Measuring the calibration

The positions of the top and bottom of the slices can be measured in pixel coordinates from any IFU observation. There is a different vertical offset for each grism but you do not need to worry about this because the offset between the positions given and the actual spectra is measured each time a flat field frame is reduced.

The x-shift values can easily be measured from an arc spectrum. Values should be chosen to get the arc lines in the _ext file produced by the _EXTRACT_SLICES_ primitive as close to being straight as possible. Integer values should be used because the fine alignment will happen automatically when all the rows of the spectrum are scrunched to a common wavelength scale. Inaccurate values will shift the spectral lines making it difficult for iarc to wavelength calibrate all the rows. Using non-integer values will cause the spectrum to be resampled twice in the dispersion direction, which is neither necessary or desirable. Positive values will move the spectrum in the positive x direction.

The y-shift value is the most difficult to measure. This shifts each slice along its length which is necessary for accurate image reconstruction. This can be measured by using telescope offsets to scan a star across the field of view of the IFU. The star should be stepped across the field in steps of approximately one slice width (0.25 arcsec) then returned to the initial position to check for systematic drifts. The integrations should be long enough to smooth the seeing effects. The datacubes or extracted images can be added together to produce a horizontal line across the field of view. The offset from one slice to another can be measured and offsets applied. A positive value of the y-offset will move the slice in the positive y direction. Note that the plate scale varies slightly from one slice to another (maximum variation $\sim 3\%$, corresponding to $\sim 1.5$ pixels over the length of the slice) so the offsets should be measured at the centre of the slices along their length. Alternatively they can be measured at two positions equal distances from the centre of the slices in the y direction and the average offset used.