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Startrace frames are taken semi-annual. The template generates 11 star spectra along the slit for the LR grating, another eleven using the MR grating and further eleven images of the source. The purpose is to model the spectrum curvature at different positions of the source on the slit. The results are the polynomial coefficients of the optical distortion model for distortions in y-direction:


The QC parameters of the optical distortion in x-direction is retrieved from arc frames. On 2006-01-01 there was a major pipeline upgrade:

The startrace recipe generates three coefficients IL1, IL2, IL3 (or IM1, IM2, IM3 for the MR grating) that describe the relation between the y-position of a point source in imaging in the FOV and the y-position of corresponding spectrum. The difference between the source position in imaging and the source spectrum position in spectroscopy is expressed by three coefficients : IL1, IL3, and ILR (LR grating) and IM1, IM2 and IM3 (MR grating). There is

Y(image) = I1 + I2 * Y(spectrum) + I3 * Y(spectrum) * Y(spectrum).

top optical distortion

QC1 parameters

parameter QC1 database: table, name procedure
coefficients of the polynomial for the optical distortion isaac_strace,
qc_distLR1, qc_distLRX, qc_distLRY, qc_distLRXY, qc_distLRXX, qc_distLRYY,
qc_distMR1, qc_distMRX, qc_distLMRY, qc_distMRXY, qc_distMRXX, qc_distMRYY,

- The 11 star spectra are fit by a polynomial in x and y. The polynomial describes the optical distortion in y-direction.The LR coefficients are for the LR grating and the MR coefficients are for the MR grating. The coefficients are monitored. The optical distortion in x-direction is derived from arc lines of the calibration lamp.

top applied optical distortion (effective distortion)

QC1 parameters

parameter QC1 database: table, name procedure
coefficients of the polynomial for the optical distortion are applied to five selected pixel.   - The polynomial coefficients are downloaded from the QC1 DB and are applied (on the fly) to the center of the array (pixel 512,512), and the four detector quadrant center pixels (pixel 256,567 and their permutations). These values show how much the effective optical distortion y-shifts amount in spectroscopy.

top imaging spectroscopy y-position offsets

QC1 parameters

parameter QC1 database: table, name procedure
coefficients of the polynomial to translate source positions on the detector in imaging into spectroscopy isaac_strace, qc_corr_IL1, qc_corr_IL2, qc_corr_IL3; qc_corr_IM1, qc_corr_IM2, qc_corr_IM3 - The 11 y-positions of the source images acquired in the slit (in imaging mode) and the y-positions of the corresponding spectra (in spectroscopy mode) are fit via a second order function. For each of both gratings separately:
Y(image) = I1 + I2 * Y(spectrum) + I3 * Y(spectrum) * Y(spectrum).


In the cpl-based pipeline used since 2006 it is taken into account that the spatial (y-coordinate) is opposite with respect to the LR grating. In 2005 and earlier, there was for e.g. IL1 = 1026.05, IL2 = -0.983 and IL3 = -2.68e-5. From 2006 on the values will be e.g. IL1 = -25.1223, IL2 = 1.0345 and IL3 = -2.9e-5. Nothing has changed for the IM1, IM2 and IM3 QC parameters, they keep their original definition.