipe. Several lamp=on and lamp=off frames taken with different DITs are fit. There are 4 products of this recipe: the Aframe, the Bframe, the Cframe and a QC frame with the stdev of the fit. The A,B and C frames contain for each pixel the coefficients a,b, c where
DIT = A * flux_meas + B * flux_meas^2 + C * flux_meas^3
where flux_meas is the measured flux. We divide this equation by A, since we want coefficients to be independent on the actual lamp flux:
DIT/A = flux_meas + B / A * flux_meas^2 + C / A * flux_meas^3
and if we assume that the array is linear for low fluxes, then the first term (flux_meas) is the
linear part and A is dependent on the calibration lamp flux.
The second (B / A * flux_meas^2) and the third term (C / A * flux_meas^3)
are the higher orders (= nonlinear terms) and describe the deviation from linearity.
The A, B, and C frames are submitted together with the imaging flat to the LWarm zeropoint recipe.
We monitor the coefficient frames for three read modes (DET.NCORRS.NAME) of the LWarray :
UCHB DCLB and UCHB (e.g. UCHB = UncorrelatedHighBias).
The zeropoint recipe applies the following correction:
counts_corr = counts_meas + B/A * counts_meas^2 + C/A * counts_meas^3
The effective linearity is:
lin_eff = ( counts_corr / counts_meas )  1 = B/A * counts_meas + C/A * counts_meas^2
and using a reference of counts_meas = 10000 ADU:
lin_eff ( 10000 ADU ) = B/A * 10^4 + C/A * 10^8
or using a reference of counts_meas = 5000 ADU:
lin_eff ( 5000 ADU ) = B/A * 5e3 + C/A * 5e3 * 5e3
Occasionally the SWarm Hawaii detector suffers from the soacalled oddeven column effect The odd columns are 12% brighter than the even columns. Moreover the effect cannot be flatfielded, since the amplitude of the flux differences between odd and even columns is a function of flux and detetcor quadrant. In addition to that the nonlinearity relation of the odd and pixel is a function of time. The details are described on the twilight flat QC page. Another feature are sporadic sudden changes in the amplitude by about 10 ADU also found in sky observation stacks.
The odd even column effect is monitored quantitatively
 by the oddeven QC parameters extracted from twilight flats,
 by the robust fixed pattern noise derived from daily darks
 by the oddeven QC parameters extracted from linearity frames (not yet public)
The oddeven column effect can be corrected
 The isaac_img_jitter recipe has a oddeven command line option, that takes the fourier transform of each quadrant, replaces the line in the powerspectrum that corresponds to the oddeven column effect by zero, and transforms the power spectrum back into a pixel image.
 The isaac_img_detlin recipe records the nonlinearity for each pixel individually. The raw frames can be preprocessed using the three products of the isaac_img_delin recipe. The three products can be recognized by their PRO.CATG=DETLIN_A and DETLIN_B and DETLIN_C respectively. The correction is applied to the raw frames in the same manner as described above for the LWarm zeropoints:
corr_frame = raw_frame + DETLIN_B / DETLIN_A * raw_frame^2 + DETLIN_C / DETLIN_A * raw_frame^3
The limitation of this approach are: a) the function oddeven(flux) is a function of time. b) the linearity frames taken in 200802 and 200803 span a small flux range of 600 to 5000 ADU; only the linearity frames taken from 200804 on cover a larger flux range up to 10000 ADU.
top nonlinearity parameters 
QC1 parameters
parameter 
QC1 database: table, name 
procedure 
linear coefficient 
isaac_line, qc_detlin_meda 
 the median of the coefficient frame A, is very close to the lamp flux 
quadratic term coefficient 
isaac_line, qc_detlin_medb 
 the median of the coefficient frame B 
cubic term coefficient 
isaac_line, qc_detlin_medc 
 the median of the coefficient frame C 
linear fit error 
isaac_line, qc_detlin_medq 
 the median of the fit error map Q 
lamp stability 
isaac_line, qc_detlin_lamp 
 the lamp flux is derived from from two flats of the same DIT but taken at the beginning and at the end of the sequence; The QC parameter is 2 (ab)/(a+b) 
normalized nonlinearity 
isaac_line, qc_detlin_cob 
 coefficient ratio: qc_detlin_medb / qc_detlin_meda 
effective linearity 
isaac_line, qc_detlin_eff 
 effective nonlinearity at 10 000 ADU =
B/A * 10000 + C/A * 10000 * 10000 
Trending
For the Hawaii array the calibrations are taken twice a year in DoubleCorr read mode. For Aladdin array the calibrations are taken monthly for three read modes: Uncorrelated at High Bias, DoubleCorrelated at Low Bias and DoubleCorrelated at High Bias.
